Minyak bumi (bahasa Inggris: petroleum, dari bahasa Latin: petrus ), dijuluki juga sebagai emas hitam adalah cairan kental, coklat gelap, atau kehijauan yang mudah terbakar, yang berada di lapisan atas dari beberapa area di kerak bumi. Minyak bumi dan gas alam berasal dari jasad renik lautan, tumbuhan dan hewan yang mati sekitar 150 juta tahun yang lalu. Sisa-sisa organisme tersebut mengendap di dasar lautan, kemudian ditutupi oleh lumpur. Lapisan lumpur tersebut lambat laun berubah menjadi batuan karena pengaruh tekanan lapisan di atasnya. Sementara itu, dengan meningkatnya tekanan dan suhu, bakteri anaerob menguraikan sisa-sisa jasad renik tersebut dan mengubahnya menjadi minyak dan gas.
Proses pembentukan minyak bumi dan gas ini memakan waktu jutaan tahun. Minyak dan gas yang terbentuk meresap dalam batuan yang berpori seperti air dalam batu karang. Minyak dan gas dapat pula bermigrasi dari suatu daerah ke daerah lain, kemudian terkosentrasi jika terhalang oleh lapisan yang kedap.
Walupun minyak bumi dan gas alam terbentuk di dasar lautan, banyak sumber minyak bumi yang terdapat di daratan. Hal ini terjadi karena pergerakan kulit bumi, sehingga sebagian lautan menjadi daratan.

Dewasa ini terdapat dua teori utama yang berkembang mengenai asal usul terjadinya minyak bumi, antara lain:

1. Teori Anorganik (Abiogenesis)

Barthelot (1866) mengemukakan bahwa di dalam minyak bumi terdapat logam alkali, yang dalam keadaan bebas dengan temperatur tinggi akan bersentuhan dengan CO2 membentuk asitilena. Kemudian Mandeleyev (1877) mengemukakan bahwa minyak bumi terbentuk akibat adanya pengaruh kerja uap pada karbida-karbida logam dalam bumi. Yang lebih ekstrim lagi adalah pernyataan beberapa ahli yang mengemukakan bahwa minyak bumi mulai terbentuk sejak zaman prasejarah, jauh sebelum bumi terbentuk dan bersamaan dengan proses terbentuknya bumi. Pernyataan tersebut berdasarkan fakta ditemukannya material hidrokarbon dalam beberapa batuan meteor dan di atmosfir beberapa planet lain. Secara umum dinyatakan seperti dibawah ini:

Berdasarkan teori anorganik, pembentukan minyak bumi didasarkan pada proses kimia, yaitu :

a. Teori alkalisasi panas dengan CO2 (Berthelot)

Reaksi yang terjadi:

alkali metal + CO2 karbida

karbida + H2O ocetylena

C2H2 C6H6 komponen-komponen lain

Dengan kata lain bahwa didalam minyak bumi terdapat logam alkali dalam keadaan bebas dan bersuhu tinggi. Bila CO2 dari udara bersentuhan dengan alkali panas tadi maka akan terbentuk ocetylena. Ocetylena akan berubah menjadi benzena karena suhu tinggi. Kelemahan logam ini adalah logam alkali tidak terdapat bebas di kerak bumi.

b. Teori karbida panas dengan air (Mendeleyef)

Asumsi yang dipakai adalah ada karbida besi di dalam kerak bumi yang kemudian bersentuhan dengan air membentuk hidrokarbon, kelemahannya tidak cukup banyak karbida di alam.

2.Teori Organik (Biogenesis)

P.G. Mackuire yang pertama kali mengemukakan pendapatnya bahwa minyak bumi berasal dari tumbuhan. Beberapa argumentasi telah dikemukakan untuk membuktikan bahwa minyak bumi berasal dari zat organik yaitu:
- Minyak bumi memiliki sifat dapat memutar bidang polarisasi,ini disebabkan oleh adanya kolesterol atau zat lemak yang terdapat dalam darah, sedangkan zat organik tidak terdapat dalam darah dan tidak dapat memutar bidang polarisasi.
- Minyak bumi mengandung porfirin atau zat kompleks yang terdiri dari hidrokarbon dengan unsur vanadium, nikel, dsb.
- Susunan hidrokarbon yang terdiri dari atom C dan H sangat mirip dengan zat organik, yang terdiri dari C, H dan O. Walaupun zat organik menggandung oksigen dan nitrogen cukup besar.
- Hidrokarbon terdapat di dalam lapisan sedimen dan merupakan bagian integral sedimentasi.
- Secara praktis lapisan minyak bumi terdapat dalam kambium sampai pleistosan.
- Minyak bumi mengandung klorofil seperti tumbuhan.

Proses pembentukan minyak bumi terdiri dari tiga tingkat, yaitu:
1. Pembentukan sendiri, terdiri dari:
- pengumpulan zat organik dalam sedimen
- pengawetan zat organik dalam sedimen
- transformasi zat organik menjadi minyak bumi.
2. Migrasi minyak bumi yang terbentuk dan tersebar di dalam lapisansedimen terperangkap.
3. Akumulasi tetes minyak yang tersebar dalam lapisan sedimen hingga berkumpil menjadi akumulasi komersial.

Proses kimia organik pada umumnya dapat dipecahkan dengan percobaan di laboratorium, namun berbagai faktor geologi mengenai cara terdapatnya minyak bumi serta penyebarannya didalam sedimen harus pula ditinjau. Fakta ini disimpulkan oleh Cox yang kemudian di kenal sebagai pagar Cox diantaranya adalah:
Minyak bumi selalu terdapat di dalam batuan sedimen dan umumnya pada sedimen marine, fesies sedimen yang utama untuk minyak bumi yang terdapat di sekitar pantai.
Minyak bumi memeng merupakan campuran kompleks hidrokarbon.
Temperatur reservior rata-rata 107°C dan minyak bumi masih dapat bertahan sampai 200°C. Diatas temperatur ini forfirin sudah tidak bertahan.
Minyak bumi selalu terbentuk dalam keadaan reduksi ditandai adanya forfirin dan belerang.
Minyak bumi dapat tahan pada perubahan tekanan dari 8-10000 psi.
Proses transformasi zat organik menjadi minyak bumi.

Ada beberapa hal yang mempengaruhi peristiwa diatas, diantaranya:
1. Degradasi thermal
Akibat sedimen terkena penimbunan dan pembanaman maka akan timbul perubahan tekanan dan suhu. Perubahan suhu adalah faktor yang sangat penting.
2. Reaksi katalis
Adanya katalis dapat mempercepat proses kimia.
3. Radioaktivasi
Pengaruh pembombanderan asam lemak oleh partikel alpha dapay membentuk hidrokarbon parafin. Ini menunjukan pengaruh radioaktif terhadap zat organik.
4. Aktifitas bakteri.
Bakteri mempunyai potensi besar dalam proses pembentukan hidrokarbon minyak bumi dan memegang peranan dari sejak matinya senyawa organik sampai pada waktu diagnosa, serta menyiapkan kondisi yang memungkinkan terbentuknya minyak bumi.

Zat organik sebagai bahan sumber
Jenis zat oragink yang dijadikan sumber minyak bumi menurut para ahli dap[at disimpulkan bahwa jenis zat organik yang merupakan zat pembentuk utama minyak bumi adalah lipidzat organik dapat terbentuk dalamkehidupan laut ataupun darat dan dapat dibagi menjadi dua jenis, yaitu: yang berasal dari nabati dan hewani.

Sumber : http://mochijar.blogspot.com/2009/02/proses-pembentukan-minyak-bumi.html

mungkin kita membutuhkannya. Apabila Anda mengalami kejadian ini maka cara menentukan

skala peta dengan langkah-langkah sebagai berikut:

1.	Membandingkan dua jarak tempat di peta dengan jarak kedua tempat di lapangan
	Contoh: Jarak antara Jakarta dan Bekasi di lapangan 20 km (2.000.000 cm). Di peta jarak keduanya 50 cm. Tentukan skala petanya! Jawab: Skala peta tersebut = = 40.000 Sehingga skala petanya = 1 : 40.000. Membandingkan dengan peta lain yang luasnya sama dan telah diketahui skalanya. Contoh:
	- Ukur jarak 2 tempat yang diketahui dalam kedua peta itu. Peta I = jarak A – B = 20 cm Peta II = jarak A – B = 4 cm - Pada peta I jarak A – B dilapangan: = 2 x 50.000 cm = 100.000 cm - Pada peta I jarak AB x cm 20 x x = 20x = 20x cm = 200.000 cm = 10.000 cm Jadi skala peta I = 1 : 10.000
	Dari penyelesaian contoh soal tersebut dapat dibuat kesimpulan rumusan sebagai berikut:
	J1 = Jarak yang sudah diketahui skalanya J2 = Jarak yang belum diketahui skalanya P1 = Penyebut skala peta yang sudah diketahui P2 = Penyebut skala peta yang dicari Bila data-data soal di atas dimasukkan ke rumus diperoleh: Jadi skala petanya = 1 : 10.000
2.	Membandingkan kenampakan-kenampakan dalam peta yang sudah pasti ukurannya.
	Contoh: Dalam peta terdapat lapangan sepak bola panjang lapangan 100 meter = 10.000 cm. Jadi skala lapangan sepak bola tersebut 1 : 10.000
3.	Menentukan dua titik di peta yang belum ada skalanya (peta x) misalnya titik A – B dengan arah Utara - Selatan.
	Setelah itu menghitung jarak dua titik dan selisih derajat garis lintangnya. Perlu Anda ingat bahwa jarak tiap 10 garis lintang = 111 km dan 10= 60 detik Contoh: Jarak A - B di peta x = 50 cm Selisih garis lintangnya = 30 detik Berapa skala peta x? Penyelesaian: 30 detik = x 111 km = 55,5 km = 5.550.000 cm 50 cm di peta x = 5.550.000 cm di lapangan Skala di peta x = 50 : 5.550.000 Jadi skala peta = 1 : 1.110.000
4.	Pada peta Topografi (peta Kontur) di Indonesia berlaku rumus:
	CI (Contour Interval) adalah selisih ketinggian antara dua garis kontur yang dinyatakan dalam meter. Contour Interval sering disebut jarak antara garis kontur. Garis Kontur yaitu garis-garis pada peta yang menghubungkan titik-titik yang memiliki ketinggian yang sama dari permukaan air laut. Perhitungan CI misalnya: Pada peta kontur Indonesia yang berskala 1 : 100.000, berapakah CI nya? Jawab: CI = x 100.000 = 50 meter Kembali ke contoh peta kontur yang belum ada skalanya! Contoh: Suatu peta kontur dengan Ci = 50 meter Berapakah skala peta tersebut! Jawab: Ci = 50 m 50 = x Penyebut skala Jadi penyebut skala = 100.000, ini berarti skala peta kontur tersebut 1 : 100.000
	Apabila Anda ingin mengukur jarak pada peta baik lurus atau berbelok-belok, lakukanlah hal-hal berikut: a. Gunakan seutas benang yang agak besar (misal: benang kasur) b. Berilah tanda pada peta di bagian yang diukur. c. Ukurlah dengan benang yang sudah dipersiapkan. d. Tekuklah benang mengikuti jarak obyek yang diukur, seperti jalan yang berbelok,benang juga harus ikut dibelokkan. e. Jarak yang diukur pada peta misalnya 50 cm (antara kota A dengan kota B). f. Sesuaikan dengan skala garis misalnya skala yang ada 1 : 50.000, maka jarak antara kota A dan B dilapangan = 50 cm x 50.000 = 2.500.000 cm = 25 km. Sampai disini apakah Anda sudah memahami materi tentang skala peta. Apabila sudah memahami segeralah mengerjakan tugas 1. Selamat belajar. Sumber : http://www.e-dukasi.net/mol/mo_full.php?moid=125&fname=geo103_06.htm

A mathematical contour plot, of the function f(x)=sin(x²+y²)cos(x)sin(y). Along the x-axis at -?/2 and ?/2 it is constant zero, as it is on the y-axis at all integer multiples of ?—hence the lines; the origin-centered circles are x²+y² = ?, x²+y² = 2? …

Most everyday use of the term is in cartography. A contour map (topographic map) uses contour lines (often just called a “contour”) to join points of equal elevation (height) and thus show valleys and hills, and the steepness of slopes.

Elevation contour map

More generally, a contour line for a function of two variables is a curve connecting points where the function has a same particular value. The prefix iso-, from the Greek prefix ???? (”equal”), is used from descriptive names for map lines that join points of equal value. The gradient of the function is always perpendicular to the contour lines. When the lines are close together the length of the gradient is large: the variation is steep. If adjacent contour lines are of the same line width, the direction of the gradient cannot be determined from the contour lines alone. However if contour lines rotate through three or more widths, or if the lines are numerically labelled, then the direction of the gradient can also be determined from the contour lines.

Contour lines are curved or straight lines on a map describing the intersection of a real or hypothetical surface with one or more horizontal planes. The configuration of these contours allows map readers to infer relative gradient of a parameter and estimate that parameter at specific places. Contour lines may be either traced on a visible three-dimensional model of the surface, as when a photogrammetrist viewing a stereo-model plots elevation contours, or interpolated from estimated surface elevations, as when a computer program threads contours through a network of observation points of area centroids. In the latter case, the method of interpolation affects the reliability of individual isolines and their portrayal of slope, pits and peaks (see Davis, 1986, Statistics and data analysis in geology).

Types of contour lines

Contour lines are often given specific names beginning “iso-” (from Greek ???? (isos), meaning ‘equal’) according to the nature of the variable being mapped, although in many usages the word “contour line” is most commonly used. Specific names are most common in meteorology, where multiple maps with different variables may be viewed simultaneously.

In general, an isogon is a line along which an angle is held constant. “Iso-” can be replaced with “isallo-” to specify a contour line connecting points where a variable changes at the same rate during a given time period.

Meteorology

Isohyetal map

Meteorological contour lines are based on generalization from point data received from weather stations. Weather stations are seldom exactly positioned at a contour line (when they are, this indicates a measurement precisely equal to the value of the contour). Instead, lines are drawn to best approximate the locations of exact values, based on the scattered information points available.

Meteorological contour maps may present collected data such as actual air pressure at a given time, or generalized data such as average pressure over a period of time, or forecast data such as predicted air pressure at some point in the future

Thermodynamic diagrams use multiple overlapping contour sets (including isobars and isotherms) to present a picture the major thermodynamic factors in a weather system.

Barometric pressure

An isobar (from ????? or baros, meaning ‘weight’) is a line of equal or constant pressure on a graph, plot, or map; an isopleth or contour line of pressure. In meteorology, the barometric pressures shown are reduced to sea level, not the surface pressures at the map locations. The distribution of isobars is closely related to the magnitude and direction of the wind field, and can be used to predict future weather patterns. Isobars are commonly used in television news weather reporting, though more commonly in Europe than in the United States.

An isostere is a line of constant atmospheric density

Temperature and related subjects

The 10°C mean isotherm in July, marked by the red line, is commonly used to define the Arctic region border

An isotherm (from ????? or therm?, meaning ‘heat’) is a line that connects points on a map that have the same temperature. Therefore, all points through which an isotherm passes have the same temperatures at the time indicated. Generally, isotherms representing 5°C or 10°F temperature differences are used, but any interval may be chosen.

An isogeotherm is a line of equal mean annual temperature. An isocheim is a line of equal mean winter temperature, and an isothere is a line of equal mean summer temperature.

An isohel (from ????? or helios, meaning ’sun’) is a line of equal or constant solar radiation.

Precipitation and air moisture

An isohyet or isohyetal line (from ????? or huetos, meaning ‘rain’) is a line joining points of equal precipitation on a map. A map with isohyets is called an isohyetal map.

An isohume is a line of constant relative humidity, while a isodrosotherm (from ?????? or drosos, meaning ‘dew’, and ????? or therme, meaning ‘heat’) is a line of equal or constant dew point.

An isoneph is a line indicating equal cloud cover.

An isochalaz is a line of constant frequency of hail storms.

Snow cover is frequently shown as a contour-line map.

Wind

An isotach (from ??? or tach, meaning ’speed’) is a line of constant wind speed. In general, an isogon is a line along which an angle is held constant. In meteorology, the term refers to a line of constant wind direction.

Freeze and thaw

An isopectic line denotes equal dates of ice formation each winter, and an isotac denotes equal dates of thawing.

Physical geography and oceanography

Elevation and depth

Topographic map with isohypses of height

Contours are one of several common methods used to denote elevation or altitude and depth on maps. From these contours, a sense of the general terrain can be determined. They are used at a variety of scales, from large-scale engineering drawings and architectural plans, through topographic maps up to continental-scale maps.

“Contour line” is the most common usage in cartography, but isobath for underwater depths on bathymetric maps and isohypse for elevations are also used. The process of drawing isohypse contour lines on a map is called isopletion.

In cartography, a contour interval is any space between contour lines, representing a difference in elevation between the lines. When calculated as a ratio against the map scale, a sense of the hilliness of the terrain can be derived.

Magnetism

In general, an isogon is a line along which an angle is held constant. In geomagnetism, the term refers to a line of constant magnetic declination (variance of magnetic north from geographic north). Isogonic lines are lines connecting those parts where the declination of the Earth’s magnetic field is the same in amount. They are similar to isoclinic lines, which are lines connecting points of equal magnetic inclination. The line drawn through the points of zero magnetic declination is called the agonic line.

Oceanography

Besides ocean depth, oceanographers use contour to describe diffuse variable phenomena much as meteorologists do with atmospheric phenomena. In particular, isobathytherms are lines showing depths of water with equal temperature, and isohalines show lines of equal ocean salinity.

Environmental science

In discussing pollution, density maps can be very useful in indicating sources and areas of greatest contamination. Contour maps are especially useful for diffuse forms or scales of pollution. Acid precipitation is indicated on maps with isoplats. Some of the most widespread applications of environmental science contour maps involve mapping of environmental noise, air pollution, soil contamination, thermal pollution and groundwater contamination.

Social sciences

In economics, contour lines can be used to describe features which vary quantitatively over space. An isochrone shows lines of equivalent drive time or travel time to a given location. An isotim shows equivalent transport costs from the source of a raw material, and an isodopane shows equivalent cost of travel time.

Indifference curves are used to show bundles of goods to which a person would assign equal utility. In political science an analogous method is used in understanding coalitions (for example the diagram in Laver and Shepsle’s work^[1])

Isolines can also be used to delineate qualitative differences. An isogloss, for example, is used in mapping the geographic spread of linguistic features.

Contour lines are also used in non-geographic charts in economics. An isoquant is a line of equal production quantity, and an isocost shows equal production costs.

Thermodynamics, engineering, and other sciences

Various types of graphs in thermodynamics, engineering, and other sciences use isobars (for showing constant pressure), isotherms (for constant temperature), isochors (for constant specific volume), or other types of iso-lines (or curves), even though these graphs are usually not related to maps. Such iso-lines are useful for representing more than two dimensions (or quantities) on two-dimensional graphs. Common examples in thermodynamics are some types of phase diagrams.

Other phenomena

• isochasm: aurora equal occurrence
• isochor: volume
• isodose: radiation intensity
• isophene: biological events occurring with coincidence such as plants flowering
• isophote: illuminance

History

The idea of lines that join points of equal value was rediscovered several times. In 1701, Edmond Halley used such lines (isogons) on a chart of magnetic variation.^[2] The Dutch engineer Nicholas Cruquius drew the bed of the river Merwede with lines of equal depth (isobaths) at intervals of 1 fathom in 1727, and Philippe Buache used them at 10-fathom intervals on a chart of the English Channel that was prepared in 1737 and published in 1752. The use of such lines to describe a land surface (contour lines) was studied theoretically by Ducarla in 1771, and Charles Hutton used them when calculating the volume of a hill in 1777. In 1791, a map of France by J. L. Dupain-Triel used contour lines at 20-metre intervals, hachures, spot-heights and a vertical section. In 1801, the chief of the Corps of Engineers, Haxo, used contour lines at the larger scale of 1:500 on a plan of his projects for Rocca d’Aufo. ^{[3] [4] [5]}

By around 1843, when the Ordnance Survey started to regularly record contour lines in Great Britain and Ireland, they were already in general use in European countries. Isobaths were not routinely used on nautical charts until those of Russia from 1834, and those of Britain from 1838. ^{[6] [7] [3]}

When maps with contour lines became common, the idea spread to other applications. Perhaps the latest to develop are air quality and noise pollution contour maps, which first appeared in the USA, in approximately 1970, largely as a result of national legislation requiring spatial delineation of these parameters. In 2007, Pictometry was the first to allow users to dynamically generate elevation contour lines to be laid over oblique images.

Technical construction factors

To maximize readability of contour maps, there are several design choices available to the map creator, principally line weight, line color, line type and method of numerical marking.

Line weight is simply the darkness or thickness of the line used. This choice is made based upon the least intrusive form of contours that enable the reader to decipher the background information in the map itself. If there is little or no content on the base map, the contour lines may be drawn with relatively heavy thickness. Also, for many forms of contours such as topographic maps, it is common to vary the line weight and/or color, so that a different line characteristic occurs for certain numerical values. For example, in the topographic map above, the even hundred foot elevations are shown in a different weight from the twenty foot intervals.

Line color is the choice of any number of pigments that suit the display. Sometimes a sheen or gloss is used as well as color to set the contour lines apart from the base map. Line colour can be varied to show other information; on some Swiss maps, the contour lines are changed from brown to grey to indicate bare rock and scree.

Line type refers to whether the basic contour line is solid, dashed, dotted or broken in some other pattern to create the desired effect. Dotted or dashed lines are often used when the underlying base map conveys very important (or difficult to read) information. Broken line types are used when the location of the contour line is inferred.

Numerical marking is the manner of denoting the arithmetical the values of contour lines. This can be done by placing numbers along some of the contour lines, typically using interpolation for intervening lines. The direction of these text labels is often used to indicate the direction of the slope. Alternatively a map key can be produced associating the contours with their values.

Plan view versus profile view

Most commonly contour lines are drawn in plan view. or as an observer in space would view the earth’s surface: ordinary map form. However, some parameters can often be displayed in profile view showing a vertical profile of the parameter mapped. Some of the most common parameters mapped in profile are air pollutant concentrations and sound levels. In each of those cases it may be important to analyze (air pollutant concentrations or sound levels) at varying heights so as to determine the air quality or noise health effects on people at different elevations, for example, living on different floor levels of an urban apartment. One can see an example of vertical contours in the article on noise barriers. In actuality, both plan and profile view contour maps are used in air pollution and noise pollution studies.

Labeling contour maps

Contour map labeled aesthetically in an “elevation up” manner.

Labels are a critical component of elevation maps. A properly labeled contour map helps the reader to quickly interpret the shape of the terrain. If numbers are placed close to each other, it means that the terrain is steep. Labels should be placed along a slightly curved line “pointing” to the summit or nadir, from several directions if possible, making the visual identification of the summit or nadir easy.^[8][9]

Manual labeling of contour maps is a time-consuming process, however, there are a few software systems that can do the job automatically and in accordance with cartographic conventions, called automatic label placement.

References

A topographic map is a type of map characterized by large-scale detail and quantitative representation of relief, usually using contour lines in modern mapping, but historically using a variety of methods. Traditional definitions require a topographic map to show both natural and man-made features,^{[1] [2]}

The Centre for Topographic Information provides this definition of a topographic map:

However, in the vernacular and day to day world, the representation of relief (contours) is popularly held to define the genre, such that even small-scale maps showing relief are commonly (and erroneously, in the technical sense) called “topographic.” According to Cartographer’s Kraak and Ormeling,

The study or discipline of topography, while interested in relief, is actually a much broader field of study which takes into account all natural and man made features of terrain.

History

Topographic maps are based on topographical surveys. Performed at large scales, these surveys are called topographical in the old sense of topography, showing a variety of landmark and landscape information.^[3] This is in contrast to older cadastral surveys, which primarily show property and governmental boundaries. The first multi-sheet topographic map series of an entire country, the Carte géométrique de la France, was completed in 1789.^[4] Topographic surveys were prepared by the military to assist in planning for battle and for defensive emplacements (thus the name and history of the United Kingdom’s Ordnance Survey).^[5] As such, elevation information was of vital importance.

As they evolved, topographic map series became a basic national resource in modern nations in planning infrastructure and resource exploitation. In the United States, the national map-making function which had been shared by both the Army Corps of Engineers and the Department of the Interior migrated to the newly created United States Geological Survey in 1879, where it has remained since.^{[6] [7]}

Uses

Topographic maps have multiple uses in the present day: any type of geographic planning or large-scale architecture; earth sciences and many other geographic disciplines; mining and other earth-based endeavours; and recreational uses such as hiking or, in particular, orienteering, which uses highly detailed maps in its standard requirements.

Map conventions

The various features shown on the map are represented by conventional signs or symbols. For example, colors can be used to indicate a classification of roads. These signs are usually explained in the margin of the map, or on a separately published characteristic sheet.^[8]

Topographic maps are also commonly called contour maps or topo maps. In the United States, where the primary national series is organized by a strict 7.5 minute grid, they are often called topo quads or quadrangles.

Topographic maps conventionally show topography, or land contours, by means of contour lines. Contour lines are curves that connect contiguous points of the same altitude (isohypse). In other words, every point on the marked line of 100 m elevation is 100 m above mean sea level.

There are several rules to note when viewing topographic maps:

• The rule of V’s: sharp-pointed vees usually are in stream valleys, with the drainage channel passing through the point of the vee, with the vee pointing upstream. This is a consequence of erosion.
• The rule of O’s: closed loops are normally uphill on the inside and downhill on the outside, and the innermost loop is the highest area. If a loop instead represents a depression, some maps note this by short lines radiating from the inside of the loop, called “hachures”.
• Spacing of contours: close contours indicate a steep slope; distant contours a shallow slope. Two or more contour lines merging indicates a cliff.

Of course, to determine differences in elevation between two points, the contour interval, or distance in altitude between two adjacent contour lines, must be known, and this is given at the bottom of the map. In most cases, contour intervals are consistent throughout a map. Sometimes dashed contour lines are present; these represent half the noted contour interval.

These maps usually show not only the contours, but also any significant streams or other bodies of water, forest cover, built-up areas or individual buildings (depending on scale), and other features and points of interest.

Today, topographic maps are prepared using photogrammetric interpretation of aerial photography. Older topographic maps were prepared using traditional surveying instruments.

Publishers of national topographic map series

Most countries have some sort of national mapping program. Those listed below are only a small selection. Several commercial vendors supply international topographic map sets.

Canada

The Centre for Topographic Information produces topographic maps of Canada at scales of 1:50,000 and 1:250,000. They are known as the National Topographic System (NTS).^[9] A government proposal to discontinue publishing of all hardcopy or paper topographic maps in favor of digital-only mapping data was shelved in 2006 after intense public opposition.^[10]

Denmark

The National Survey and Cadastre of Denmark is responsible for producing topographic and nautical geodata of Denmark, Greenland and the Faroe Islands.^[11]

Finland

The National Land Survey of Finland produces topographic maps of Finland at 1:20,000 and 1:50,000.^[12]

France

The Institut Géographique National (or IGN) produces topographic maps of France at 1:25,000 and 1:50,000.^[13] In addition, topographic maps are freely accessible online, through the Géoportail website.

India

The Survey of India is responsible for all topographic control, surveys and mapping of India.^[14]

Japan

The Geographical Survey Institute of Japan is responsible for base mapping of Japan. Standard map scales are 1:25,000, 1:50,000, 1:200,000 and 1:500,000 ^[15]

New Zealand

Land Information New Zealand is the government agency responsible for providing up-to-date topographic mapping. LINZ topographic maps cover all of New Zealand, offshore islands, some Pacific Islands and the Ross Sea Region.^[16] Vector data[1] from the New Zealand Topographic Database (NZTopo) is also available. NZTopoOnline[2] is a publicly accessible, free online service.

Switzerland

Swisstopo (the Federal Office of Topography) produces topographic maps of Switzerland at seven different scales.

United Kingdom

The Ordnance Survey (or OS) produces topographic map series covering the United Kingdom at 1:25,000 and 1:50,000 scales. The 1:25,000 scale is known as the “Explorer” series, and include an “OL” (Outdoor Leisure) sub-series for areas of special interest to hikers and walkers. It was formerly known as the “Pathfinder” series. The 1:50,000 scale is known as the “Landranger” and carries a distinctive pink cover. More detailed mapping as fine as 1:10000 cover some parts of the country.^[17] The 1:25K and 1:50K metric scales are easily coordinated with standard romer scales on currently available compasses and plotting tools. Ordnance survey maintains a mapping database from which they can print specialist maps at virtually any scale.^[18]

United States

The United States Geological Survey (or USGS), a civilian Federal agency, produces several national series of topographic maps which vary in scale and extent, with some wide gaps in coverage, notably the complete absence of 1:50,000 scale topographic maps or their equivalent. The largest (both in terms of scale and quantity) and best-known topographic series is the 7.5-minute, 1:24,000 scale, quadrangle, a non-metric scale virtually unique to the United States. Each of these maps covers an area bounded by two lines of latitude and two lines of longitude spaced 7.5 minutes apart. Nearly 57,000 individual maps in this series cover the 48 contiguous states, Hawaii, U. S. territories, and areas of Alaska near Anchorage, Fairbanks, and Prudhoe Bay. The area covered by each map varies with the latitude of its represented location due to convergence of the meridians. At lower latitudes, near 30° north, a 7.5-minute quadrangle contains an area of about 64 square miles (166 km²). At 49° north latitude, 49 square miles (127 km²) are contained within a quadrangle of that size. As a unique non-metric map scale, the 1:24,000 scale naturally requires a separate and specialized romer scale for plotting map positions.^[19] In recent years, budget constraints have forced the USGS to rely on donations of time by civilian volunteers in an attempt to update its 7.5-minute topographic map series, and USGS stated outright in 2000 that the program was to be phased out in favor of their National Map^[20] (not to be confused with the National Atlas of the United States produced by the Department of the Interior, one of whose bureaus is USGS).

An older series of maps, the 15-minute series, was once used to map the contiguous 48 states at a scale of 1:62,500, but was discontinued some time ago for maps covering the continental U.S. Each map was bounded by two parallels and two meridians spaced 15 minutes apart - the same area covered by four maps in the 7.5-minute series. The 15-minute series, at a scale of 1:63,360 (one inch representing one mile), remains the primary topographic quadrangle for the state of Alaska (and only for that particular state). Nearly 3,000 maps cover 97% of the state.^[19] The U.S.A. remains virtually the only developed country in the world without a standardized civilian topographic map series in the standard 1:25,000 or 1:50,000 metric scales, making coordination difficult in border regions (the U.S. military does issue 1:50,000 scale topo maps of the continental U.S., though only for use by members of its defense forces).

The next-smallest topographic series, in terms of scale, is the 1:100,000 series. These maps are bounded by two lines of longitude and two lines of latitude. However, in this series, the lines of latitude are spaced 30 minutes apart and the lines of longitude are spaced 60 minutes, which is the source of another name for these maps; the 30 x 60-minute quadrangle series. Each of these quadrangles covers the area contained within 32 maps in the 7.5-minute series. The 1:100,000 scale series is unusual in that it employs the Metric system primarily. One centimeter on the map represents one kilometer of distance on the ground. Contour intervals, spot elevations, and horizontal distances are also specified in meters.

The final regular quadrangle series produced by the USGS is the 1:250,000 scale topographic series. Each of these quadrangles in the conterminous United States measures 1 degree of latitude by 2 degrees of longitude. This series was produced by the U.S. Army Map Service in the 1950s, prior to the maps in the larger-scale series, and consists of 489 sheets, each covering an area ranging from 8,218 square miles (21,285 km²) at 30° north to 6,222 square miles (16,115 km²) at 49° north.^[19] Hawaii is mapped at this scale in quadrangles measuring 1° by 1°.

USGS topographic quadrangle maps are marked with grid lines and tics around the map collar which make it possible to identify locations on the map by several methods, including the graticule measurements of longitude and latitude, the township and section method within the Public Land Survey System, and cartesian coordinates in both the State Plane Coordinate System and the Universal Transverse Mercator coordinate system.

Other specialty maps have been produced by the USGS at a variety of scales. These include county maps, maps of special interest areas, such as the national parks, and areas of scientific interest.

A number of Internet sites have made these maps available on the web for affordable commercial and professional use. Because works of the U.S. Government are in the public domain, it is also possible to find many of these maps for free at various locations on the Internet. Georeferenced map images are available from the USGS as digital raster graphics (DRGs), in addition to digital data sets based on USGS maps (notably Digital Line Graphs (DLGs) and digital elevation models (DEMs)).

Global 1-kilometer map

This map is derived from GTOPO30 data that describes the elevation of Earth’s terrain at intervals of 30 arcseconds (approximately 1 km). It uses color and shading instead of contour lines to indicate elevation.

Each tile is available at a resolution of 1800 × 1800 pixels (approximate file size 1 MB, 60 pixels = 1 degree, 1 pixel = 1 minute)

Source : http://en.wikipedia.org/wiki/Topographic_map

Development

An important cartographic element preceding thematic mapping was the development of accurate base maps. Improvements in accuracy proceeded at a gradual pace, and even until the mid-17th century, general maps were usually of poor quality. Still, base maps around this time were good enough to display appropriate information, allowing for the first thematic maps to come into being.

Examples of early thematic cartographers

Edmond Haley’s map of ocean currents.

John Snow

The most widely touted example of early thematic mapping comes from London physician John Snow. Though disease had been mapped thematically, Snow’s cholera map in 1855 is the best known example of using thematic maps for analysis. Essentially, his technique and methodology anticipate principles of a geographic information system (GIS).

John Snow’s Cholera map (dot style).

Starting with an accurate base map of a London neighborhood which included streets and pump locations, Snow mapped out the incidents of cholera death. The emerging pattern centered around one particular pump on Broad Street. At Snow’s request, the handle of the pump was removed, and new cholera cases ceased almost at once. Further investigation of the area revealed the Broad Street pump was near a sewer line.

Uses of thematic maps

Thematic maps serve three primary purposes. First, they provide specific information about particular locations. Second, they provide general information about spatial patterns. Third, they can be used to compare patterns on two or more maps. Common examples are maps of demographic data such as population density. When designing a thematic map, cartographers must balance a number of factors in order to effectively represent the data. Besides spatial accuracy, and aesthetics, quirks of human visual perception and the presentation format must be taken into account.

Of equal importance is audience. Who will “read” the thematic map and for what purpose helps define how it should be designed. A political scientist might prefer having information mapped within clearly delineated county boundaries (choropleth maps). A state biologist could certainly benefit from county boundaries being on a map, but nature seldom falls into such smooth, man-made delineations. In which case, a dasymetric map charts the desired information underneath a transparent county boundary map for easy location referencing.

Displaying data

In constructing any type of thematic map (or any map for that matter) it is understood that location is a key feature. After selecting the physical area to examine, the next step is collecting data sets.

Data dealing with one subject is called univariate, which examines occurrences of a single type of event. The distribution of population, cancer rates, and rainfall are all examples of univariate data.

Bivariate mapping shows the distribution of two sets of data to explore possibilities of correlations. For example, we can examine population density in relation to textile manufacturing. Other examples could be cancer rates and population density, or rainfall and elevation.

More than two sets of data leads to multivariate mapping. Taking three or more data sets and displaying the result on a map helps determine possible correlations between different phenomena. For instance, our bivariate example maps two data sets, rainfall and elevation. If we add another variable such as population density, our map becomes multivariate rather than bivariate.

Map makers must be careful in designing thematic maps that display too much information or suggest phenomenon have a correlation when in fact they do not.

Methods of thematic mapping

Geographers use many methods to create thematic maps, but five techniques are especially noted.

Choropleth

Choropleth map of water use.

The most commonly used method of thematic mapping. Choropleth maps are particularly suited for charting phenomena that are evenly distributed within each enumeration unit (set area).

Raw data , e.g. population distribution, should not be mapped with this technique. If a derived values can be otained from raw data, (such as population densities), then the choropleth can apply.

Proportional symbol

Also known as graduated symbols, these maps represent data associated with point locations (i.e., cities or counties). The data is displayed with proportionally sized symbols to graphically represent a realistic difference in occurrence. If the raw data cannot be dealt with as a ratio or proportion, then they should be portrayed with the proportional symbol technique.

Isarithmic

Isarithmic map of barometric pressure.

These maps, also known as contour maps, depict smooth continuous phenomena such as precipitation. They are also well-suited to displaying three-dimensional values such as elevation i.e; on topographic maps.

Dot

A map using dots to show the presence of a feature or occurrence and display a spatial pattern. Dr. Snow used this method in his famous map. One dot represented one death. Note, though, that a dot is not required to represent a single unit and may indicate any number of entities; 14 armadillos, 7 dwarves, 100 voters.

Dasymetric

Dasymetric map of climate and plant hardiness zones.

These maps utilize areal symbols. However, although boundaries are displayed on dasymetric maps, these geographic units may span multiple theme values. Plots often represent extremes in the data sets, without much coverage in between. For that reason, and because they can be difficult to generate, dasymetric maps are not very common.

References

• Muehrcke, P…et al. Map Use, The University of Chicago Press, 4th Edition, 2001
• Petchenik, B. B. From Place to Space: The Psychological Achievement in Thematic Mapping, American Cartographer 1, 1979
• Robinson, A. Early Thematic Mapping in the History of Cartography, The University of Chicago Press, 1982
• Robinson, A…et al. Elements of Cartography, Wiley, 6th Edition, 1995
• Slocum, T….et al. Thematic Cartography and Geographic Visualization, Prentice Hall, 2nd Edition, 2005
• Thrower, N. Maps and Civilization: Cartography in Culture and Society, The University of Chicago Press, 1996

Source : http://en.wikipedia.org/wiki/Thematic_map

The stratigraphic contour lines are drawn on the surface of a selected deep stratum, so that they can show the topographic trends of the strata under the ground. It is not always possible to properly show this when the strata are extremely fractured, mixed, in some discontinuities, or where they are otherwise disturbed.

Strike and dip symbols consist of (at minimum) a long line, a number, and a short line which are used to indicate tilted beds. The long line is the strike line, which shows the true horizontal direction along the bed, the number is the dip or number of degrees of tilt above horizontal, and the short line is the dip line, which shows the direction of tilt.

Mapped global geologic provinces

A geologic map or geological map is a special-purpose map made to show geological features.

History

The oldest preserved geologic map is the Turin papyrus, made around 1150 BCE for gold deposits in Egypt.

A fascinating story of the first modern geologic map is told in The Map that Changed the World, by Simon Winchester. It’s the story of William Smith, a canal digger who created the first geologic map of Great Britain in 1819, but ended up in debtor’s prison and lived homeless for 10 years until he was recognized for his work by King William IV in 1831. (Harper-Collins publishers, 2202. ISBN 0-06-093180-9)

Maps and Mapping across the globe

United States

In the United States, geologic maps are usually superimposed over a topographic map (and at times over other base maps) with the addition of a color mask with letter symbols to represent the kind of geologic unit. The color mask denotes the exposure of the immediate bedrock, even if obscured by soil or other cover. Each area of color denotes a geologic unit or particular rock formation (as more information is gathered new geologic units may be defined). However, in areas where the bedrock is overlain by a significantly thick unconsolidated burden of till, terrace deposits, loess deposits, or other important feature, these are shown instead. Stratigraphic contour lines, fault lines, strike and dip symbols, are represented with various symbols as indicated by the map key. Whereas topographic maps are produced by the United States Geological Survey in conjunction with the states, geologic maps are usually produced by the individual states. There are almost no geologic map resources for some states, while a few states, such as Kentucky, are extensively mapped geologically.

United Kingdom

In the United Kingdom the term geological map is used. The UK and Isle of Man have been extensively mapped by the British Geological Survey since 1835; a separate Geological Survey of Northern Ireland (drawing on BGS staff) has operated since 1947.

Two 1:625,000 scale maps cover the basic geology for the UK. More detailed sheets are available at scales of 1:250,000, 1:50,000 and 1:10,000. The 1:625,000 and 1:250,000 scales show both onshore and offshore geology (the 1:250,000 series covers the entire UK continental shelf), whilst other scales generally cover exposures on land only.

Sheets of all scales (though not for all areas) fall into two categories:

Superficial deposit maps (previously known as solid and drift maps) show both bedrock and the deposits on top of it.

Bedrock maps (previously known as solid maps) show the underlying rock, without superficial deposits.

The maps are superimposed over a topographic map base produced by Ordnance Survey, and use symbols to represent fault lines, strike and dip or geological units, boreholes etc. Colors are used to represent different geological units. Explanatory booklets (memoirs) are produced for many sheets at the 1:50,000 scale.

Small scale thematic maps (1:1,000,000 to 1:100,000) are also produced covering geochemistry, gravity anomaly, magnetic anomaly, groundwater, etc.

Source : http://en.wikipedia.org/wiki/Geologic_map

Many maps are static two-dimensional, geometrically accurate representations of three-dimensional space, while others are dynamic or interactive, even three-dimensional. Although most commonly used to depict geography, maps may represent any space, real or imagined, without regard to context or scale; e.g. Brain mapping, DNA mapping, and extraterrestrial mapping.

Geographic maps

A celestial map from the 17th century, by the Dutch cartographer Frederik de Wit.

Cartography, or map-making is the study and, often, practice, of crafting representations of the Earth upon a flat surface (see History of cartography), and one who makes maps is called a cartographer.

Road maps are perhaps the most widely used maps today, and form a subset of navigational maps, which also include aeronautical and nautical charts, railroad network maps, and hiking and bicycling maps. In terms of quantity, the largest number of drawn map sheets is probably made up by local surveys, carried out by municipalities, utilities, tax assessors, emergency services providers, and other local agencies. Many national surveying projects have been carried out by the military, such as the British Ordnance Survey (now a civilian government agency internationally renowned for its comprehensively detailed work).

A map can also be any document giving information as to where or what something is.

Orientation of maps

The Hereford Mappa Mundi, about 1300, Hereford Cathedral, England. A classic “T-O” map with Jerusalem at centre, east toward the top, Europe the bottom left and Africa on the right.

The term orientation refers to the relationship between directions on a map and compass directions. The word orient is derived from oriens, meaning east. In the Middle Ages many maps, including the T and O maps, were drawn with east at the top. Today the most common, but far from universal, cartographic convention is that North is at the top of a map. Examples of maps not oriented to north are:

• Reversed maps, also known as Upside-Down maps or South-Up maps, which generally show Australia and New Zealand at the top of the map instead of the bottom.
• Polar maps of the Arctic or Antarctic regions are conventionally centred on the pole, in which case the direction north would be towards or away from the centre of the map, respectively.
• Buckminster Fuller’s Dymaxion maps are based on a projection of the Earth’s sphere onto an icosahedron. The resulting triangular pieces may be arranged in any order or orientation.
• Maps from non-Western traditions are oriented a variety of ways. Old maps of Edo show the Japanese imperial palace as the “top”, but also at the centre, of the map. Labels on the map are oriented in such a way that you cannot read them properly unless you put the imperial palace above your head.
• Medieval European T and O maps such as the Hereford Mappa Mundi were centred on Jerusalem with east at the top. Indeed, prior to the reintroduction of Ptolemy’s Geography to Europe around 1400, there was no single convention in the West. Portolan charts, for example, are oriented to the shores they describe.
• Route and channel maps have traditionally been oriented to the road or waterway they describe.
• Many maps used in the Society for Creative Anachronism show the west at the top, in honour of the Society starting in California.^{[citation needed]}

Scale and accuracy

Sample detail of a 1:50,000 topographic map of the Swiss Alps (Swiss Federal Office of Topography).

Many but not all maps are drawn to a scale, expressed as a ratio such as 1:10,000, meaning that 1 of any unit of measurement on the map corresponds to 10,000 of that same unit in reality. This allows the reader to estimate the sizes of, and distances between, depicted objects. A larger scale (i.e. the second number of the ratio is smaller) shows more detail and supports more accurate estimates, thus requiring a larger map to show the same area. Highly detailed maps covering areas ranging upward in size from small cities or counties to entire countries or continents are now often published as books, or computer software (with numerous tools to aid the user, including user-adjustable scale and customized search engines), for convenient handling. Printed versions may include a comprehensive index, tables of distances between cities, and possibly even a cross reference of important destinations. Computer software based maps provide numerous tools to aid the user, including user-adjustable scale (a.k.a “zoom”) and customized search engines to locate street addresses.

Historically, large maps were presented (but not necessarily published, due to prohibitive labor costs) as scrolls, a famous example of which is the recently rediscovered hand-made copy of the Tabula Peutingeriana^[1].

For modern examples, published maps designed for the hiker (e.g. USGS Topographic maps, a.k.a. “Topos”) are often scaled at the ratio of approximately 1:25,000^{[citation needed]}, while maps designed for the motorist to display major highways might be scaled at 1:250,000 or 1:1,000,000^{[citation needed]}. In any case, a properly made map will either state its scale, or declare that it is not scaled and can not be reliably used to deduce distances.

Cartogram: The EU distorted to show population distributions.

Maps which use some quality other than physical area to determine relative size are called cartograms.

A famous (non-cartogram) example of a map without scale is the London Underground map, which best fulfills its purpose by being less physically accurate and more visually communicative to the hurried glance of the commuter. This is not a cartogram (since there is no consistent measure of distance) but a topological map that also depicts approximate bearings. The simple maps shown on some directional road signs are further examples of this kind.

In fact, most commercial navigational maps, such as road maps and town plans, sacrifice an amount of accuracy in scale to deliver a greater visual usefulness to its user, for example by exaggerating the width of roads. With the end-user similarly in mind, cartographers will censor the content of the space depicted by a map in order to provide a useful tool for that user. For example, a road map may or may not show railroads, smaller waterways or other prominent non-road objects, and if it does, it may show them less clearly (e.g. dashed or dotted lines/outlines of various colors) than highways. Known as decluttering, the practice makes the subject matter the user is interested in easier to read, usually without sacrificing measurement accuracy. Software-based maps often allow the user to toggle decluttering between ON, OFF and AUTO as needed. In AUTO the degree of decluttering is adjusted as the user changes the scale being displayed.

Topographic maps, show elevation above (or depression below) sea level as contour lines, a specific type of Isoline. Isolines on any map or chart indicate the constant labeled value, such as elevation, temperature, or rainfall, for that particular line. Depending on the type of a map, alternative representations of elevation (or depression) exist as well.

Scale and accuracy

Sample detail of a 1:50,000 topographic map of the Swiss Alps (Swiss Federal Office of Topography).

Many but not all maps are drawn to a scale, expressed as a ratio such as 1:10,000, meaning that 1 of any unit of measurement on the map corresponds to 10,000 of that same unit in reality. This allows the reader to estimate the sizes of, and distances between, depicted objects. A larger scale (i.e. the second number of the ratio is smaller) shows more detail and supports more accurate estimates, thus requiring a larger map to show the same area. Highly detailed maps covering areas ranging upward in size from small cities or counties to entire countries or continents are now often published as books, or computer software (with numerous tools to aid the user, including user-adjustable scale and customized search engines), for convenient handling. Printed versions may include a comprehensive index, tables of distances between cities, and possibly even a cross reference of important destinations. Computer software based maps provide numerous tools to aid the user, including user-adjustable scale (a.k.a “zoom”) and customized search engines to locate street addresses.

Historically, large maps were presented (but not necessarily published, due to prohibitive labor costs) as scrolls, a famous example of which is the recently rediscovered hand-made copy of the Tabula Peutingeriana^[1].

For modern examples, published maps designed for the hiker (e.g. USGS Topographic maps, a.k.a. “Topos”) are often scaled at the ratio of approximately 1:25,000^{[citation needed]}, while maps designed for the motorist to display major highways might be scaled at 1:250,000 or 1:1,000,000^{[citation needed]}. In any case, a properly made map will either state its scale, or declare that it is not scaled and can not be reliably used to deduce distances.

Cartogram: The EU distorted to show population distributions.

Maps which use some quality other than physical area to determine relative size are called cartograms.

A famous (non-cartogram) example of a map without scale is the London Underground map, which best fulfills its purpose by being less physically accurate and more visually communicative to the hurried glance of the commuter. This is not a cartogram (since there is no consistent measure of distance) but a topological map that also depicts approximate bearings. The simple maps shown on some directional road signs are further examples of this kind.

In fact, most commercial navigational maps, such as road maps and town plans, sacrifice an amount of accuracy in scale to deliver a greater visual usefulness to its user, for example by exaggerating the width of roads. With the end-user similarly in mind, cartographers will censor the content of the space depicted by a map in order to provide a useful tool for that user. For example, a road map may or may not show railroads, smaller waterways or other prominent non-road objects, and if it does, it may show them less clearly (e.g. dashed or dotted lines/outlines of various colors) than highways. Known as decluttering, the practice makes the subject matter the user is interested in easier to read, usually without sacrificing measurement accuracy. Software-based maps often allow the user to toggle decluttering between ON, OFF and AUTO as needed. In AUTO the degree of decluttering is adjusted as the user changes the scale being displayed.

Topographic maps, show elevation above (or depression below) sea level as contour lines, a specific type of Isoline. Isolines on any map or chart indicate the constant labeled value, such as elevation, temperature, or rainfall, for that particular line. Depending on the type of a map, alternative representations of elevation (or depression) exist as well.

World maps and projections

Main article: World map

Map of large underwater features. (1995, NOAA)

Maps of the world or large areas are often either ‘political’ or ‘physical’. The most important purpose of the political map is to show territorial borders; the purpose of the physical is to show features of geography such as mountains, soil type or land use. Geological maps show not only the physical surface, but characteristics of the underlying rock, fault lines, and subsurface structures.

Maps that depict the surface of the Earth also use a projection, a way of translating the three-dimensional real surface of the geoid to a two-dimensional picture. Perhaps the best-known world-map projection is the Mercator Projection, originally designed as a form of nautical chart.

Airplane pilots use aeronautical charts based on a Lambert conformal conic projection, in which a cone is laid over the section of the earth to be mapped. The cone intersects the sphere (the earth) at one or two parallels which are chosen as standard lines. This allows the pilots to plot a great-circle route approximation on a flat, two-dimensional chart.

• Azimuthal or Gnomonic map projections are often used in planning air routes due to their ability to represent great circles as straight lines. Reginald Buckminster Fuller patented one such Gnomonic projection in 1946 as the Dymaxion Map.

• Richard Edes Harrison produced a striking series of maps during and after World War II for Fortune magazine. These used “bird’s eye” projections to emphasize globally strategic “fronts” in the air age, pointing out proximities and barriers not apparent on a conventional rectangular projection of the world.

Electronic maps

A USGS digital raster graphic.

From the last quarter of the 20th century, the indispensable tool of the cartographer has been the computer. Much of cartography, especially at the data-gathering survey level, has been subsumed by Geographic Information Systems (GIS). The functionality of maps has been greatly advanced by technology simplifying the superimposition of spatially located variables onto existing geographical maps. Having local information such as rainfall level, distribution of wildlife, or demographic data integrated within the map allows more efficient analysis and better decision making. In the pre-electronic age such superimposition of data led Dr. John Snow to discover the cause of cholera. Today, it is used by agencies as diverse as wildlife conservationists and militaries around the world.

Even when GIS is not involved, most cartographers now use a variety of computer graphics programs to generate new maps.

Interactive, computerised maps are commercially available, allowing users to zoom in or zoom out (respectively meaning to increase or decrease the scale), sometimes by replacing one map with another of different scale, centred where possible on the same point. In-car satellite navigation systems are computerised maps with route-planning and advice facilities which monitor the user’s position with the help of satellites. From the computer scientist’s point of view, zooming in entails one or a combination of:

1. replacing the map by a more detailed one
2. enlarging the same map without enlarging the pixels, hence showing more detail by removing less information compared to the less detailed version
3. enlarging the same map with the pixels enlarged (replaced by rectangles of pixels); no additional detail is shown, but, depending on the quality of one’s vision, possibly more detail can be seen; if a computer display does not show adjacent pixels really separate, but overlapping instead (this does not apply for an LCD, but may apply for a cathode ray tube), then replacing a pixel by a rectangle of pixels does show more detail. A variation of this method is interpolation.

For example:

• Typically (2) applies to a Portable Document Format (PDF) file or other format based on vector graphics. The increase in detail is, of course, limited to the information contained in the file: enlargement of a curve may eventually result in a series of standard geometric figures such as straight lines, arcs of circles or splines.
• (2) may apply to text and (3) to the outline of a map feature such as a forest or building.
• (1) may apply to the text (displaying labels for more features), while (2) applies to the rest of the image. Text is not necessarily enlarged when zooming in. Similarly, a road represented by a double line may or may not become wider when one zooms in.
• The map may also have layers which are partly raster graphics and partly vector graphics. For a single raster graphics image (2) applies until the pixels in the image file correspond to the pixels of the display, thereafter (3) applies.

See also Webpage (Graphics), PDF (Layers), Mapquest, Google Maps, Google Earth or Yahoo! Maps.

Labeling

To communicate spatial information effectively, features such as rivers, lakes, and cities need to be labeled. Over centuries cartographers have developed the art of placing names on even the densest of maps. Text placement or name placement can get mathematically very complex as the number of labels and map density increases. Therefore, text placement is time-consuming and labor-intensive, so cartographers and GIS users have developed automatic label placement to ease this process.^[2][3]

Source : http://en.wikipedia.org/wiki/Map

2. Memberitahukan berapa centimeter di atas peta yang sama dengan satu kilometer di atas permukaan bumi.

3. Menarik suatu garis, dimana di atas garis dibuat suatu skala dengan bagian-bagian yang menyatakan satu kilometer di atas permukaan bumi.

Faktor-faktor yang mempengaruhi pemilihan suatu skala peta :

· Tujuan pembuatan peta

· Luas daerah yang akan dipetakan

· Kepadatan unsur-unsur/detail

· Bentuk topografi

· Karakteristik daerah

Klasifikasi skala peta berdasarkan jenis peta :

1. Peta Topografi

· Tidak ada skala yang ideal (luas daerah, kerapatan unsur-unsur di permukaan tanah, potensi).

· Terdapat standarisasi skala pada daerah-daerah batas administrasi.

· Terdapat pertimbangan ekonomis berdasarkan karakteristik daerah.

1. Skala sangat besar = 1:1000 s/d 1:5000 (untuk perencanaan teknis)

2. Skala besar = 1:5000 s/d 1:25000

3. Skala menengah = 1:25000 s/d 1:100000

4. Skala kecil = 1:100000 s/d 1:1 000 000

· Skala yang dianjurkan oleh PBB :

1:25000 1:100000 1:250000 1:1 000 000

1:50000 1:200000 1:500000

2. Peta Kadaster (peta kantor, peta umum, peta lapangan)

· Daerah kepadatan tinggi 1:250

· Daerah kepadatan sedang 1:500

· Daerah kepadatan rendah 1:1000

3. Peta Teknis

· Untuk tujuan teknis (pengelolaan jaringan (pipa air minum, jaringan listrik, telepon, gorong-gorong) dan administratif di pemerintah kota

? Skala besar 1:1000 s/d 1:5000

? Skala menengah 1:5000 s/d 1:10000

· Untuk peta dasar (kepadatan detil dan tata guna tanah)

? Daerah kota 1:500 s/d 1:1000

? Pinggiran kota 1:1000 s/d 1:2000

? Pedesaaan 1:2000 s/d 1:4000

? Daerah lainnya 1:10000

Sumber : http://wikantika.wordpress.com/2008/05/06/apa-itu-skala-peta/