What Is A Coordinate System
Coordinate systems enable geographic datasets to use common locations for integration. A coordinate system is a reference system used to represent the locations of geographic features, imagery, and observations such as GPS locations within a common geographic framework.
Each coordinate system is defined by:
- Its measurement framework which is either geographic or planimetric .
- Unit of measurement .
- The definition of the map projection for projected coordinate systems.
- Other measurement system properties such as a spheroid of reference, a datum, and projection parameters like one or more standard parallels, a central meridian, and possible shifts in the x- and y-directions.
Using Globes Vs Projecting On A Plane
The globe is the only way to represent the earth without distorting one or more of the above-mentioned metric properties. Globes have the advantage of being true to metric properties and able to provide a true picture of spatial relationships on the earths surface. The disadvantages of the globe are that it is impractical to make large-scale maps with it, it is difficult to measure on a globe, one cant see the whole world at once and it is difficult to handle and transport a globe around .
The flat map has the disadvantage of always distorting one or more of the metric properties and it is more difficult to get a true picture of the spatial relationships between objects. Flat maps have numerous advantages however it is not practical to make large or even medium scale globes, it is easier to measure on a flat map, easy to carry around, and one can see the whole world at once.
Scale in particular is effected by the choice between using a globe vs. a plane. Only a globe can have a constant scale throughout the entire map surface and the scale for flat maps will vary from point to point and may also vary in different directions from a single point . The scale for a flat map can only be true along one or two lines or points . The scale factor is therefore used to measure the difference between the idealized scale and the actual scale at a particular point on the map.
Scale Distortions On Map Projections
Map projections without distortions would represent the correct distance, direction, shapes, and areas on a map. However, map projections have distortions which depend largely on the size of the area being mapped. Scale distortions on maps are shown on the map by an ellipse of distortion or using scale factor which is the ratio of the scale at a given point to the true scale. Distortions on maps of countries or cities are not evident to the eye and can only be identified when computing distances and areas.
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Types Of Coordinate Systems
There are two common types of coordinate systems used in GIS:
- A global or spherical coordinate system such as latitudelongitude. These are often referred to as geographic coordinate systems.
- A projected coordinate system based on a map projection such as transverse Mercator, Albers equal area, or Robinson, all of which provide various mechanisms to project maps of the earth’s spherical surface onto a two-dimensional Cartesian coordinate plane. Projected coordinate systems are sometimes referred to as map projections.
Coordinate systems provide a framework for defining real-world locations. In ArcGIS, the coordinate system is used as the method to automatically integrate the geographic locations from different datasets into a common coordinate framework for display and analysis.
What Are Map Projections And Distortions
Representing the earthâs surface in two dimensions causes distortion in the shape area distance or direction of the data. A map projection uses mathematical formulas to relate spherical coordinates on the globe to flat planar coordinates. Different projections cause different types of distortions.
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Choosing A Proper Projection
Choosing the proper projection for a map is vital to correctly presenting the map’s information or message. A proper projection helps the viewer correctly interpret the information contained on the map, while the misuse of projection gives viewers a skewed perspective of the area or information they are viewing. Poor use of projection can have a variety of negative consequences, such as the viewer gaining an inaccurate mental perception of an area or incorrect understanding of the map’s message, or a user becoming lost. Maps are created for a variety of purposes, and as a result, there is no single map projection that is better than the rest. Therefore, when choosing a map projection, it is useful to consider a number of factors such as the following:
Elements Of Map Projection
a. Reducedearth:A model of the earth is represented by the help of a reduced scale on a flat sheet of paper. This model is called the reduced earth. This model should be more or less spheroid having the length of polar diameter lesser than equatorial and on this model the network ofgraticulecan be transferred.
b. Parallels oflatitude:These are the circles running round the globe parallel to the equator and maintaining uniform distance from the poles. Each parallel lies wholly in its plane which is at right angle to the axis of the earth. They are not of equal length. They range from a point at each pole to the circumference of the globe at the equator. They are demarcated as 0º to 90º North and South latitudes.
c. Meridians oflongitude:These are semi-circles drawn in north-south direction from one pole to the other, and the two opposite meridians make a complete circle, i.e. circumference of the globe. Each meridian lies wholly in its plane, but all intersect at right angle along the axis of the globe. There is no obvious central meridian but for convenience, an arbitrary choice is made, namely the meridian of Greenwich, which is demarcated as 0° longitudes. It is used as reference longitudes to draw all other longitudes
d. Globalproperty:In preparing a map projection the following basic properties of the global surface are to be preserved by using one or the other methods:
Distance between any given points of a region
Shape of the region
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What Is An Example Of Map Projection
For example, Albers Equal Area Conic and the Lambert Conformal Conic projections are conic projections. Both of these map projections are well-suited for mapping long east-west regions because distortion is constant along common parallels. … While the area is distorted, the scale is mostly preserved.
Why Is Winkel Tripel Better Than Mercator
Differences: The Mercator projection is a more accurate projection than Winkel Tripel, however the poles cannot be represented in Mercator. In Winkel Tripel the latitide and longtitude lines curve as they move away from the Equator and the Prime Meridian. In Mercator the the latitide and longtitude lines stay straight.
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Selecting A Map Projection
Cartographers at National Geographic discuss how they select an appropriate map projection for the September 2012 issue.
- The “Map of the Moon,” published in the February 1969 issue of National Geographic Magazine, was the first map to show the entire lunar surfaceincluding the far side of the moonon a single sheet of paper.
- In 1922, the National Geographic Society adopted the Van der Grinten projection, which depicts the globe by projecting it in a circle rather than a rectangle or an ellipse, common in other projections. The Van der Grinten projection was used by National Geographic until 1988.
- Many popular online map services like Google Maps and ArcGIS Online use a variation of the Mercator projection. This projection is very good for preserving angles in maps, but is not good for viewing areas of the world close to the North and South Poles.
- In 1995, the Winkel Tripel projection replaced the Robinson projection on the Society’s signature world maps. Long used in various European atlases, the Winkel Tripel, first published as a map supplement in National Geographic Magazine in April 1995, is one of the most accurate representations of the round globe on flat paper.
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A Special Case Universal Transverse Mercator System
It took another 200 years for the next development in take place for the Mercator projection.
Again, like Lamberts revolutionary change to the way that the Mercator projection was calculated this development was a change in how the Transverse Mercator projection was used. In 1947 the North Atlantic Treaty Organisation developed the Universal Transverse Mercator coordinate system .
NATO recognised that the Mercator/Transverse Mercator projection was highly accurate along its Standard Parallel/Central Meridian. Indeed as far as 5° away from the Standard Parallel Central Meridian there was minimal distortion.
Like the World Aeronautical Charts, the UTM system was able to build on the achievements of the International Map of the World. As well as developing an agreed, international specification the IMW had developed a regular grid system which covered the entire Surface of the Earth. For low to mid-latitudes the IMW established a grid system that was 6° of longitude wide and 4° of latitude high.
Please note that this is not a new revised projection, but a series of maps using the same projection . This is not commonly appreciated and UTM is often wrongly described as a projection in its own right it is not it is a projection system.
This is why UTM is regarded as a Special Case.
World wide, including Australia, this UTM system is used by mapping agencies for local and national, topographic maps.
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Classification Of Map Projections
Map Projections may be classified on the following bases:
a. Drawing techniques:On the basis of method of construction, projections are generally classified into perspective, non-perspective and conventional or mathematical.Perspective projectionscan be drawn taking the help of a source of light by projecting the image of a network of parallels and meridians of a globe on developable surface.Nonperspectiveprojections are developed without the help of a source of light or casting shadow on surfaces, which can be flattened.Mathematical or conventionalprojections are those, which are derived by mathematical computation, and formulae and have little relations with the projected image.
Figure 4.1 Conversions from a Globe to a flat surface produces distortions in area, shape and directions.
Figure 4.2 A conical projection from a Globe to a Flat Map
d. Source of light:On the basis of location of source of light, projections may be classified as gnomonic, stereographic and orthographic.Gnomonic projectionis obtained by putting the light at the centre of the globe.Stereographic projectionis drawn when the source of light is placed at the periphery of the globe at a point diametrically opposite to the point at which the plane surface touches the globe.Orthographic projectionis drawn when the source of light is placed at infinity from the globe, opposite to the point at which the plane surface touches the globe.
Commonly Used Map Projections
|Best Used in areas over the Poles or for small scale continental mapping|
|Lambert Conformal Conic||Best Used in mid-latitudes e.g. USA, Europe and Australia|
|Mercator||Best Used in areas around the Equator and for marine navigation|
|Robinson||all attributes are distorted to create a more pleasant appearance||the 1963Best Used in areas around the Equator|
|Transverse Mercator||Best Used for areas with a north-south orientation|
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What Is A Map Projection
A map projection is a method for taking thecurved surface of the earth and displaying it on something flat,like a computer screen or a piece of paper. Map makers have devisedmethods for taking points on the curved surface of the earth and”projecting” them onto a flat surface. These methods enable mapmakers to control the distortion that results from creating a flatmap of the round earth.
Every map projection has some distortion.Equal areaprojections attempt to show regions that are the same size onthe Earth the same size on the map but may distort the shape.Conformal projections favor the shape of features on the map butmay distort the size.
What is a map projection? The EqualArea Projection on the left correctly shows the relative sizes ofregions on the map. For example, Alaska is about twice the size ofTexas. The Conformal Projection on the right correctly shows theshapes of areas such as Alaska and Texas. However, the sizes ofareas on the map are distorted. Alaska now looks much larger than itreally is.
List Of Map Projections
This is a summary of map projections that have articles of their own on Wikipedia or that are otherwise notable. Because there is no limit to the number of possible map projections, there can be no comprehensive list.
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Which Projection Is Best
The mathematics of projection do not permit any particular map projection to be best for everything. Something will always be distorted. Thus, many projections exist to serve the many uses of maps and their vast range of scales.
Modern national mapping systems typically employ a transverse Mercator or close variant for large-scale maps in order to preserve conformality and low variation in scale over small areas. For smaller-scale maps, such as those spanning continents or the entire world, many projections are in common use according to their fitness for the purpose, such as Winkel tripel, Robinson and Mollweide. Reference maps of the world often appear on compromise projections. Due to distortions inherent in any map of the world, the choice of projection becomes largely one of aesthetics.
Thematic maps normally require an equal area projection so that phenomena per unit area are shown in correct proportion.However, representing area ratios correctly necessarily distorts shapes more than many maps that are not equal-area.
The Mercator projection, developed for navigational purposes, has often been used in world maps where other projections would have been more appropriate. This problem has long been recognized even outside professional circles. For example, a 1943 New York Times editorial states:
Arcgis Automatically Integrates Datasets Whose Coordinate Systems Are Known
All geographic datasets used in ArcGIS are assumed to have a well-defined coordinate system that enables them to be located in relation to the earth’s surface.
If your datasets have a well-defined coordinate system, then ArcGIS can automatically integrate your datasets with others by projecting your data on the fly into the appropriate frameworkfor mapping, 3D visualization, analysis, and so forth.
If your datasets do not have a spatial reference, they cannot be easily integrated. You need to define one before you can use your data effectively in ArcGIS. The spatial reference or coordinate system is metadata. It describes the coordinate framework that the data is already using.
When you define the coordinate system for a dataset using the Define Projection tool or the dataset property page, you are updating the metadata to identify the current coordinate system. The dataset’s extent and coordinate values will not change. The dataset must already be using the coordinate system. To change a dataset’s coordinate system, including its extent and values, use the Project or Project Raster tools.
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What Is Map Projection
Map projection is defined as the systematic drawing of a network or graticules of parallels and meridians on a developable surface portraying a part or whole of the earths surface with a specific scale and in accordance with a set of geometric and mathematical principles to satisfy certain objectives of the user.
Hence, a map projection is a technique by which we can represent the three-dimensional figure of the earth on a two-dimensional paper or plane.
Properties Of Map Projection:
When three-dimensional earth is represented on a flat surface some distortion always occurred. In a map projection, it is always desired to preserve all the properties. But it is impossible to keep all the properties in a single projection. In preparing a map projection the following basic properties of the three-dimensional earth are to be preserved by using different methods.
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Cylindrical Projection Transverse Mercator
Johann Heinrich Lambert was a German French mathematician and scientist. His mathematics was considered revolutionary for its time and is still considered important today. In 1772 he released both his Conformal Conic projection and the Transverse Mercator projection.
The Transverse Mercator projection is based on the highly successful Mercator projection. The main strength of the Mercator projection is that it is highly accurate near the Equator and the main problem with the projection is that distortions increase away from the Equator. This set of virtues and vices meant that the Mercator projection is highly suitable for mapping places which have an east-west orientation near to the Equator but not suitable for mapping places which have are north-south orientation .
Lamberts stroke of genius was to change the way the imaginary piece of paper touched the Earth instead of touching the Equator he had it touching a line of Longitude . This touch point is called the Central Meridian of a map. This meant that accurate maps of places with north-south orientated places could now be produced. The map maker only needed to select a Central Meridian which ran through the middle of the map.