Mapping and visualization > Using ArcMap
About coordinate systems and map projections |
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Release 9.3
Last modified December 3, 2008 |
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The features on a map reference the actual locations of the objects they represent in the real world. The positions of objects on the earth's spherical surface are measured in geographic coordinates. While latitude and longitude can locate exact positions on the surface of the earth, they are not uniform units of measure; only along the equator does the distance represented by one degree of longitude approximate the distance represented by one degree of latitude. To overcome measurement difficulties, data is often transformed from three-dimensional geographic coordinates to two-dimensional projected coordinates.
Learn how to specify a coordinate system in ArcMap
Map projections
Because the earth is round and maps are flat, getting information from a curved surface to a flat one involves a mathematical formula called a map projection, or simply a projection.
This process of flattening the earth will cause distortions in one or more of the following spatial properties:
- Distance
- Area
- Shape
- Direction
No projection can preserve all these properties; as a result, all flat maps are distorted to some degree. Fortunately, you can choose from many different map projections. Each is distinguished by its suitability for representing a particular portion and amount of the earth's surface and by its ability to preserve distance, area, shape, or direction. Some map projections minimize distortion in one property at the expense of another, while others strive to balance the overall distortion. As a mapmaker, you can decide which properties are most important and choose a projection that suits your needs.
Learn more about map projections
Do you need to display your data with a projected coordinate system?
If your spatial data references locations with latitude and longitude—for example, decimal degrees—you can still display it on your map. ArcMap draws the data by simply treating the latitude-longitude coordinates as planar x,y coordinates. If your map doesn't require a high level of locational accuracy—you won't be performing queries based on location and distance, or you just want to make a quick map—you might decide not to transform your data to a projected coordinate system. If, however, you need to make precise measurements on your map, you should choose a projected coordinate system.
Reasons for using a projected coordinate system
The following are some reasons for using a projected coordinate system:
- You want to make accurate measurements from your map and be sure that the spatial analysis options you use in ArcMap calculate distance correctly. Latitude-longitude is a good system for storing spatial data but not as good for viewing, querying, or analyzing maps. Degrees of latitude and longitude are not consistent units of measure for area, shape, distance, and direction.
- You are making a map in which you want to preserve one or more of these properties: area, shape, distance, and direction.
- You are making a small-scale map such as a national or world map. With a small-scale map, your choice of map projection determines the overall appearance of the map. For example, with some projections, lines of latitude and longitude will appear curved; with others, they will appear straight.
- Your organization mandates using a particular projected coordinate system for all maps.
What type of map projection should you choose?
Here are a few things to consider when choosing a projection:
- Which spatial properties do you want to preserve?
- Where is the area you're mapping? Is your data in a polar region? An equatorial region?
- What shape is the area you're mapping? Is it square? Is it wider in the east–west direction?
- How big is the area you're mapping? On large-scale maps, such as street maps, distortion may be negligible because your map covers only a small part of the earth's surface. On small-scale maps, where a small distance on the map represents a considerable distance on the earth, distortion may have a bigger impact, especially if you use your map to compare or measure shape, area, or distance.
Answering these questions will determine what map projection and projected coordinate system you'll want to use to display your data.
Map projection classifications
Map projections can be generally classified according to what spatial attribute they preserve.
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Equal area projections preserve area. Many thematic maps use an equal area projection. Maps of the United States commonly use the Albers Equal Area Conic projection.
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Conformal projections preserve shape and are useful for navigational charts and weather maps. Shape is preserved for small areas, but the shape of a large area, such as a continent, will be significantly distorted. The Lambert Conformal Conic and Mercator projections are common conformal projections.
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Equidistant projections preserve distances, but no projection can preserve distances from all points to all other points. Instead, distance can be held true from one point (or a few points) to all other points or along all meridians or parallels. If you will be using your map to find features that are within a certain distance of other features, you should use an equidistant map projection.
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Azimuthal projections preserve direction from one point to all other points. This quality can be combined with equal area, conformal, and equidistant projections, as in the Lambert Equal Area Azimuthal and the Azimuthal Equidistant projections.
- Other projections minimize overall distortion but don't preserve any of the four spatial properties of area, shape, distance, and direction. The Robinson projection, for example, is neither equal area nor conformal but is aesthetically pleasing and useful for general mapping.
Learn more about map projections