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About coordinate systems and map projections

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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:

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:

What type of map projection should you choose?

Here are a few things to consider when choosing a projection:

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.

Learn more about map projections

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