Choosing a map projection

Which map projection you choose for a particular map depends on the map's purpose and the spatial properties you want to preserve.

Map purpose

If your map will be used for general reference or in an atlas, you will usually want to balance shape and area distortion. In this case, a compromise projection such as the Robinson projection may be the best choice. On the other hand, if your map has a specific purpose, you may need to use a projection that preserves a specific spatial property.

Preserving spatial properties

As you have learned, projections can be categorized based on the three types of surfaces that a map can be projected onto: cylinder, cone, and plane. Projections can also be categorized based on the spatial property they preserve. The table below shows five projection categories.

The first four categories are based on the four spatial properties: shape, area, distance, and direction—the fifth category is a "compromise" category. Projections in this category attempt to minimize distortion of all properties, without preserving any single property.

Characteristics of Map Projections
Projection Category	Properties	Common Uses
Conformal	Preserves local shapes and angles	Topographic maps, navigation charts, weather maps
Equal Area	Preserves areas	Dot density maps, thematic maps
Equidistant	Preserves distance from one or two specified points to all other points on the map	Maps of airline distances, seismic maps showing distances from an earthquake epicenter
Azimuthal	All directions are true from a single specified point (usually the center) to all other points on the map	Navigation and route planning maps
Compromise	No point is completely distortion free; distortion is minimized near the center and along the equator	World maps

There are other factors to consider when choosing a map projection, including the size of the area you're mapping, the orientation (east-west or north-south), and the particular portion of the earth that is covered. Some of these factors are beyond the scope of this course, but here are a few points to keep in mind:

· ArcMap comes with many predefined map projections organized by world, continent, and country. You can easily find an appropriate map projection for your area of interest and assign it to your data.

· When you're working at a large scale, (a county or a district within a province or state), distortion doesn't play a big role and almost any projection that is centered on your area of interest will be appropriate.

· In some situations, the decision of which map projection to use has already been made. For example, there may be a standard projection for your project, your organization, or the part of the world you are mapping (e.g., the State Plane and UTM coordinate systems are standard for mapping the U.S. states).