Understanding distortion

Converting locations from a spherical surface to a flat surface causes distortion, regardless of the projection surface that is used. The four spatial properties on a map that are subject to distortion are:

· Shape

· Area

· Distance

· Direction

Each map projection is good at preserving one or more (but not all) of the four spatial properties.

Different map projections preserve different spatial properties and produce different-looking maps.

Shape

If a map preserves shape, then shapes, such as outlines of countries, look the same on the map as they do on the earth. A map that preserves shape is called "conformal." On conformal maps, compass directions are true for a limited distance around any given location.

Area

If a map preserves area, then the size of a feature on the map is the same relative to its size on the earth. On an equal area map, if you draw a shape and move it around the map, no matter where you place it, its size will be the same.

Distance

If a map preserves distance, then a line between one point on the map and another is the same distance as it is on the earth (taking scale into consideration). Most maps have one or two lines of true scale. An equidistant map preserves true scale for all straight lines passing through a single specified location. For example, if the map is centered on Moscow, a linear measurement from Moscow to any other point on the map would be correct.

Direction

Direction, or azimuth, is measured in degrees of angle from north. An azimuthal projection preserves direction for all straight lines passing through a single, specified location. On an azimuthal map, directions from one central location to all other points on the map will be shown correctly.

The azimuth from A to B is 22 degrees. If the azimuth value from A to B is the same on a map as it is on the earth, then the map preserves direction from A to B.

Anyone who uses maps should know which projections are being used and which spatial properties are distorted and to what extent. When choosing a map projection, think about which properties you want to preserve. If your map is large-scale (shows a relatively small area of the earth), the effect of a map projection will be much less than if your map is small-scale (shows a large portion of the earth's surface).