### About understanding projection horizons

Each geographic or projected coordinate system has a valid area of use. The area of use may include the entire earth or, because of mathematical or practical limitations, a portion of the earth. These areas of use are the basis of x,y domain produced when using ISpatialReferenceResolution.ConstructFromHorizon.

The area of use for a geographic coordinate system is between –360 and +360 for the longitude, or x-coordinates, and –90 and +90 for the latitude, or y-coordinates. When working with the x,y domain of a spatial reference, the minimum values of –400 are used for simplicity. The unit of measure is decimal degrees. Other units of measure use the equivalent converted values.

When working with projected coordinate systems, the valid area of use is often called the projection horizon. Like the horizon you see outside, a projection horizon defines the limits of what is possible for a projected coordinate system. Projected coordinate systems that use a particular map projection, such as transverse Mercator, will have similar horizons, but the parameter values and geographic coordinate systems will determine the precise values.