**About understanding projection horizons**

Each geographic coordinate system (GCS) or projected coordinate system (PCS) has a valid area of use. The area of use can include the entire earth or because of mathematical or practical limitations, a portion of the earth. These areas of use are the basis of the 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 in decimal degrees. Other units of measure use the equivalent converted values.

When working with a PCS, there are the following two horizons:

- The PCS horizon is the valid extent in the units of the PCS. Like the horizon you see outside, a projection horizon defines the limits of what is possible for a PCS. A PCS that use a particular map projection, such as transverse Mercator, have similar horizons but the parameter values and GCS determine the precise values.
- The geographic horizon or sometimes the projection horizon, is the valid area of use in geographic coordinates (latitude-longitude). You can use the geographic horizon to clip features before converting it to a PCS.