**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.