# Geographic Coordinate System

A geographic coordinate system (GCS) is a method of describing points on an idealized model of the earth. Geodetic locations on the surface of the earth are usually given in degrees west-east longitude (sometimes designated with the Greek letter $$\lambda$$ ) and south-north latitude (sometimes designated with the Greek letter $$\phi$$ ).  A GCS underlies every projected coordinate system.

The geographic coordinates system is composed of three parts:

### Prime Meridian

This is the west-east origin of 0 longitude.  Most (but not all) GCS use Greenwich as the prime meridian.

### Angular Unit of Measure

Most GCS use degrees as the unit of measure.  A few use radians.

### Datum

This is the underlying model of the earth used.

Not all longitudes and latitude are the same!  Each model of the earth will map locations on the surface of the earth to different longitudes and latitudes.

VSP provides translation capabilities between a few select datums.  Translating from one datum to another is necessary when connecting to the Map Tile Servers.  The Map Tile Servers use WGS84 and must be translated to your chosen coordinate system for display.  VSP may support more translations in the future as demand and resources warrant.