Geographic coordinate system Filter Geographic coordinate system topics

Name of the geographic coordinate system (or datum), not to be confused with projected coordinate systems. Grid systems vary, but the most common is a square grid with grid lines numbered sequentially from the origin at the bottom left of the map. The grid numbers on the east-west (horizontal) axis are called Eastings, and the grid numbers on the north-south (vertical) axis are called Northings.

 

The Geodetic Glossary (National Geodetic Survey, National Ocean Service, National Oceanic and Atmospheric Administration, Rockville, MD, September 1986) pp. 54, defines geodetic datum as:

1. "A set of constants specifying the coordinate system used for geodetic control, i.e., for calculating the coordinates of points on the Earth."
2. "The datum, as defined in (1), together with the coordinate system and the set of all points and lines whose coordinates, lengths, and directions have been determined by measurement or calculation."

These differing definitions require the word "datum" to be used with caution. The first definition of datum is synonymous with grid coordinate system. The second definition of datum is synonymous with a list of coordinates of the control points. When the first definition is used, the published coordinates of control points be modified when allowed by better measurements. Using the second definition, a change in coordinates results in a new datum. NGS has used the first definition for NAD 1983.

 

Grids may be arbitrary, or based on specific distances, for example the United Kingdom Ordnance Survey maps use a one-kilometre square grid spacing.