Spatial - Ed.com

Projections allow for reconciling the difference between the (sort of) round Earth and flat maps and computer monitors.

A projection is the mathematical equivalent of shining a light through a transparent globe. The resulting, 2D image on the wall is the basis of a ‘projected’ coordinate system.

A projection consists of a coordinate, datum and an ellipsoid.

1. Every location needs a COORDINATE (X, Y, Z)

Locations described in a geographic coordinate system are expressed in degrees of latitude and longitude, which originate at the Prime Meridian and Equator.

Locations described in a projected coordinate system are expressed in meters (or feet) both north and east of designated origins. In the Universal Transverse Mercator (UTM) system, the nothing is measured from the South Pole or Equator and the easting is measured from a ‘false easting’. The State Plane System is a similar system covering smaller areas with its origins based within each state.

2. Every coordinate needs a DATUM

A datum is simply a point of reference. It is where the coordinate grid is tied to the ground. As technology allows for measurements that are more precise, datums are recalculated and updated.

North American Datum 1927 (NAD27) is a local datum based on land-based measurements. It is centered at Meade Ranch, KS and is optimized for North America. USGS topo maps are still based on NAD27 with a slow move to NAD83.

North American Datum 1983 (NAD83) and World Geodetic System 1984 (WGS-84) are global datums are based on space-based measurements. These are the most common datums. GPS satellites broadcast in WGS-84 and most GPS receivers, including Trimble receivers, receive data only in WGS-84.

ALWAYS state the associated datum when reporting a set of coordinates.

3. Every datum is linked to an ELLIPSOID

An ellipsoid is the mathematical model describing the shape of the earth. It is not necessary to specify the ellipsoid for every datum since each datum is linked to a single ellipsoid. The WGS-84 datum is based on the GRS-80 ellipsoid, NAD83 datum on the GRS-80 ellipsoid and NAD27 datum on the Clarke 1866 ellipsoid.