A geodetic (or geographic) spatial reference frame is related to the ellipsoid used to model the earth, for example the latitude/longitude system.
Geodetic spatial reference frame (latitude/longitude). An optional argument may be used to specify the ellipsoid using one of the ellipsoid codes that are defined here. If no ellipsoid is specified, then "WE" is assumed, i.e. the WGS84 ellipsoid. An optional "WGS84" string can be specified if you wish all elevations to relative to the WGS84 geoid (i.e., mean sea level) otherwise, all elevations will be relative to the ellipsoid. An example spatial reference frame definition of this format is [ "GD", "WD" ], for a geodetic spatial reference frame based upon the WGS72 ellipsoid with all elevations being relative to that ellipsoid.
(<latitude>, <longitude>, <elevation>) or (<longitude>, <latitude>, <elevation>). The order of latitude and longitude is controlled by the geoSystem field. If "latitude_first" is specified, the order is latitude then longitude. If "longitude_first" is specified, the order is longitude then latitude. If neither is specified, "latitude_first" is the default. Elevation is always specified third. Latitude and longitude are given in units of degrees. Latitude is in the range −90..+90, and longitude can be in the range −180..+180 or 0..360 (0 deg longitude is the same point in both cases). Longitudinal values are relative to the Greenwich Prime Meridian. Elevation is given in units of metres above the ellipsoid (the default) or above the WGS84 geoid (if you supplied the "WGS84" parameter in the geoSystem field). For example, (37.4506, −122.1834, 0) is the latitude/longitude coordinate for Menlo Park, California, USA.
Supported earth geoids
Datum information is required to accurately transform coordinates from a reference system based on one datum to a reference system based on another datum (e.g. from OSGB36 geographicals to WGS84 geographicals).