A geodetic datum is a coordinate system and a reference surface. Important examples include:
A model of mean global sea level based on gravitational potential. The geoid represents mean sea level without winds, currents, and tides through both oceans and continents. The geoid is used as a vertical datum for measuring orthometric height above sea level.
g.region n=90:00:00N s=90:00:00S e=360:00:00E w=00:00:00W r.in.gdal -r input=EGM08_25.tif output=emg r.colors map=emg color=viridis r.relief input=emg output=relief zscale=500 r.shade shade=relief color=emg output=shade brighten=50 d.legend raster=emg at=5,35,2,3 font=Lato-Regular fontsize=16
The World Geodetic System 1984 (WGS84) uses the Earth Gravitational Model 1996 (EMG96) as its geoid reference. the Global Positioning System (GPS) uses WGS84 as its coordinate reference system (CRS)
A map projection transforms a globe into a planar map that can be measured.
The Universal Transverse Mercator (UTM) coordinate system divides the world into 60 longitudinal zones and 20 latitudinal zones, 6 degrees wide and 12 degrees tall.
The State Plane Coordinate System of 1983 is based on the North American Datum of 1983 (NAD 83). Each state in the US has at least one zone in either the Transverse Mercator, Lambert Conformal Conic, or Oblique Mercator projection. To reduce distortion, many states have multiple zones.