WHAT IS A DATUM / MAP PROJECTION?
Datum-A set of parameters and control points used to accurately define the three dimensional shape of the earth [eg. as a spheroid]. The datum is the basis of planar co-ordinate system. For eg. the North American Datum for 1983 (NAD83) is the datum for map projections and coordinates within the united states and throughout North America Map Projection-A device for representing all or part of a rounded surface on a flat sheet. Since this cannot be done without distortion, the cartographer must choose the map characteristic (area, shape, scale, direction) which is to be shown accurately at the expense of others.” Map Projections Used by the U. S. Geological Survey, Geological Survey Bulletin 1532, Second Edition, U. S. Government Printing Office, Washington, D.C., 1982, p. 5. The map projection geometrically or mathematically generates the grid of lines of latitude and longitude.
