What is datum, and why do I need to be aware of it?
A ‘datum’ is a frame of reference when plotting locations on a globe. Just as “Hollywood” can be “Hollywood, California” or “Hollywood, Florida”, Latitude / Longitude and UTM can be in “NAD 1927” or “NAD 1983”. Properly defined, NAD 27 is North American Datum of 1927. A reference point datum based on the Clarke Ellipsoid of 1866, a fixed position and orientation were named starting at Meade’s Ranch in Kansas. Based upon this information latitudes and longitudes were calculated for all points based primarily on triangulation. NAD 1983 is a datum based on the GRS 80 Ellipsoid, and fixed position and orientation at the center of the earth. The measurements used in NAD 27 were adjusted- this time using a computer. This results in a different latitude and longitude for a location when compared to its NAD 27 values, with the difference often amounting to hundreds of feet. Most quality maps with latitude, longitude, or UTM measurements on them also display which datum was used. For example, m
