A WORD ABOUT DATUM

When asked “what is the reference frame used by GPS?”, one will answer of course: “WGS 84”. But when asked which revision of WGS 84, there might not be an answer at all. Furthermore, GPS, SBAS (WAAS, EGNOS, MSAS, GAGAN, etc), Omnistar, radio beacon, local base stations,at etc, all use different datum or reference frames. It is very important to understand the differences between the various implementations within a specific datum and make sure that the maps and GPS data are in the same revision of the datum used.