Dear Dr. Map: Why does a GPS receiver operated on the zero meridian at Greenwich indicate a longitude differing by about 100 meters from zero?
A: At the International Meridian Conference held in October 1884 in Washington, D.C. at the request of President Chester A. Arthur, 41 delegates from 25 Nations decided “That the Conference proposes to the Governments here represented the adoption of the meridian passing through the centre of the transit instrument at the Observatory of Greenwich as the initial meridian for longitude.”(Check out the whole text: scanned and on-line at: http://www.ucolick.org/~sla/leapsecs/scans-meridian.html ). What they didn’t say was on what spheroid (or ellipsoid, see a past column!) the world’s mapping should proceed from that point on. Each country had already chosen its own point of origin or “datum”, and only much later, in the satellite era, was a global standard devised that was earth-centered (WGS66). Over time a host of different spheroids were used, culminating with today’s dominant WGS84 standard, which I am guessing was in use on the aforementioned GPS receiver. When different earth size and shape models are used, there are small differences in positions and elevations around the earth, as each point on the earth’s surface maps up or down onto the spheroid. I would say that the person who submitted this question measured the ellipsoid difference at the Greenwich observatory at 100 meters. Dr. Map stopped by the GO on a recent trip to London (See Photo).
