Why didn’t my GPS receiver read longitude 0° 00' 00" when I visited Greenwich Observatory and stood on the prime meridian?
The NAVSTAR satellites of the Global Positioning System can provide your location to an accuracy of 3 meters (10 feet) or better. The coordinates you get are based on a particular geodetic datum, such as the World Geodetic System 1984 (WGS84) or North American Datum 1927 (NAD27). Each of these — and there are hundreds — is for a particular 3-D model of Earth’s size and spheroidal shape and the coordinate specifications adopted for use in one or more geodetic surveys. The lines of latitude and longitude and the elevation contours that you see on a topographic map are always referenced to a specific datum (look at the fine print under the chart’s title).
In the case of the widely used WGS84, the meridian line for longitude 0° is offset about 100 meters east of the world’s prime meridian traditionally marked by the Airy transit telescope at the old Royal Observatory in Greenwich, England. GPS receivers often use WGS84 as the default datum, and they’ll show the Airy transit telescope to be near longitude 5" west. But some GPS units allow you to change the datum internally. If you switch it to Ordnance Survey of Great Britain 1936 (GB36), the longitude reading will be much closer to zero. If you’re comparing GPS coordinates to a topographic map, you need to set the datum in the GPS unit to match that used for the chart. Similarly, when reporting your position to high accuracy, such as for timings at an occultation or solar eclipse, be sure to include the datum you’re using.
— Edwin L. Aguirre