GPS and Other Coordinate Systems in Archaeology & Archaeoastronomy - LexiLine Journal 475

The prime meridian of GPS and Google Earth, which uses WGS84 (1984), runs 102.5 meters east of the Prime Meridian of 1884 at Greenwich. This is not an "error". Read at the Flamsteed Astronomy Society why that is so.

Determining just where we are or where a given place is located is not as simple as it may initially seem to anyone who uses GPS or other survey systems, including the navigation systems in your automobiles. Large discrepancies exist between the various systems, so that awareness of conversion problems is necessary in archaeological and archaeoastronomical work as also for navigation.

GPS (Global Positioning System) operates by means of satellites which determine the position of your GPS receiver. If we define a particular located position as "X", then position X must be a given a value within some kind of a specific coordinate system, and in the case of GPS that position X is given in terms of latitude and longitude, as calculated by WGS84 (World Geodetic System 1984), a standard which was revised by the geopotential model EGM96 (Earth Gravity Model 1996 ) and is in future revision in 2008 as EGM2008 (initially EGM06).

There are numerous other survey systems used for mapping and cartography around the globe and the National Geospatial Intelligence Agency offers the program Geographic Translator (GEOTRANS) for conversion of "twenty-five different coordinate systems, map projections, grids, and coding schemes, and over two hundred different datums...."

The Universal Transverse Mercator Coordinate System is used for mapping the world, now based on WGS84.

A different survey system is ED50, which is used in mapping Western Europe, excluding Britain, Ireland, Sweden and Switzerland, who have their own mapping systems. ED50 can differ by as much as 100 meters west and south from WGS84, i.e. GPS.

Similarly large up to 100-meter+ differences from WGS84 can also be found in the mapping coordinate systems used by Great Britain and Ireland.

For Great Britian, beside GPS, the major mapping system in use is:
OSGB36 - Ordinance Survey Great Britain 1936-1962 - The British national grid reference system.

For Ireland, the major mapping system besides GPS in use is The Irish grid reference system (which also includes Northern Ireland).

Coordinate conversion viz. transformation forms for both the British and Irish coordinate systems are available at the nearby.org.uk Coordinate Converter and also at the Ordnance Survey Coordinate Transformer.

If we plug in e.g. SU123422 (the grid reference for Stonehenge), nearby.org.uk converts this to
51.178904N Long: 1.825418W
which is very close to latitudinal and longitudinal values found online
51.178816N Longitude: 1.826563W (Megalithic Portal)
51.178381, -1.824018, Stonehenge (Stone Search)
51°10´44´´N, 1°49´34´´W (Wikipedia)

For megalithic sites specifically:

Stones Search:
1) generates GPS-suitable output as a comma-separated-variable (CSV) format with the fields latitude, longitude, site name

or at the standard search form
2) generates the British or Irish grid reference

Other Specialized Converters are:

GPS Coordinate Converter with MAP feature showing location (we entered WGS84 data 51.178381, -1.824018 for Stonehenge and received GPS N 51 10.703 W 1 49.441 and latitude and longitude in degrees minutes and seconds N51 10 42W1 49 26)
Coordinate Converter Latitude, Longitude <=> UTM (with choice of Datum)
Geographic/UTM Coordinate Converter
JEEEP.com (Translate coordinates WGS-84, NAD-83, and NAD-27 to and from Latitude/Longitude and UTM)
UK Street Map Coordinate Converter
Archaeoptics (Easting Northing)
Map Window GIS (Open Source Programmable Geographic Information System Tools)
Latitude Longitude Converter
Find Out Where You Are (latitude & longitude, national grid references (NGR), Maidenhad locators (QRA)