Tony Fisher's Geodesy Page


Great Circle Distances

To find out the great-circle distance between two points specified by latitude and longitude, use the Automated Data Service from the U.S. Naval Observatory.

Log in as user ads. At the first prompt type loran. At the second prompt type ldx. When you've finished, type bye.

Map Projections

There's some excellent software called PROJ, an extremely comprehensive and well-documented program which computes projections (and nothing else). It is available free of charge. PROJ handles projections I'd never heard of. PROJ was developed by Gerald I. Evenden <gie@charon.er.usgs.gov> of the U.S. Geological Survey.

Another interesting package is GMT (generic mapping tools). These programs calculate map projections, and much else besides. They are very well documented and available free of charge for down-loading, as a compressed tar file. GMT was developed by Pål Wessel and Walter H.F. Smith of the University of Hawaii. Much less comprehensive is my humble offering:

For conversion in both directions between lat-lon and x-y according to the Ordnance Survey projection, which is Transverse Mercator centred on 2°W. with a scale factor applied on the central meridian, you can use a set of coordinate conversion programs written by me and available as C source. (``A poor thing, sir, but mine own.'')

Further Information

The University of Texas PCL Cartographic Resources pages have lots of interesting information: data on the figure of the Earth, equations for datum shifts, a brief guide to projections, etc.
Tony Fisher / fisher@minster.york.ac.uk