ael,
The OS national grid has been around for a long time and has been adjusted and tweaked as measurement accuracy has improved. The official transform from the national grid (OSGB36) to GPS lat lon (WGS84) is known as OSTN02. OSTN02 uses a large look up table to account for the strange shape of OSGB36. There are of course various mathematical approximations of OSTN02 which are easier to use. I am not sure which transformation Garmin uses but it seems to be accurate enough.
Various software libraries exist to do the conversion. I have played with the perl library Geo::Coordinates::OSTN02 and ported it to python, if you are interested.
Regards, TimSC On 09/05/11 11:44, ael wrote:
I have been encouraging a friend to use OSM. He has just mailed in puzzlement after trying to find an OS grid reference (presumably looking at mapnik). I saw http://help.openstreetmap.org/questions/2056/using-the-ordnance-survey-national-grid-with-openstreetmap but just converting a single grid reference to a good approximation to (lat,long) isn't *that* difficult, surely? My Garmin does it a less than a ms or so with very low computing power. Or am I being naive? ael
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