Author Topic: Garmin Topo France  (Read 1579 times)

Raph

Garmin Topo France
« on: 16 April, 2009, 09:04:35 pm »
Last year I spent ages trying to find out what the grid reference system was on IGN maps - put out questions of forums and got no answers.

Finally found what it is - recent IGN maps have a reference to being "GPS compatible" on the cover - all this means is that there's a grid that corresponds to one of the grid systems on the GPS. Before you say why not use degrees and minutes, there isn't a grid on the map for that - the 25,000 scale maps have degrees and minutes at the edge, but no grid (or some have a very sparse one), and the 50,000 scale maps don't even have that. The GPS compatible IGN grid system is a square grid, lines 1km apart (I think - I haven't actually checked this!), very similar to the OS grid, though not in separate "tiles", i.e. it's all one system. It's "UTM UPS" - and the reference seems to vary according to how recent the maps. The one that's accurate with the most recent maps is WGS84, though WGS72 might be as good. Other references, listed under "map datum", seem to be up to half a kilometre out either in longitude or latitude. Comparing landmarks between GPS mapping and a 25,000 scale map, the two WGS settings seem to be about 15 metres out in longitude and just about accurate in latitude. A real life GPS (especially the ancient GPSmap60C I've got) is never going to be all that accurate anyway so it's good enough for me.

So to sum up with less waffling - for Garmin Topo France to line up with IGN maps - grid system UTM UPS, map datum WGS84 or 72.