Dear Geoff, I share your lament...
> I thought that I understood the principles > of the OSGB but since Keith said that it is > not strictly based on a transverse Mercator > projection, I am now confused again. Well, the Ordnance Survey certainly WAS based on a transverse Mercator projection, and had you used the past tense ["used" not "uses"] this would pretty much be right: > I have long believed that OSGB uses a TM > projection of Airy's ellipsoid with a central > meridian of 2 degrees West... In some ways it is a great tribute to Ordnance Survey that they have very much kept up with the times. Airy's ellipsoid was, and still is, a good fit for the UK, but it isn't even geocentric! These days the GRS80 ellipsoid is used pretty much everywhere. This is a best-fit for the whole Earth. What the Ordnance Survey do these days is to use GPS and WGS84 and all those things and, for the benefit of people who still like grid references, provide transformation functions. The most illuminating description I have read is at: http://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-b ritain.pdf You get a very good explanation about how all national map makers are perpetually wrestling with their own history. I do wonder how long grid references will survive. It seems a long time since I spent happy evenings staring at 1km squares. > What should I now believe? Tempora mutantur, nos et mutamur in illis. Frank --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
