As both Frank and Keith have pointed out, discussions of map projections can lead to confusion. 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.
I have long believed that OSGB uses a TM projection of Airy's ellipsoid with a central meridian of 2 degrees West and a scale factor of 0.9996. The national grid system is then simply drawn as an overlay on this flat map. Despite the revolution in data collection brought on by satellites using different geodetic datums, I thought that these basic facts were still true. What should I now believe? Geoff -----Original Message----- From: sundial [mailto:[email protected]] On Behalf Of Keith Christian Sent: 17 August 2014 19:01 To: [email protected] Subject: Re: Map required As Frank King mentioned, the subject of map 'projections' can be a tricky one - and most of us probably think of "school atlas" maps, where it is likely to be one based on "conical with two standard parallels", where (as Tony Moss said), Latitude lines will be slightly curved, and Longitude converging to the North. However, Tony definitely wants 'orthogonal' (i.e. straight and at right-angles) lines of Latitude and Longitude - which rather restricts the projections you can use, though Tony did not give his reasons WHY the map had to have these specific properties. Strictly speaking, the UK Ordnance Survey 'National Grid' is not based on a "Transverse Mercator" projection (as Frank King suggested) - but here are two websites, with more information. http://en.wikipedia.org/wiki/Map_projection http://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid I hope they might be useful, and not even more confusing! Sincerely, Keith Christian. --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
