Dear Frank, Thank you for your interesting and, as always, thoughtful response. I was aware of the excellent Ordnance Survey document "A guide to coordinate systems in Great Britain" and I had referred to it before I wrote my original posting but thank you for drawing attention to it.
I still believe that in order to produce flat maps of the UK the OSGB first of all models the geoid in the vicinity of the UK using Airy's ellipsoid, locates the geographical features on this model and then uses a transverse Mercator projection to map these features onto a plane. As I understand it, the location of the geographical features is indeed achieved by satellite systems using the ETRS89 datum but this is then used to calculate positions in the OSGB36 TRF which may then be transferred to maps using a TM projection. Satellites have brought accuracy and complexity to geodesy but the process of mapping our chosen ellipsoid onto a plane surface is still as vital an element in the production of maps as it was in the time of Mercator. Best wishes, Geoff -----Original Message----- From: Frank King [mailto:[email protected]] Sent: 18 August 2014 16:58 To: Geoffrey Thurston Cc: [email protected]; Frank King Subject: Re: Map required 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 ----- No virus found in this message. Checked by AVG - www.avg.com Version: 2014.0.4745 / Virus Database: 4007/8056 - Release Date: 08/18/14 ----- No virus found in this message. Checked by AVG - www.avg.com Version: 2014.0.4745 / Virus Database: 4007/8056 - Release Date: 08/18/14 --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
