Re: [Gpsbabel-misc] Accuracy of BNG > GPX (WGS84) transformations
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robertl
From: tsteven4 <tst...@gm...> - 2024-08-06 16:02:32
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As described in guide-coordinate-systems-great-britain.pdf (ordnancesurvey.co.uk) <https://www.ordnancesurvey.co.uk/documents/resources/guide-coordinate-systems-great-britain.pdf> , using the helmert transformation (6.2) and parameters (6.6) results much closer matches to the web sites you used. I use the approximation they mentioned of changing the signs of the parameters for reversing "small" transformations. However, note the reference says > The error is up to 3.5 metres (95%) both horizontally and vertically If you really want the best conversion you need to use OSTN15(6.3). I don't see gpsbabel implementing OSTN15, but I can imagine gpsbabel implementing the helmert transformation. > <?xml version="1.0" encoding="UTF-8"?> > <gpx version="1.0" creator="GPSBabel - https://www.gpsbabel.org" > xmlns="http://www.topografix.com/GPX/1/0"> > <time>2024-08-06T15:49:13.516Z</time> > <bounds minlat="50.816527458" minlon="-2.122918364" > maxlat="50.827399509" maxlon="-2.050323375"/> > <wpt lat="50.827399509" lon="-2.050323375"> > <time>2024-08-05T06:00:00Z</time> > <name>Badbury Rings Trig Point</name> > <cmt>Test Point 1</cmt> > <desc>Test Point 1</desc> > </wpt> > <wpt lat="50.816527458" lon="-2.122918364"> > <time>2024-08-05T06:00:00Z</time> > <name>Spettisbury Rings Trig Point</name> > <cmt>Test Point 2</cmt> > <desc>Test Point 2</desc> > </wpt> > </gpx> On 8/5/2024 11:37 AM, Nims via Gpsbabel-misc wrote: > Spettisbury Rings Trig Point,Test Point 2,ST 91437 01894,2024/08/05 |