[Gpsbabel-misc] Accuracy of BNG > GPX (WGS84) transformations
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robertl
From: <deh...@gm...> - 2024-08-05 17:37:54
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<html><head></head><body><div style="font-family: Verdana;font-size: 12.0px;"><div>Hi Robert,</div> <div> </div> <div>I've been converting Universal CSV lists of BNG coordinates to GPX XML files with GPSBabel and am wondering about the accuracy of the conversion. Here's my test input which lists a couple of trig points (triangulation stations) in the county of Dorset:</div> <div> </div> <div>name,comment,bng,date<br/> Badbury Rings Trig Point,Test Point 1,ST 96552 03097,2024/08/05<br/> Spettisbury Rings Trig Point,Test Point 2,ST 91437 01894,2024/08/05</div> <div> </div> <div>The trig points are marked on British Ordnance Survey maps and also visible in satellite images, allowing me to check accuracy in various mapping apps. </div> <div>Using the GPSBabel GUI (v.1.9.0) under Windows 10, the file is processed successfully with the following feedback:</div> <div> </div> <div>gpsbabel -w -i unicsv -f G:/Trig Points.csv -o gpx -F G:/Trig Points.gpx<br/> Translation successful</div> <div> </div> <div>This is the output:</div> <div><?xml version="1.0" encoding="UTF-8"?><br/> <gpx version="1.0" creator="GPSBabel - https://www.gpsbabel.org" xmlns="http://www.topografix.com/GPX/1/0"><br/> <time>2024-08-05T16:54:46.903Z</time><br/> <bounds minlat="50.816463984" minlon="-2.122941465" maxlat="50.827336109" maxlon="-2.050345019"/><br/> <wpt lat="50.827336109" lon="-2.050345019"><br/> <time>2024-08-04T23:00:00Z</time><br/> <name>Badbury Rings Trig Point</name><br/> <cmt>Test Point 1</cmt><br/> <desc>Test Point 1</desc><br/> </wpt><br/> <wpt lat="50.816463984" lon="-2.122941465"><br/> <time>2024-08-04T23:00:00Z</time><br/> <name>Spettisbury Rings Trig Point</name><br/> <cmt>Test Point 2</cmt><br/> <desc>Test Point 2</desc><br/> </wpt><br/> </gpx></div> <div> </div> <div>All well and good but when I check the generated coordinates in mapping apps, they are a few metres off target. Furthermore, they differ from coordinates generated by various online BNG > WGS84 conversion tools which are themselves consistent. For example:</div> <div> </div> <div>British Geological Society<br/> https://webapps.bgs.ac.uk/data/webservices/convertForm.cfm<br/> Badbury Rings Trig Point = 50.827400 , -2.050323<br/> Spettisbury Rings Trig Point = 50.816527 , -2.122918</div> <div> </div> <div>Grid Reference Finder<br/> https://gridreferencefinder.com<br/> Badbury Rings Trig Point = 50.827400 , -2.0503234<br/> Spettisbury Rings Trig Point = 50.816527 , -2.1229184</div> <div> </div> <div>Mapserve<br/> https://www.mapserve.co.uk/conversion-tools<br/> Badbury Rings Trig Point = 50.8273995082622 , -2.05032333478781<br/> Spettisbury Rings Trig Point = 50.8165274566942 , -2.1229183177272</div> <div> </div> <div>The difference isn't huge but it seems to be more than a rounding error. So, I'm wondering what might need to be done to harmonise the GPSBabel conversion with these others.</div> <div> </div> <div>Many thanks,<br/> Nims</div></div></body></html> |