Re: [Gpsbabel-misc] Accuracy of BNG > GPX (WGS84) transformations
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Re: [Gpsbabel-misc] Accuracy of BNG > GPX (WGS84) transformations
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From: Robert L. <rob...@gp...> - 2024-08-05 22:20:20
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I'm not UNhappy leaving such transformations to other tools like ArcGIS or ogr2ogr and keeping us focused on file format conversions. It's a bit of a parlor trick that we do this at all. It's a digital, global world, and fixing country specific datum protections isn't really our focus. I'm happy leaving that to GDAL just like they happy to leave the details of a zillion GPSes and file formats to us. If someone brings a PR to our doorstep to improve things (that doesn't involve including a copy of GDAL), great, but it's not strategic for us. On Mon, Aug 5, 2024, 5:02 PM tsteven4 <tst...@gm...> wrote: > We actually use the standard molodensky (5 parameter) transformation, but > not the 7 parameter Molodensky-Badekas or the OSTN02 transformation > mentioned in the reference below. It isn't clear to me what transformation > the online tools you referenced used. > On 8/5/2024 2:43 PM, tsteven4 wrote: > > I suspect but have not proven this is because we use a abridged molodensky > transformation using the 3 parameters dX, dY dZ values shown below. The > citation claims this is only accurate to +/- 20m. We don't use PROJ.4, and > we don't have more complex transformations available. > > See Transformations in GDAL/OGR (edina.ac.uk) > <https://digimap.edina.ac.uk/help/gis/transformations/transform_gdal_ogr/> > which says > > PROJ.4 contains a large number of transformations and parameters. However > by default OGR uses a 3 parameter shift because it covers the largest > available area - see blogpost on how transformations are chosen in Proj4. > <http://fwarmerdam.blogspot.com/2010/03/in-last-few-weeks-i-believe-i-have-made.html> > However, this is only accurate to +/- 20 metres over the whole of the > UK. It is equivalent to the ArcGIS transformation called > OSGB_1936_To_GS_1984_1 and has parameters of dX: 375, dY-111, dZ 431 > > > On 8/5/2024 11:37 AM, Nims via Gpsbabel-misc wrote: > > Hi Robert, > > 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: > > name,comment,bng,date > Badbury Rings Trig Point,Test Point 1,ST 96552 03097,2024/08/05 > Spettisbury Rings Trig Point,Test Point 2,ST 91437 01894,2024/08/05 > > 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. > Using the GPSBabel GUI (v.1.9.0) under Windows 10, the file is processed > successfully with the following feedback: > > gpsbabel -w -i unicsv -f G:/Trig Points.csv -o gpx -F G:/Trig Points.gpx > Translation successful > > This is the output: > <?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" <http://www.topografix.com/GPX/1/0>> > <time>2024-08-05T16:54:46.903Z</time> > <bounds minlat="50.816463984" minlon="-2.122941465" > maxlat="50.827336109" maxlon="-2.050345019"/> > <wpt lat="50.827336109" lon="-2.050345019"> > <time>2024-08-04T23:00:00Z</time> > <name>Badbury Rings Trig Point</name> > <cmt>Test Point 1</cmt> > <desc>Test Point 1</desc> > </wpt> > <wpt lat="50.816463984" lon="-2.122941465"> > <time>2024-08-04T23:00:00Z</time> > <name>Spettisbury Rings Trig Point</name> > <cmt>Test Point 2</cmt> > <desc>Test Point 2</desc> > </wpt> > </gpx> > > 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: > > British Geological Society > https://webapps.bgs.ac.uk/data/webservices/convertForm.cfm > Badbury Rings Trig Point = 50.827400 , -2.050323 > Spettisbury Rings Trig Point = 50.816527 , -2.122918 > > Grid Reference Finder > https://gridreferencefinder.com > Badbury Rings Trig Point = 50.827400 , -2.0503234 > Spettisbury Rings Trig Point = 50.816527 , -2.1229184 > > Mapserve > https://www.mapserve.co.uk/conversion-tools > Badbury Rings Trig Point = 50.8273995082622 , -2.05032333478781 > Spettisbury Rings Trig Point = 50.8165274566942 , -2.1229183177272 > > 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. > > Many thanks, > Nims > > > _______________________________________________ > Gpsbabel-misc mailing list http://www...@li... > To unsubscribe, change list options, or see archives, visit:https://lists.sourceforge.net/lists/listinfo/gpsbabel-misc > > _______________________________________________ > Gpsbabel-misc mailing list http://www.gpsbabel.org > Gps...@li... > To unsubscribe, change list options, or see archives, visit: > https://lists.sourceforge.net/lists/listinfo/gpsbabel-misc > |