Re: [Gpsbabel-misc] Accuracy of BNG > GPX (WGS84) transformations

[tsteven4 <tst...@gm...>]

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