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

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<html><head></head><body><div style="font-family: Verdana;font-size: 12.0px;"><div>Hi Robert,</div>

<div>&nbsp;</div>

<div>I&#39;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&#39;s my test input which lists a couple of trig points (triangulation stations) in the county of Dorset:</div>

<div>&nbsp;</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>&nbsp;</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.&nbsp;</div>

<div>Using the GPSBabel GUI (v.1.9.0) under Windows 10, the file is processed successfully with the following feedback:</div>

<div>&nbsp;</div>

<div>gpsbabel -w -i unicsv -f G:/Trig Points.csv -o gpx -F G:/Trig Points.gpx<br/>
Translation successful</div>

<div>&nbsp;</div>

<div>This is the output:</div>

<div>&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;<br/>
&lt;gpx version=&quot;1.0&quot; creator=&quot;GPSBabel - https://www.gpsbabel.org&quot; xmlns=&quot;http://www.topografix.com/GPX/1/0&quot;&gt;<br/>
&nbsp; &lt;time&gt;2024-08-05T16:54:46.903Z&lt;/time&gt;<br/>
&nbsp; &lt;bounds minlat=&quot;50.816463984&quot; minlon=&quot;-2.122941465&quot; maxlat=&quot;50.827336109&quot; maxlon=&quot;-2.050345019&quot;/&gt;<br/>
&nbsp; &lt;wpt lat=&quot;50.827336109&quot; lon=&quot;-2.050345019&quot;&gt;<br/>
&nbsp; &nbsp; &lt;time&gt;2024-08-04T23:00:00Z&lt;/time&gt;<br/>
&nbsp; &nbsp; &lt;name&gt;Badbury Rings Trig Point&lt;/name&gt;<br/>
&nbsp; &nbsp; &lt;cmt&gt;Test Point 1&lt;/cmt&gt;<br/>
&nbsp; &nbsp; &lt;desc&gt;Test Point 1&lt;/desc&gt;<br/>
&nbsp; &lt;/wpt&gt;<br/>
&nbsp; &lt;wpt lat=&quot;50.816463984&quot; lon=&quot;-2.122941465&quot;&gt;<br/>
&nbsp; &nbsp; &lt;time&gt;2024-08-04T23:00:00Z&lt;/time&gt;<br/>
&nbsp; &nbsp; &lt;name&gt;Spettisbury Rings Trig Point&lt;/name&gt;<br/>
&nbsp; &nbsp; &lt;cmt&gt;Test Point 2&lt;/cmt&gt;<br/>
&nbsp; &nbsp; &lt;desc&gt;Test Point 2&lt;/desc&gt;<br/>
&nbsp; &lt;/wpt&gt;<br/>
&lt;/gpx&gt;</div>

<div>&nbsp;</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 &gt; WGS84 conversion tools which are themselves consistent. For example:</div>

<div>&nbsp;</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>&nbsp;</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>&nbsp;</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>&nbsp;</div>

<div>The difference isn&#39;t huge but it seems to be more than a rounding error. So, I&#39;m wondering what might need to be done to harmonise the GPSBabel conversion with these others.</div>

<div>&nbsp;</div>

<div>Many thanks,<br/>
Nims</div></div></body></html>