[Talk-GB] "GPS trace" tracking county boundary

Chris Hodges chris at c-hodges.co.uk
Mon Dec 14 19:50:20 UTC 2020


Either a datum mix-up or different roundings used in the constants for 
the back-and-forth conversion.  Either way that's not a real trace

On 14/12/2020 19:33, Colin Smale wrote:
>
> On 2020-12-14 20:21, Edward Bainton wrote:
>
>> With plenty of portages...
>> Glad I'm not going mad. Does it say anything useful or interesting 
>> that the "GPS trace" is a few metres away from the boundary as marked 
>> on the map? (Sorry if this has been answered recently: there was 
>> extensive discussion on alignment not long ago, but too technical for 
>> me to follow easily.)
> If my suspicion is correct that a converted version of the OS 
> Boundary-Line data was uploaded as GPX, then the small shift just 
> indicates that the exact parameters used to convert from the OS 
> coordinate system (Eastings and Northings) to the system used by GPS 
> and OSM were different.
>
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