[josm-dev] Use of mixed map datum in my GPSr and JOSM

[David Earl]

On Thursday, 29 September 2011, Bob Hawkins <bobhawkins at waitrose.com> wrote:
> I live in Great Britain and use Ordnance Survey British National Grid and
map datum in my GPSr for compatibility with my printed map when walking.  I
use Mercator projection in JOSM because of its portrayal of rectangular
shapes.  Should I be concerned about the position of my GPS trace over
existing data in JOSM set to either Mercator or WGS84, or does some
automatic correction take place?  I ask because I understand there is a +70m
to +120m difference longitudingly and -70m to +50m latitudingly between
Ordnance Survey OSGB36 and WGS84.  My personal observation, however,
generally tells me there is a good positional match.


GPX is in WGS84 by definition, so anything that creates such a file should
be converting whee necessary. (More likely is that your GPS works in some
canonical system, such as WGS84, and converts on the fly to OSGB36, so
everything is a conversion).

That's not to say everything gets this right. I have seen software which
just does GPX in whatever datum the numbers happen to arrive in, but such
software is wrong. So just a caution to keep a skeptical eye on your data,
but it sounds like it is OK.

An easy test would be to record the same short journey in tow different
datums and compare the two GPX tracks. Obviously they won't match perfectly
because of inheretnGPS inaccuracy, but they shouldn't be tens or hundreds of
metres different.

David