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.
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