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

[Bob Hawkins]

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.