I wouldn’t (myself) use the elevation at all, it is just that survey_point asked for it. Further, the Ordnance Survey’s Coordinate transformation tool asks for an ellipsoid height when converting from degrees and decimal minutes. The is well beyond my knowledge of geodesy!
The Garmin quadrangulates to determine altitude relative to the WGS geoid, with the handicap that it can only receive data from satellites above the horizon. It has no altimeter. I had just assumed that consumer level kit would not give a usable altitude.
So I will look for a map and see if I can find a height. I will be looking for a 150 contour as seen on geograph or UK Grid Reference Finder
Is there a way of finding out where mapping parties are taking place? I have kept an eye out.
اوپن سٹریٹ میپ