The Ordnance Survey bases all elevations including mapped contour lines and spot heights on the mean sea level at Newlyn ( see Ordnance Datum ) .
The Ordnance Survey is the United Kingdom 's mapping agency .
The Ordnance Survey , the United Kingdom 's mapping agency , bases all elevations including mapped contour lines and spot heights on the mean sea level at Newlyn ( see Ordnance Datum ) .