The spot heights in the Ordnance Survey MasterMap sample data over Exeter with feature code 15404 are presented as an integer value in metres. But how accurate are these spot heights? I had assumed that spot heights are GPS measurements that are accurate to a few centimetres.
Looking at the referenced document, it appears that they haven't been determined by GPS measurement, but instead by levelling
These are non-physical points, the altitude of which (relative to Ordnance Datum) has been determined by levelling. All current spot heights, are shown by a point feature or symbol. The altitude to one decimal place of a metre is shown by a textual description. The latest information on Ordnance Survey’s GPS, triangulation and control points can be found at http://www.ordnancesurvey.co.uk/oswebsite/gps/.
Bench marks are represented in the Topography Layer as a symbol to mark their position. The values for these bench marks are available from http://benchmarks.ordnancesurvey.co.uk/ It should be noted that the bench mark information is historic, and Ordnance Survey cannot guarantee its accuracy.