1

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.

(First posted on the Ordnance Survey forums)

  • Is there no product information/metadata supplied with the data? – nmtoken Nov 18 '19 at 12:12
  • I checked the MasterMap topography product guide but I couldn't find an answer to my question. – hertzsprung Nov 18 '19 at 12:21
  • I think the first thing I'd do is see if the Environment Agency have one or two metre DTMs for your area of interest. If they have you could load the DTMs under the Master Map data and click on the spot heights, to show the elevation at the point clicked. I'd start here: environment.data.gov.uk/DefraDataDownload/?Mode=survey. The EA say All LIDAR data has a vertical accuracy of +/-15cm RMSE. – nhopton Nov 18 '19 at 14:42
3

Looking at the referenced document, it appears that they haven't been determined by GPS measurement, but instead by levelling

Spot height

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.

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