Accuracy of Heights from Ordnance Survey Maps

1) Spot heights at summits have an accuracy of about ±3.3m at all map scales, in agreement with the published figure. 2) Levelled heights from old 6-inch maps can be a few metres below the summit but are unlikely to be more than a few tenths of a metre higher. 3) The accuracy of old and new 1:50k maps is similar, but where heights have changed the old height has a tendency to be too low. 4) Spot heights at cols have an expected error range of –3.0 to +4.2m (height tends to be overestimated). 5) Interpolated col heights in the DoBIH have an expected error range of –5.2 to +6.6m. 6) About 0.5% of summit heights on current maps are extreme values that are unlikely to arise from normal measurement error. 7) Errors of up to 0.31m were found in 21 flush bracket heights.