Surveying Articles:

Ordnance Survey Accuracy

Whilst the Ordnance Survey data is very useful for large scale mapping and navigation purposes, it has limitations due to its nature and it is not likely to be accurate enough for any development purposes.

A measured land survey at a scale of 1:200 should be accurate to + / - 5 cm. The Ordnance Survey at 1:1250 scale in an urban or defined rural town quote +/- 40cm. In rural areas this accuracy level falls to +/- 1.2m.

Example of relative accuracy
If the distance between a building corner and a fence post is 60m, then on a 1:200 measured land survey the distance would be represented on the survey drawing by a scaled distance of between 59.95m and 60.05cm. Using the Ordnance Survey 1:1250 the distance would be represented by a distance of between 59.20m and 60.80m.

This is why a land survey at 1:100 or 1:200 is recommended for most schemes. The danger is that if a scheme is designed using the Ordnance Survey mapping, the developer may then discover that the plot actually will not quite fit everything in !

Sometimes we are asked to overlay our surveys onto the Ordnance Survey map for the area, either by using GPS or a graphical best fit method. We are always happy to do this and can supply the Ordnance Survey data if required, however if the fit is not particularly good it is because of the differences in accuracy.

Scale 1:1250 1:2500 (resurvey/reformed) 1:2500 (rural) 1:10 000

Expected Accuracy at 95% confidence level 0.8m 0.9m 2.4m 7.1m

(Information taken from Ordnance Survey ‘Accuracy of Large Scale Mapping’ Sept 1998 and current OS website).