Accuracy of our legacy control data

Find out about the accuracy of our legacy control data - passive GPS stations, benchmarks and trigs.

Accuracy statement for our current OS Net stations and transformation

Passive stations

The standard error of the passive network is 0.055m in plan and 0.066m in ellipsoidal height at 95% confidence level.

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Benchmarks and their heights haven't been regularly maintained for over 40 years. The figures here are a guide to the accuracy standards for when benchmarks were being maintained.

The figures in the table only relate to marks heighted relative to ODN (Ordnance Datum Newlyn). The table shows the expected maximum error between BMs on the same levelling line up to 4km apart. For levelling lines less than 1km, treat as 1km.

Order of vertical control Maximum error (mm)
Geodetic (1st order) +/- 2mm x sq. root of distance levelled (km)
Secondary (2nd order) +/- 5mm x sq. root of distance levelled (km)
Tertiary (3rd order) +/- 12mm x sq. root of distance levelled (km)

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Triangulation stations

OSGB36 triangulation stations are no longer maintained. The figures here are a guide to the accuracy standards for when they were being maintained.

The table below shows the expected accuracy of stations in relation to others of the same type with the circular area of a given diameter.

Type of control station

Expected accuracy

(1 sigma, m)

Diameter of circular

area (km)

Primary trig station Error free N/A
Secondary trig station 0.06m 15km
Tertiary trig station 0.05m 7km
Minor control station 0.04m 3km
Chain survey station 0.08m 3km
Map detail station 0.06m 2km

The expected accuracy (1 sigma) of one type of control station in relation to another of a different type is shown in the table below. In these cases the expected accuracy applies within the smallest diameter of circular area (from table above) pertaining to the stations used.


Primary trig station

Secondary trig station

Tertiary trig station

Minor Control station

Chain survey station 

Map detail station

Primary trig station 0.00m          
Secondary trig station 0.06m 0.06m         
Tertiary trig station 0.08m  0.08m 0.05m       
Minor control station 0.09m 0.09m 0.07m  0.04m     
Chain survey station 0.12m 0.12m 0.11m  0.09m  0.08m   
Map detail station 0.11m 0.11m 0.09m  0.08m  0.10m  0.06m 

E.g. the expected accuracy between a secondary trig station and a minor control station = 0.09m. This is with a circular area of diameter of 3km which is the lesser of 15km for secondary trig and 3km for minor control (from the 1st table).

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