What do 'absolute positional accuracy' and 'relative positional accuracy' mean?

Absolute positional accuracy and relative positional accuracy are OS MasterMap^® quality measures. Absolute positional accuracy is a measure of the average discrepancy between the true National Grid positions of features and their surveyed positions in our data (map position).  The true position is determined by a more accurate method than the survey capture method. Relative positional accuracy is a measure of the average discrepancy in distances between surveyed features approximately 60 m apart. 

We sometimes use the RMSE (root mean square error) measure to summarise these discrepancies. However, in the Topography Layer of OS MasterMap we state 99% confidence limits that are easier to interpret than RMSE. For example, where the OS MasterMap Topography Layer accuracy of position attribute has a value of '1.0m' (representing urban data capture standards), the 99% confidence level of absolute positional accuracy is 1.0 metre.  This means that in 99 cases out of 100, the true position of a coordinated point lies within a circle of radius 1.0m centred on the map position. More information can be found in the user guide.

Go to the OS MasterMap user guide page.

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