Positional Accuracy Improvement (PAI) Programme

As part of an ongoing commitment to data quality, Ordnance Survey (OS) undertook a national Positional Accuracy Improvement (PAI) programme between 2001 and 2006. This programme improved the absolute accuracy (absolute accuracy is the position of features in relation to the OS National Grid) of all maps covering the 1:2500 scale rural mapping. The PAI programme resulted in a more consistent accuracy standard of mapping for rural areas.

The reason for the OS undertaking such a programme was that the majority of 1:2500 scale mapping was originally based on overhaul mapping from the 1950s. The overhaul process took the existing County Series mapping, each based on an individual projection, and converted it to the National Grid. There were limitations in that conversion process that resulted in relatively poor absolute accuracy. However, the relative accuracy of features (distances between features close to each other) was very good and was retained during the PAI programme.

Showing the update in absolute position from pre-PAI map (red) to post-PAI map (green)

Details of the PAI programme

Work started in April 2001 on the two elements to the national programme; one covering a list of rural towns, the other covering the remaining rural areas. The rural towns completed in December 2004 and the programme for all other 1:2500 scale rural areas completed in March 2006. The PAI programme delivered a re-survey of the listed rural towns with a much improved absolute accuracy, as well as an overall improvement of the absolute accuracy in all other 1:2500 scale rural areas.

Implications for RoS

As a result of the PAI programme, RoS received a large number of new map versions across the 1:2,500 areas of Scotland, and this meant that even larger numbers of registered titles had to be updated to the new map detail. The defined rural towns were each treated as small projects by Map Base Maintenance, with each registered title and index entry being considered, often by checking the original extent deed, and re-mapped as appropriate using Map Updates (MUs). For the remainder of the 1:2500 areas, a larger project team was set up and any titles updated during that project were re-worked using a PAI application.

Should you be investigating the extent of an older registered title and note that a PAI application version exists, it may be helpful to compare the pre- and post-PAI map versions on the Plan Viewer to see what updates were made to the title.