Positional accuracy improvement programme

Background information

November 2004

Positional accuracy improvement (PAI)

As part of our ongoing commitment to data quality, Ordnance Survey have recently completed a national PAI programme. This programme was developed to enable us to capture data at 1:2500 scale to a greater absolute accuracy (absolute accuracy is the position of features in relation to the Ordnance Survey National Grid). The PAI programme has resulted in an improved and more consistent accuracy standard of mapping data for rural areas. Furthermore, it has future proofed the data for the addition of new building development and other change, as well as providing a better relationship between Ordnance Survey 1:2500 scale map data and customers' own GPS-positioned resources.

Work started in April 2001 on the two elements to the national programme; one covering 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.

PAI delivery process

The PAI programme has delivered:

210 rural towns (approximately 2 500 km²) to an absolute accuracy of ± 0.4 metres root mean square error (RMSE); and
all other 1:2500 scale rural areas (approximately 155 000 km²) to an overall absolute accuracy of ± 1.1 metres RMSE.

Supply of PAI data

Supply of PAI data to Ordnance Survey Land Line^® customers is now complete. For interest sakes and for those who made the decision not to do anything until the programme was completed all those on maintenance agreements were sent the following:

positional accuracy improved map data; and
associated link files.

The supply was undertaken monthly on clearly labelled silver CDs. To alert customers to the fact that this was a PAI supply, the CDs were supplied in a DVD case with an orange coloured insert which is also labelled "Land-Line PAI"

Prior to October 2004 the data was not supplied in a distinctive box but the CD itself had an orange label to differentiate it from a customer’s normal supply:

PAI supply CD

Normal supply CD

For OS MasterMap^® aim is to incorporate the final PAI data by June 2006. Customers on maintenance agreements can access the latest PAI as change only update. The relevant link files can be downloaded from the Ordnance Survey ftp server.

Why PAI?

The reasons for undertaking such a programme are that the majority of 1:2500 scale mapping is 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 ± 2.8 metres RMSE absolute accuracy. However, the relative accuracy of features (distances between features close to each other) was very good, ± 1.2 metres RMSE, and was retained.

Since the overhaul PAI has happened on a local level when new map detail has been incorporated. But with the advent of GIS and customers associating their data with Ordnance Survey data, it was recognised that it was no longer right for us to supply discrete moved areas to fit in new developments without informing customers. Additionally, with both Ordnance Survey and the GI community embracing and using advances in data capture technology, there is increased demand for better accuracy.

Both Ordnance Survey and its customers were aware of the accuracy limitations of the overhaul mapping, and regular consultation confirmed that both Ordnance Survey and its customers would benefit from improving accuracy levels. After seven consultation and information papers (1997–2000) and a PAI seminar held in February 2000 there was consensus that a national programme of PAI in areas mapped at 1:2500 scale should proceed. The need was recognised in the context of:

future proofing our large-scale map data to accommodate ongoing development and change;
providing higher-quality data to customers; and
providing better relationships with customers' GPS-positioned assets.

The conclusions were that we should:

Publish a national programme of PAI activity that will provide customers with details of when, where and how the changes will be implemented.
Complete PAI in one pass per 1 km by 1 km area. PAI activities must not introduce errors in content detail.

Further consultation has demanded that Ordnance Survey undertake the PAI programme once only and complete it as quickly as possible.

Communication and consultation will continue to be a key feature of the programme.

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