A national programme to improve the positional accuracy by Ordnance Survey

A national programme to improve the positional accuracy by Ordnance Survey

Ordnance Survey is set to embark upon a national programme to improve the positional accuracy of its detailed mapping data of rural areas at 1: 2,500 scale.

As a result of new technology, such as the Global Positioning System (GPS) and digitally scanned aerial photography, Ordnance Survey can now collect mapping data at even greater accuracy than in the past. These new methods of surveying contrast greatly with those used many years ago when the large-scale maps of rural areas were compiled and revised. This in turn created its own problem - how to fit this higher accuracy work alongside old surveyed detail.

The national programme, which has full backing from key users of Ordnance Survey data, will use this wealth of new technology to improve the positional accuracy of its 1:2,500 scale mapping.

The mapping agency believes that it is important to tackle this now to ensure its 1:2,500 scale rural areas map data meets modern quality standards and to ensure that the accuracy level will be suitable and acceptable in the future when new developments and other changes need to be incorporated.

Ordnance Survey's Jim Page says: "To put the task ahead into context, the area of Britain is just over 230,000 square kilometres and Ordnance Survey maps around 158,000 of them at 1:2,500 scale. The biggest impact of the improvement programme will be on less than two per cent of the 1: 2,500 scale mapping. These cover small towns and peri-urban areas and will be completely resurveyed or reformed to a higher accuracy to achieve the same quality standard as our current 1:1,250 scale mapping.

"There will be changes on the majority of the other 1:2,500 scale maps as well to meet a consistent absolute accuracy standard, but the number of features affected will be less. As geometric fidelity and relative accuracy will be maintained, typical land parcel areas are not expected to change by more than two per cent. Nonetheless, we know it will be a pain for some of our Land-Line data customers, who will be most affected by these improvements, but we hope they will understand that it's short-term pain for long-term gain."

The improvement programme will, inevitably, have a knock-on effect for existing users of Ordnance Survey's rural Land-Line data at 1:2,500 scale who have linked their own data to it using geographical information systems (GIS). For instance, whilst in most cases areas of land are unlikely to change in size and shape, their position may shift in relation to the National Grid. Thus users may find that there is a mismatch between the National Grid position of their overlaid data and the Ordnance Survey 1:2,500 scale base map.

Because of this potential impact on customers, Ordnance Survey has consulted extensively with its major users during 2000 as part of the development of a national programme of positional accuracy improvement. During this time, Ordnance Survey has also been working with system suppliers to develop software that will help customers move their data automatically to coincide with the new survey position. This has involved developing coordinate records or link files recording the current location of major features in the map data as well their position after positional accuracy improvement. System suppliers Intelliscan and Dotted Eyes have been particularly active in this development process.

Jim Page adds: "We want to be open and informative about the challenges ahead but we also want to stress the positive long-term benefits of the positional accuracy programme. Among these benefits will be a more consistent accuracy standard of map data for rural areas as well as greater compatibility in adding new developments. This in turn will ensure a much better relationship between the position of map data and customers' own GPS positioned resources."

The national positional accuracy improvement programme of 1:2,500 scale map data will start on 1 April 2001. Up-to-date information about the programme's progress can be found on the Internet at www.ordnancesurvey.co.uk/positional .