GIS Thoughts

Did you feel the earth move..

I’ve just got back from the inaugural meeting of the British Computer Society’s Geospatial Interest Group, another example of GI hitting the mainstream. I was asked a number of questions on positioning technology and the accuracy of data, which reminded me that the OS has just completed it’s Positional Accuracy Improvement (PAI) programme.

Like most other national mapping agencies and other creators of national geospatial data products, the data contained in the OS National Database dates from a time before absolute positioning technology, where the location of a feature was ‘measured’ relative to a fixed control network and other “stable” geographical features.

For most purposes this has not been a problem, however with the introduction of GPS and the fact that the data was originally collected using local country projection systems, random location errors of up to 10 metres became obvious in rural areas.

The OS was left with the dilemma, ignore the issue – or correct it. This is not as obvious a decision as you might expect, in the UK most organisation who had captured they own information had done so in relation to features as positioned on OS maps, and these features might now be moving.

There really was no choice for the OS, as with many other organisations the widespread use of GPS meant the data has to be corrected, so five years ago the project was started and it was completed last month !

Out of the 244,000 sq Km of Great Britain, over 160,000 sq km had to be corrected using photogrammetric survey to increase positional accuracy to around a metre in most cases. To allow user defined data to be adjusted geometrically, no less than 450 million change vectors have been calculated for use in transformation software.

The OS may be one of the first organisation to address this issue, but I’m sure many other organisations will have to follow the example, the relative accuracy’s of mapping data used in the early days of digital cartography are no longer ‘fit for purpose” in the era of widespread GPS use, so for many people the earth may well be moving!

Written and submitted from home, using my home 802.11 network.

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