Video Mapper

Use of GPS with Video Mapper

The geo-referencing, or location of the video images produced by VideoMapper, relies on GPS. This technology is not as straightforward as many people think. There is great potential for error and inaccuracy if it is not used correctly. Merrett Survey Partnership have many years of experience with GPS and this expertise has been applied to get the best out of VideoMapper.

There is no single solution to the problem of computing your position. A variety of GPS hardware can be applied to the VideoMapper system. This is one of the aspects that make it so flexible. Achievable accuracy is dependant on the equipment and methods you apply. Similar data collection systems rely on DGPS - where the corrections, necessary to obtain accurate results, are broadcast to the user - often via a communications satellite. This works well in many parts of the world - but not everywhere.

The DGPS signal can be blocked by trees, or tall buildings in an urban environment. Inertial systems can be deployed to keep a track of position when the GPS or DGPS signal is lost, but the accuracy of these devices falls off rapidly after a short distance. Many of them do not record height data.

Better results can be obtained by post processing the GPS data (this involves collecting data in the 'rover' instrument, and also at a local base station), The post processing, together with a few 'tricks of the trade' means that results can be obtained in many of the locations where DGPS systems will not work.

DGPS coverage is not always guaranteed. We have learnt by experience, that even when the service supplier confirms coverage, this is not always guaranteed to work. You have to test it for yourself. However, VideoMapper uses it's own base station, so is not reliant on an external service, and can be used in any part of the world.

Accuracy is also improved by keeping the base station as close as possible to the rover. So VideoMapper can provide accuracies better than those offered by DGPS systems.