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With VideoMapper, the digital images can be geo-referenced.
Conversely, geo-referenced data (say a highway centerline, street
furniture, or pipeline route), can be supplied with a digital photo.
The system is equally applicable for both aspects of data collection.
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Use of GPS with VideoMapper
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(For an explantion of the technical terms used below, see
the 'how GPS works' section on the next page - use link below)
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.
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