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QUALITY
CONTROL -
Merrett Survey Partnership have completed a large amount of
quality control work, for both Government and private sector
organisations, including confirmation and analysis of data
accuracy. This is undertaken using ground survey methods,
so we obtain realistic practical results (not just theoretical
accuracies). We have developed specialist software to deal
with the very large data quantities that aerial laser scanning
generates. Understanding the intricacies of ellipsoids, map
projections and height datums is essential for the accurate
analysis of the data. You must compare 'like with like'. We
have witnessed people dismiss lidar as 'inaccurate' when the
only fault was a mis-understanding of the geodesy involved.
TRANSFORMATIONS
AND GEOID / ELLIPSOID MODELLING
- Aerial laser scanner systems compute positions and heights
using GPS. The GPS system uses the WGS84 ellipsoid to reference
it's heights. If you want to accurately convert these heights
to a more usable geoidal height (eg height above sea level),
then you need an accurate model of the geoid, local to your
area of interest. You also need to tools to convert the huge
data quantities from one system to the other, for which we
have developed specialist in-house software.
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The
Ground Truthing Site (GTS) is a ground survey of a test area;
we use specially made targets that show up well in the LiDAR
due to the intensity of the light that is reflected back.
The LiDAR cloud can be compared with the surveyed GTS. A patch
of LiDAR can be statistically analysed in order to test the
vertical and horizontal accuracy of the LiDAR.
The
image on the left shows a ground truthing site with a raised
target. If you move your cursor over the image, you will see
the survey data with LiDAR points overlaid. The green coloured
points have a level greater than that of the grassy area and
the raised target is clearly visible in the scan. The levels
of the LiDAR points are compared to the surveyed levels for
vertical and horizontal accuracy analysis.
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