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Calibrating aircraft offsets

Aerial LiDAR services.

LiDAR = Light Detection and Ranging

LiDAR is a similar technique to Radar - It's an Airborne laser scanning terrain modelling system.

For information on terrestrial laser scanning, click here

DEM of MSP offices, produced by gridding LiDAR data and overlaying intensity map.

Digital elevation models are created by gridding LiDAR data. In gridded data, each cell is assigned a value based on surrounding elevation points. This gives the data a continuous distribution resulting in a smooth surface when displayed.

Once the LiDAR is in grid format, Digital Elevation Models (DEM), and contour and shaded relief maps can be created.


A closer view of a location.

X,Y,Z View of the LiDAR


Gridded View of the LiDAR



Exeter Devon England United Kingdom Great Britain


Ground control work

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. It is then possible to adjust the LiDAR to best fit the surveyed test site. We also measure the offset on the aircraft so that we know how far the GPS antenna is to the laser center.

Ground truthing site with target







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 accuracy analysis.




The graph on the left shows the differences between the LiDAR points and the surveyed test site. These results are very good because most of the differences are between 0 and 0.05m from the surveyed data.

Number of points tested: 9976
Mean height difference (m): 0.024
Standard deviation: 0.056
Root mean square error: 0.061



Processing of LiDAR

The LiDAR is processed automatically to seperate the returns into ground and vegetation files. This allows us to create 'bare earth' models without buildings or trees. The LiDAR can be processed in order to produce a variety of products including contour maps, cross-sections and digital elevation models (explained in information bar on the right). Once the point cloud has been processed into grid format (DEM), a variety of maps can be made, including shaded relief maps, vector maps (showing water flow), 3D perspective maps, slope maps, hillshade maps and ortho-drape maps. We can also compute volumes from the DEM's and have analysed differences between DEM's on beaches to quantify changes in beach volume. In combination with cross sections, we were able to show how wave action was affecting the beach profile.

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. Accuratemaps Survey Partnership have developed software solutions that allow for the conversion of huge data quantities from one co-ordinate system to another.


Aerial photograph draped over DEM
Raw LiDAR, discrete points The two images above show DEM's (continuous data) that were constructed from raw LiDAR points (discrete data). The image on the left shows a point cloud coloured by height, as you can see there are so many points that the data looks almost continuous.


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