Click here for terrestrial lidar services Lidar survey services - worldwideAirborne laser scanning servicesHow we can apply laser scanning to meet your needsLidar - industry relevant sitesgallery showing some laser scanning projectsContact Merrett Survey - laser scanning experts 

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.

MAXIMISING QUALITY - From our experience of analysing the data, why not reverse the process ? Ensure that you are obtaining the best accuracies and the best quality data before someone checks your data. In the image (right) a surveyor checks the aircraft offsets to ensure that we know how far the GPS antenna is to the laser center.


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.



The graph on the right 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

            Laser scanning experts in data capture and presentation.
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