The kit we use is a Riegl VZ-1000 with a Nikon D700 calibrated camera and on top of the camera is a Trimble R6 GPS receiver. The UAV is a Gatewing X100.
Firstly we create a site calibration with a scale factor of 1. We fix control points on and around the site using a base station and 2 additional receivers. The wgs84 data is processed using a network adjustment programme, this will give us accuracy of + or - 5mm. This is then uploaded to the survey controller and used to fix the location of each scan, but not the orientation.
The scanner is mounted on the roof of a 4 x 4, this stop go method of surveying is very fast. The range is set at the lowest range, for the VZ-1000, 450m. At this range setting the speed will be 122,000 points per second and a high resolution the scan will take 47 seconds. The GPS location can be recorded simultaneously. Targets set over the control points are scanned at various times these are used both to orientate the scans and verify the accuracy of the final data. Using this method, when colour is not required, we can complete 50 scans in 2 hours.
The ground control for the UAV are laid out and scanned.
Once the scan data has been collected we register the scan locations using Riscan Pro. Firstly we filter the data using an octree and deviation filter we then use a planer surface and a deviation filter. using the backsight method we register those scans that have a tie point. Those scans with only a GPS scan location are fixed using this location point and an approximate northing. These scans can be rotated using the modify scan location. When all scans positioned we use the Multi Station Adjustment routine, we lock location and orientation of those scans with a backsight and we lock only the location of all those scans without. This will then fix all scans within about 30mm (GPS error). We then run the MSA again, this time we unlock the location and orientation of all but one or two scans. We the check the results of the tie point scans (back sights on control points) against the site calibrated control to confirm the accuracy of the survey.
At this stage we can use the scanned ground control to process the images acquired by the UAV. Under normal circumstances there would be relatively few points for ground control however with the scan data any amount can be used to improve and check the accuracy of the Digital Surface Model produce by the UAV's data.
Once the scans are registered we can start to classify the data. We use Terrascan. We run a number of macros which classify, ground points, vegetation, buildings etc.
Ground points are then taken back into Riscan Pro where we produce a mesh. At this point we would extract breaklines if required. We can also out put ground contours, a grid of levels and sections etc.
We would also export the mesh as a BMP which can be used in LSS to drape over the Digital Terrain Model or as a background in AutoCad. The high resolution ortho-rectified image, acquired by the UAV, is also used in the same way in LSS and AutoCad.
For the features such as buildings, fences etc we use a classification routine in Terrascan to slice through at a known height above ground. This is then exported to Cad.
This workflow works well for certain sites but we would not be averse to using GPS or total stations, as well, where there are lots of service covers at ground level.
Also with the VZ-1000 range of 1.4km we would also mount the scanner on a tripod if site conditions suited.
If you would like to know more please feel free to give me a call, details on our web site www.miningsurveys.co.uk