We were invited by a local council to demonstrate our aerial topographic survey capabilities for a small culvert project. This was challenging as the route passed through particularly dense tree coverage. Typically multi return LiDAR would offer better penetration to identify true surface through vegetation and is typically chosen in this situation (See our article comparing Photogrammetry and LiDAR). In this instance we wanted to show photogrammetry as a valid option during winter months which would have a much smaller cost to the client over LiDAR data collection (typically costing thousands a day). The survey manager was specifically interested in a topographic drone survey to see how this compared with existing measurements of the site. The culvert ran next to an existing tarmac walkway under some small tree’s densely packed together.
In February we returned to site to conduct the survey. This was to be a geo-referenced survey and we chose to use ground control points and manually collect their locations using a survey pole and base station for later post-processing.
Firstly, the base station was set up to collect satellite data for 2 hours. In this case we used the Emlid Reach RS GNSS that we use for our own RTK drone setup. However, typically we use which ever survey equipment clients prefer such as those from Leica and Trimble. In our test we knew the accuracy of the Emlid would still be several magnitudes better than the accuracy of positioning information available from the drone. We laid out 12 ground control points along the length of the route with square/diamond patterns indicating the center point. Tthe position was then taken from this point with the rover pole and later tagged in the drone imagery.
In this case since the mission was a demonstration and close to residential property so we chose to use the DJI Mavic 2 Pro. Typically we would use a drone platform with higher image quality such as the DJI Matrice or heavy lift drone with a full frame camera. For mission planning we chose Drone Deploy with a one pass Nadir grid with standard settings at a low altitude of 20m. The mission was flow collecting 199 images of 5472×3648 resolution. Some blurring was observed due to poor winter light. After the mission was flow we measured the position of the GCPs before clearing up.
Drone Survey Data Processing
Later the survey data was post processed using RTKLib and local RiNEX data to provide accurate positions of each GCPs. This provided an accuracy beter than 2cm X,Y and under 4cm in height. The images and GCP locations were input into Pix4D and the mission processed to obtain a classified point cloud, DEM, DTM, mesh model and contour map as show below. This had an ground sampling accuracy of 0.81cm/px over the area of 2.47ha. The absolute camera position uncertainties X,Y,Z(sigma) were 0.112(0.021), 0.317(0.028) ,0.71(0.051)m. The error of a couple of the GCP measurements was high as the rover had poor line of sight to the base station on the hill through vegetation.
From the results we observed holes in the point cloud due to the dense wood branches over the pathway. This led to a poor contour model as seen below. This was disappointing but highlighted the importance of planning in accordance with the task which Drone Deploy alone didn’t consider.
Improved 3D Survey Model
We returned and manually flew and acquired photos around the problematic area this time taking additional oblique images perpendicular to the walkway. Processing these images provided an improved point cloud with the walk way visible through the trees to measure the topography along the route.
Extra processing was done to classify the point cloud this time so that high vegetation could be classified and removed from the point cloud to provide a more accurate DSM for measurement and topographical contour creation.
This example from the earlier batch of processing without point cloud classification and high vegetation removal shows peaks along the DSM profile. The profile was much more accurate after point cloud classification.
Our conclusion is that during winter months photogrammetry can provide actionable topographic survey data over woodlands, an area where LiDAR is typically required. With anticipated or non essential delivery schedules this can provide huge financial savings. Sky Tech are happy to work with local surveyors on projects such as this where drones can provide more rapid data collection over convential site walks. Please get in touch if you would like to work with us: