Water board Friesland conducted extensive levelling works in a Polder. In three areas the works gave rise to complaints, as rainwater would collect itself in areas not properly flattened. The water board asked engineering firm MUG to conduct DTM surveys using a drone to identify and analyse the problem. Engineering firm MUG has a framework agreement with the water board for surveying activities.
MUG engineering has entered into a partnership with Skeye to provide aerial surveys using drones. In this partnership Skeye will take care of the aerial acquisition and data processing whereas MUG engineering will measure the reference points, conducts the final geodetic inspection and creates additional end products for the client.
Prior to the aerial survey, ground control markers are placed in the project area. These are round circular discs with a diameter of around 50 cm. These markers were subsequently measured using RTK-GPS by MUG engineering staff in the field.
The drone measurements were performed with the Microdrone MD4-100. This type of drone is very suitable for low flights in smaller areas. The drone will take off in manual mode after which the flight is performed automatically. For this project the flight was carried out at a height of 75 m above ground. The helicopter was programmed in such a manner as to make images in pre-defined locations. At each picture position the drone stops to take a picture. The flight is programmed in such a manner as to achieve sufficient overlap to create a good stereo model. The observer can see a live preview of the images on a laptop on the ground.
Once the flight is completed the observer will check in the field if all images have been taken, are of good quality and if there is sufficient overlap.
At the office all aerial photos are placed on a high powered processing computer and a backup of the images is made. Subsequently the lens used is calibrated using photogrammetric calibration software. This package allows the modelling of all the geometrical abnormalities that occur in the lens of the camera. This model is important in order to achieve a good and accurate end results.
The creation of the height model takes place by means of photogrammetric software. In the first step all corresponding points in adjacent images are sought between the pictures. These are points in the images, which the software finds in more than one image. By means of photogrammetry the height of these points can then be determined,. The result is a 3D point cloud. Next, the locations of the ground markers are read into the software. These must then be assigned and located by hand in each of the photo images. As a result the entire point cloud is accurately placed (geo-referenced) in the local Dutch coordinate system. The point cloud is then used to create a height model. Once the terrain heights are known, an orthophoto mosaic is created from all the individual photo images. The result is a orthophoto mosaic where each pixel is positioned on its actual location in the local coordinate system.
Control and final products
MUG Engineering carried out a quality control on the supplied data by comparing the measured verification points with the nearest points on the height model. The differences in height are presented in the following histogram. The results show that the average height deviation is 1 cm. In each area, the average deviation was between 0 to +5 cm. The areas where a maximum of +5 cm was observed are the areas where winter wheat is still present in the field and this explains the difference between the height model and the verification points. The standard deviation of the heights vary between 4 and 5 cm. The largest deviations are found in the area with furrows.
Apart from the above products MUG engineering can create additional products from the 3D point cloud such as cross sections or vector based DTM’s.