This job was carried out in November 2018 for Trust Power Ltd with the scope being to survey the lake bathymetry out to the lake shelf.
The aerial survey was carried out using a DJI Matrice 210 UAV fitted with an X4S camera and depth calibration data was collected by staff from NIWA using a remote controlled Jet Boat equipped with a depth sounder a (JetTec R/C boat with Tritech PA500 Echosounder).
Ground control points were placed around the lake shore in order to assist with georeferencing of the imagery.
A o.1m resolution orthophoto was generated and the Red, Green and Blue (RGB) values were extracted from the image at each depth point.
Machine learning techniques were then applied in order to find a relationship between RGB and depth.
Overall results showed an excellent match with the calibration data giving a mean absolute error of 0.12 m and a R2 Goodness of Fit of 0.93.
A visualisation of the final DEM as well as a number of extracted profiles overlaid onto the survey profiles are presented below.
These results go out to the lake shelf which coincides with a depth of approximately 4m.
A further attempt was made to see what is the maximum depth we could get to with the data at hand. More extensive processing was required however we found that depths of up to 10m could be surveyed with reasonable confidence as shown in the figures below. Due to the limited calibration data collected at depth, we were not able to extend beyond this depth.
We would love to have more opportunities to explore and develop these techniques further. If you are interested in learning more about these techniques or trialing them at your site, please get in contact with Matthew Gardner.