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Joining two different technologies in a single software

In this project we combined two technologies: close range photogrammetry and bathymetry. With photogrammetry, we captured the images on the terrain, and we used bathymetry with sonar to measure the points on the bottom of the lake, thus measuring the terrain in full. Both measurements can then be put together into a combined model. The data acquisition is performed every three months in order to thoroughly follow the changes on the selected area (land and lake).

This way of work can also be used in similar projects as for example to the coastal areas, where the shore can be mapped with the drone and the bottom of the sea can be measured using the sonar. With these measurements we can evaluate the erosion of the sea, which is caused by the currents, waves, storms and other natural phenomenon.

The aim of the measurements in this showcase was to calculate the volume of the material that was dug out of the lake. The excess material is then later on taken to the depot. Another aim of the measurement is performed to calculate the volume of the material that can still be dug out of the lake – they can dig up to the depth of 8m. All the acquired data is then calculated in the 3Dsurvey software. The measurements are done every 3 months in order to easily monitor and accordingly plane the work process. When the results are calculated in the software, they are able to dig out of the lake the maximum allowance of the material and then they discharge the material that is not usable back to the lake. In this way they calculate their services much more efficiently and much faster as in the past.

And what exactly is needed to be done in the 3Dsurvey in order to do the measurement of the materials? 

With the help of drone, they have to perform the photogrammetry in order to make a digital model of the terrain. Using the swimming robot boat with the sonar, they perform the bathymetry in order to make the model of the bottom of the lake. The result of the both measurements is orthophoto plan of the entire area and Digital Surface Model from the lake bottom.

The drone used for this measurement was the Phantom 4 PRO. In 2 consecutive flights it made 1150 images, which were then used to measure the point cloud, DSM and orthophoto.

To measure the bottom of the lake the autonomous measuring boat (swimming robot) was used. It is a self-propelled surveying boat that captures large underwater areas with CPU from the drone technology. Sonar which is placed on the boat is measuring the depth of the lake and is also directly connected to the survey grade GPS with the dish. 

The path of the boat is planned in advance, similarly as the one on the drone. When the boat is in the water it is floating and collecting data autonomously. On the photo below you cans see the path of the boat.

The results of this measurements are the 3D points of the bottom of the river, which can easily be exported as *.txt file and it looks like x,y,z position of the points.

We simply imported the points to the 3Durvey as follows: Point cloud -> Load -> and select *.txt file. Because the points of the sonar are colourless, we decided to use the Heightmap for a better representation and we also enlarged the points with the “Point render size”.

In this way we are able to see the path of the boat very well. We can also see the way it was measuring the data and the depth of the lake, which is demonstrated thoroughly with the colour of the points. There is one limitation to the boat, which is the fact that it can measure the data on the minimal depth of 0,5 m. To add the boundary of the lake we added also the point cloud of the terrain which was calculated with the help of photography and with the Merge function. At the end all the data of the photogrammetry and the sonar were collected in one point cloud.

Becasue we have a good density of the points from the data of the sonar (one point for 1 meter and the width between the lines is 2 – 10 meters) we were able to calculate the unified model using the Average points method.

If the data from the sonar were insufficient or if there was just one point every 10 meters, we would have to calculate the DSM for sonar separately, following the method of ‘’Connected points’’ and DSM for photogrammetric data using the standard method of the Average points. Both DSMs would then be combined in one.

In case you would like to check the results in GIS application follow the link: https://www.mapservice.de/caddydeponie/demo/

Results:

  • Mapping of the entire area
  • Digital model of the terrain and the lake
  • Calculated profile of the lake
  • Calculated free volume of the lake

Project Tools & Specs

  • Drone & Software: Phantom 4 PRO + 3Dsurvey & Swimming robot with sonar
  • Camera: Integrated 20 MPIX
  • Sonar: Deeper Smart Sonar PRO+ and Airmar Triducer DST800
  • Boat GNSS: Drotek RTK Solution Base: XXL RTK Rover Sirius RTK
  • Orientation:GPS measurements – 12 GCP points / 
  • Total number of photos: 1150
  • Flying height: 80m
  • DOF resolution: 2cm
  • Area size: 15 Hectares
  • Data processing time on super computer: BA: 4h, Orientation: 25 min, Dense reconstruction: 15h (on Extreme). DOF, volumes, profiles – on the GO.
  • Prices: up to 10000€ for a Swimming Surveyor 2 including the training

Results:

  • Point cloud
  • Cross sections/Profile line from lake bottom
  • Orthophoto
  • DSM
  • Model of the lake bottom
  • Volume calculations