Τοπογραφία

Low Cost Mapping with DJI Mini 2

In this post we propose a method to perform low cost mapping with DJI Mini 2 drone using free tools in order to create missions or flying over waypoints. Recently a number of drone applications have included in their supported drones the DJI Mini 2. Thus, it is straightforward to create a flight plan with waypoints in order to map an area of interest such as a road or a river triggering the camera using the intervalometer every 2 or 3 seconds according to flying speed of the drone.

DJI mini 2 flting over waypoints

Mapping with DJI Mini 2

As said before, planning a linear mission is straightforward. Furthermore, below we describe how we could plan a correct and efficinet rectangular mission over an area. In order to do this we need to estimate the image footprint on the ground so the distance between every flight line can be calculated.

After a camera calibration the mini 2 camera parameters are (these are different for every drone):
Focal Length  4.91mm
Width   6.476mm / 4000pixels
Height  4.857mm  / 3000pixels
Pixel Size 0.0016mm

Using the collinearity equations we calculated the flight height, GSD and image footprint for these cases:

  • GSD 0.01m – Height 30m – 40m x 30m footprint – 80% side overlap 8m / flight line
  • GSD 0.02m – Height 60m – 80m x 60m footprint – 80% side  overlap 16m / flight line
  • GSD 0.03m – Height 90m – 120m x 90m footprint – 80% side  overlap  24m / flight line

Forward overlap is calculated according to flight speed and trigger interval e.g. 20 kph = 5.56m/sec

For 80% forward overlap from 90m flight height the trigger distance should be 18m so
with a flight speed of 20 kph the interval should be set to 3sec.  For 30 kph flight speed the interval should be set to 2sec

Rectangular mission for DJI Mini 2 drone

Once we estimate the desired overlap using the calculations above we can draw the rectangular mission. We use CAD or GIS software. In this case we utilize the QGIS open source GIS software to draw the mission and export it in KML file. Consequently we upload the KML file on the drone app in order to accomplish the flight.

Rectangular mission design in QGIS and exporting KML file

Then we import the kml file with the rectangular mission in the drone application and perform the flight

Import KML file with rectangular mission for DJI mini2 drone

Results

Below results are presented from another test performed using the proposed method

Point cloud created with DJI Mini 2 drone and the proposed flight planning above
DSM derived from the above point cloud

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