Contour lines at bare earth [with Zenmuse L1]

This video shows a workflow to create contour lines at bare earth using lidar data obtained with DJI’s Zenmuse L1 sensor.

First, I will explain you the advantages of lidar data. Lidar doesn’t only reflect on the top of a canopy. Some laser beams will go between the leaves or grass and reflect on the ground. This allows to acquire information that would be shrouded by vegetation when you work with a photogrammetric drone survey.

In the second step, I will use the Virtual Surveyor check points tool to analyze the accuracy of lidar Zenmuse L1 data with a set of 123 control points. This analysis shows an accuracy of 5 centimeter.

In the third and fourth step, I will explain the method to create a topographic surface from the lidar data in detail. Lidar doesn’t always reach the ground. My method doesn’t try to remove the remaining stems, cars or houses but will model a topographic surface under these objects. Particularly interesting is that the produced data are created in a CAD compatible format. My method searches the lower points in a defined grid size and allows the surveyor to remove the few points that are not at the ground level.

In the fifth step, I will show how to create contour lines from the generated topographic surface and validate its accuracy. The analysis will show a similar precision of 5 cm.

As an extra, I will do the same process but with the photogrammetric Zenmuse P1 sensor. With this sensor, I can build less on the information under the canopy and I will have to work with a larger low-pass grid to create contours at bare earth. This shows an accuracy over this tall grass area of 8 cm but a similar accuracy of 5 cm over the hard surfaces. The orthophoto from the Zenmuse P1 provides a phenomenal clear and sharp detail.

Make sure to drop your own findings in the comments below. The point cloud conversion and check points tool are available in the free Virtual Surveyor Valley version.

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