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Drone data audit

Good day,

We have conducted an aerial survey of about 21 000 Ha using an Ebee drone, with flight hights varying between 300m & 400m ASL. We allocated about 1200 GCPs roughly at 400m x 400m (some areas were not accessible).
The data was processed with Pix4d v.3.2.10 and v.3.2.4.
The following problems were identified on the data:
• Resulting DTM elevations do not match GCP elevations in all areas. GCPs were used as a framework to check the replication of the drone survey data.
• DTM elevations for the GCPs were determined using the “apply elevations from terrain layers to selected features” in Global Mapper
• Ridges created where different areas are merged
• Misalignment where 2 or more projects are merged
• When selected, GCP value changes instantly (Image below)
• DTM has wrong values along low-lying areas (water), please advise on what can be done to compensate for the wrong values

Please advise on the way forward to fixing these problems.

Thank you and Kind Regards

Hello Nick,

Regarding the DTM, the DTM, which is automatically generated by Pix4D Desktop, is based on the DSM. The DTM algorithm takes as input the DSM and it smooths it so as to remove the buildings and objects. In general, the automatic DTM generation should detect the objects and generate good results. 
Since 4.0.23 version the point cloud classification is available and it is also used as an input for the DTM generation. The point cloud classification classifies the points into various groups.These groups and any manual intervention in the classification will also be used for the DTM generation. To improve the DTM, all the terrain points should be assigned to one of the point groups (Ground and Road Surface) that are used for the DTM. For more information you can take a look at the following:
https://support.pix4d.com/hc/en-us/articles/115004864586
https://support.pix4d.com/hc/en-us/articles/202560579

Regarding the misalignment between projects, you should always make sure that the same horizontal and especially vertical coordinate system are used for the 2 projects and that there is an overlapping part between the projects.

Regards,