Elevation Calibration

I’m having some issues forcing the Z value to the absolute elevation corresponding to the GCPs. To give a bit of backstory;

  • I map the flight, fly the drone and upload photos importing GCPs and making sure that I use the right coordinate system (OSGB)
  • When everything is processed I calibrate the map if required so the ‘elevations’ tab shows elevations that tie in with my GCPs
  • If I export to a DTM (.dxf) or point cloud (.las) the X & Y co-ordinates tie in with the GCPs but the Z co-ordinates (elevations) don’t

I’m also making sure that any files i export are in the coordinate system i’m using. I’m trying to use the drone topos for surface creation in Civ 3D so it’s kind of vital that the elevations actually tie in - does anyone have any idea as to why it’s not working?

A DTM will almost always have different values at individual spots. The DSM should be really close, but the DTM process changes the point cloud and how everything is triangulated. It’s usually within 0.10ft from what I’ve seen, but it depends on what was around the GCP that the DTM process modified. We create our DTM’s outside of DroneDeploy so that we have control of this and it still happens unless you exclude areas.

I would be Ok with 0.10ft but for example a GCP on one of my latest flights should have been approx. 79.7m and it’s actually sitting about 28m on the .las & .dxf exports which obviously throws the whole model off. In the actual map / model on the drone deploy app it looks like is correctly set to 79.7m so I don’t understand why the exports are way off.

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What are you use to open the LAS and DXF files?

AutoCAD Civ 3D to open the dxf and Autodesk recap to convert the las to an rcp file before opening in civ 3D.

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The fact that you have two programs opening two different sets of data and they are matching each other, but not matching the survey grade tells me that something went awry in the exporting process like you mentioned.

Is the offset consistent across the entire site?

If you raised the exported data that amount does everything match?

The only thing I can think of is an insertion point discrepancy, but honestly I think this is going to have to be a support issue unless someone can chime in that has had this problem. I have never had an export differ from what was reported on the DD website.

Yeah the vertical offset is consistent across the model / site and if i raise the surface then everything ties in. I don’t want to have to do that though because I want to be confident with the Drone topo without tying back into a ground survey to check everything - kind of defeats the purpose of using the drone for efficiency!

I don’t think it can be an insertion point error because I’m not actually specifying an insertion point I’m just inserting to the original coordinates. I suppose it could be some kind of error with the software not talking to each other but I think it’s fairly common for people to use DD and autocad together?

I’ll hang around to see if anyone else has experience of this but thanks for the response so far.

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Email support@dronedeploy.com with the Flight ID from under Map Details. We use Carlson with Civil 3D and like I said I have never seen an export change elevations.