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Author Topic: DEM generation issue - inverse referencing  (Read 1058 times)

khufkens

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DEM generation issue - inverse referencing
« on: August 07, 2018, 06:51:27 PM »
Hi all,

I'm trying to export a DEM from old aerial photographs with 60% sidelap (basically for old school DEM generation).

However, the "build DEM" menu item stays inactive even after generating a dense point cloud, mesh and orthomosaic (following the basic tutorial). I've no clue which step I skipped. The only thing I did not do was explicitly specify GCP.

For reference, the idea is to export the DEM and do an inverse georeferencing exercise using existing DEM data of the region. This might seem backward and inaccurate but the region is located in the central Congo basin (with lots of self similar forest) and obviously few remaining ground control points (especially in the forest). Using the overall topographic structure therefore is the closest one can get to referencing a large area of forest without true control points.

However, I first have to generate the DEM which doesn't seem to work.

More on the background of the project can be read here:
http://cobecore.org/blog/photoscan-orthomosaic-first-pass/

a blog post on the equipment used can be read here:
http://cobecore.org/blog/wild-rc5-camera/

Any advice would be appreciated!

Kind regards,
Koen




Alexey Pasumansky

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Re: DEM generation issue - inverse referencing
« Reply #1 on: August 07, 2018, 07:25:31 PM »
Hello Koen,

Usually DEM generation cannot be applied to unreferenced and unscaled models.

You can georeference the model if you know the coordinate information for the camera locations or some control points in scene. To scale the model you can use known distances between the pairs of camera locations or points on the ground (defined as PhotoScan marker instances).

If you cannot follow any of these operations, you can manually adjust the model orientation using Rotate Object tool to fit Z-up orientation in space and then apply any adjustment using Scale Model tool. After these actions you'll be able to Build DEM in the current manually set coordinate system.
Best regards,
Alexey Pasumansky,
Agisoft LLC

khufkens

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Re: DEM generation issue - inverse referencing
« Reply #2 on: August 10, 2018, 12:55:00 PM »
Thanks Alexey,

This works. I'll probably rely on some markers as I noticed that with the arbitrary option I can introduce slant which I can't remove easily. In short, the local environment (relatively speaking) shows the correct topography but the general context is then lacking. Hence I guess the requirement for pegging some reference points.

I only seem to be unable to stitch all data (403 files) in one pass. Strangely the areas with the most obvious key points are left out of the analysis and doing these regions separately yields good results but throwing in all data leaves data out and results in worse DEM. Any pointers would be appreciated. I feel I'm close to there but just missing the mark on these details.

Cheers,
K