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Author Topic: DEM without geographical coordinates  (Read 1356 times)

frans80

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DEM without geographical coordinates
« on: December 17, 2018, 05:37:18 PM »
Hello,

I have taken many pictures with a classical camera above a rough soil (without GPS infos). With Photoscan pro, I can align them correctly but I can not build a DEM. According to previous posts, I have to " .. apply three transformations to the model using Rotate Object, Scale Object and Move Object instruments that would also allow to generate the DEM, however, in this case the scale of the DEM will be arbitrary". I do not see clearly how to Rotate, Scale and Move Object instruments. Can you please help me?

My final goal is to compute the standard deviation of the surface heights. I guess that this is not possible with Agisoft?

Thanks
« Last Edit: December 17, 2018, 05:45:28 PM by frans80 »

Alexey Pasumansky

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Re: DEM without geographical coordinates
« Reply #1 on: December 17, 2018, 05:52:02 PM »
Hello frans80,

 Rotate Object, Scale Object and Move Object instruments can be found on the main toolbar  while you are in the Model view.
Best regards,
Alexey Pasumansky,
Agisoft LLC

frans80

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Re: DEM without geographical coordinates
« Reply #2 on: December 17, 2018, 06:08:53 PM »
Thanks for your quick reply.
When do I have to apply the Rotate, Scale and Move Objects Instruments during the workflow (before/after Align, Build dense cloud, Build mesh, ...)?
Do I need to just click on them or do I need to add markers, ...? Do you have more information on how to use these functions?
Thanks for your help,

Alexey Pasumansky

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Re: DEM without geographical coordinates
« Reply #3 on: December 17, 2018, 06:27:33 PM »
Hello frans80,

If you do not have any coordinate information for any control points in scene, then there is no need to add markers. Just use all of the three mentioned instruments, even to apply a small transformation. After that Build DEM option would be unlocked.

I suggest to give proper model orientation (parallel to XY plane) in order to use TopXY projection when generating DEM.
Best regards,
Alexey Pasumansky,
Agisoft LLC

frans80

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Re: DEM without geographical coordinates
« Reply #4 on: December 17, 2018, 07:52:10 PM »
Thanks for your help!

Dave Martin

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Re: DEM without geographical coordinates
« Reply #5 on: December 18, 2018, 02:27:30 PM »
Hi Frans80,

Just a quick comment - you mention that your goal is compute the standard deviation of the height of the soil. Unless the surface is extremely varied, GPS co-ordinates are going to be too crude - but I think you will need some sort of reference. In the post you quoted, it does mention the key phrase "the scale of the resulting DEM will be arbitrary" - so you end up with more of a qualitative than quantitative result.

I think you will need some way of scale into your model - otherwise the resultant DEM etc. are unit-less. Without input of any absolute scale, your model may be internally consistent, and you could perhaps compare the height SD in different zones of the same DEM and say 'area A is twice as rough as area B' - but you could not compare between projects or give any meaningful figures. Without scale, you could have a DEM of soil with height variations of a few millimetres, or an aerial of the Alps with height variations of hundreds or thousands of metres.

I think you may well find you need to capture some object or points of known scale to make the most of your project.

Dave