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Author Topic: Accuracy of DEM  (Read 7068 times)

jessica_tay

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Accuracy of DEM
« on: April 26, 2017, 04:07:40 PM »
Hello all,
I am trying to create a DEM from images taken from a drone (100m height). the images are references with x,y and z(altitude) information.

My coordinates are in decimal degree and my work area is in germany.

Generation of the sparse/dense cloud and mesh are fine, but when I generate my DEM based on dense cloud or mesh, both results are odd, giving me values in the negative range.

Anyone knows how to correct this?

Thanks!

SAV

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Re: Accuracy of DEM
« Reply #1 on: April 27, 2017, 06:02:22 AM »
Hi Jessica,

I guess you are using a DJI drone. There is a known issue with the altitude information in geotagged images that you get from these (e.g., Phantom 4 or Inspire 1). DJI uses BELOW SEA LEVEL as a reference, hence it records negative altitudes, which is quite odd. Additionally, the altitude you see in Photoscan is NOT the altitude above the ground, which is 'hidden' in the EXIF information of all the pictures. Just have a look at the altitude that is shown for each picture in the reference pane after importing it into Photoscan (no processing needed to see this error/problem).

There are several ways to fix this issue:
1. Use ground control points next time
2. Adjust EXIF information (z-coordinate) of the geotagged images based on your planned flight height using third party software/freeware
3. Extract the real AGL (=elevation above ground level) from the geotagged images and then add the known elevation of the take-off point to the AGL to get the real elevation.

I hope that helps.

Regards
SAV

jessica_tay

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Re: Accuracy of DEM
« Reply #2 on: April 27, 2017, 03:35:06 PM »
Hello SAV,

Thank you for your reply. the z coordinates are already adjusted based on the planned flight, which are all around 99m to 100m.

I do have 5 ground control points but it did not make the DEM better..

How do I extract the AGL from the geotagged photos?

Thanks!!

jessica_tay

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Re: Accuracy of DEM
« Reply #3 on: April 27, 2017, 04:02:35 PM »
Hello,

I've included an image of the data that I am working with.

Can anyone tell me what is wrong with it? that the DEM is wrong?

i've created a dense cloud, then made the DEM with it. But it generate that dem..

Thanks!!

Packy_GIS

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Re: Accuracy of DEM
« Reply #4 on: April 27, 2017, 09:02:38 PM »
The "elevation" you have in your EXIF is the flight altitude (100m above ground). There should be the real elevation (unless you are flying in 0m altitude).
What I would suggest:
1. uncheck all of the images in the reference pane
2. align images using the generic option (not reference)
3. define your 6 GCPs
4. problem should be solved
Jan

jessica_tay

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Re: Accuracy of DEM
« Reply #5 on: April 27, 2017, 09:47:22 PM »
Hi Jan,

Thanks for the reply.

I realised that the images from the drone doesn't have the gps metadata.. so i dont really know how to get elevation data? Does it come with the flight log as well? maybe it is possible to extract the elevation from a previous elevation model.

I will try to define my gcp with the xyz data and try again.

Thanks anyway.

Regards,
Jessica

SAV

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Re: Accuracy of DEM
« Reply #6 on: April 28, 2017, 01:53:25 AM »
Hi Jessica,

The easiest fix is what Packy_GIS suggested. Only use your 5 GCPs to georeference the model. However, they should be properly distributed over the whole area to get reliable results and measured with a high-accuracy GPS.

Make sure that you have selected the right coordinate reference system that matches the GCPs in the reference settings (tools icon in reference pane). Additionally, adjust the marker accuracy depending on how the GCPs have been measured.

Note. After unchecking all pictures and checking all GCPs, click on the UPDATE icon in the reference pane and then on the magic wand symbol to optimize the cameras. This will adjust your sparse point cloud/tie point cloud. You'll then have to compute the dense point cloud, mesh, orthophoto and DEM again.

Btw, at the moment your DEM seems to have a general tilt (E is high, W is low).

Regards
SAV



jessica_tay

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Re: Accuracy of DEM
« Reply #7 on: April 28, 2017, 11:00:48 AM »
Hi SAV,

Thanks for the suggestion. But I should also remove the Z values? since there are flight altitude and not the elevation?

and yes, i realised that after aligning the photos, the points are not horizontal.. instead it is lying diagonally in the bounding box. Do you have any idea how to solve it? my coordinates are all taken in WGS84, in decimal degrees.

thanks!

jessica_tay

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Re: Accuracy of DEM
« Reply #8 on: April 28, 2017, 12:51:10 PM »
to those who replied,

Thank you so much! I managed to build the DEM after using all your comments and input. :)

Regards,
Jessica

SAV

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Re: Accuracy of DEM
« Reply #9 on: May 01, 2017, 03:13:28 AM »
Hi Jessica,

Good to hear that you have been able to solve the problem  :D

Regards
SAV

Packy_GIS

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Re: Accuracy of DEM
« Reply #10 on: May 01, 2017, 09:55:17 PM »
Happy to help  ;)