Making Digital Elevations Models (DEM) on the cheap

It is a little off topic but I thought many of the forum goers might find the Photosynth to 3D process I've been working on useful for creating 3D models from aerial photos.  The process is pretty simple and FREE:

·         Gather images with a lot of overlap (like most of our aerial shots)

·         Upload the files to http://www.photosynth.net (or use the open source Bundler app http://phototour.cs.washington.edu/bundler/ )

·         Use this exporter to extract the point cloud http://pspcexporter.codeplex.com/

·         Use a product like Meshlab  http://www.meshlab.net/ (hard to get good results) or VrMesh Studio ( http://www.vrmesh.com/products/overview.asp  ) to generate a mesh surface from the point cloud.

 

Here are two examples from my work in Ecuador and West Texas:

https://www.youtube.com/user/mdwillis01#p/a/u/0/2-oK5lnNA-I

and

https://www.youtube.com/user/mdwillis01#p/u/6/nJgvLll57f0 (only part of this one was done with Photoysnth).

The images were captured from kite and balloon platforms but the same workflow should work for any series of photographs.

Also, here is a Google Earth file with some of the same data http://70.114.146.89/~mwillis/Puchara_Grande.kmz (~10 Mb).  My linux box connection is slow. It'll take a bit to download.

-Mark

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Replies

  • Interesting article: http://gisws.media.osaka-cu.ac.jp/grass04/viewpaper.php?id=37
    http://gisws.media.osaka-cu.ac.jp/grass04/viewpaper.php?id=37
  • Mark,
    we are presently assembling some large gantry cranes over a flat concrete slab. The cranes are over 20m high. I will fix some targets on the slab and get some picture for you to process. I can get the targes surveyed by RTK GPS. The slab is a structural slab and is very flat (maybe +/-10mm but I can check this if we get dodgy results). If you can process them then we can see what we come up with. VRMesh is not cheap so I won't be buying a copy soon, I would rather put the cash towards an Ensomaic licence! The only problem I envisage is that the slab might not have sufficient texture - something to consider.

    I do have access to a PhotoModeler and it would be interesting to compare results.

    If you are interested then please let me know and I will organise the pics.
  • Has anyone done a check on the accuracy of this? Maybe take a few pics of something flat with some GCPs marked on it? Would be a good starting point. Limited accuracy might be good enough for use in orthorectification rather than for absolute use.
  • Mark,
    I did upload a bunch of picktures to Photosynth just to see how it worked: http://photosynth.net/view.aspx?cid=3f85fd1f-fe6f-4c10-ba6b-643b8f4...

    I was also able to extract a poincloud and to transform the poincloud into a mesh.


    Next, I would like to create elevation curves, and to drape picktures over the mesh/dtm. Can this be done in VRMesh Studio?
  • Is there a package version of Bundler anywhere? I tried following the instructions that came with the binary, but failed. It appears to run, but the output file is 0 bytes.
  • Really a good toolset :-)
  • T3
    Hey Mark,
    Great post, can't wait to grab some images and try it.

    I'm at work and havn't had a chance to poke thru all your links, so forgive me if these questions are answered else where, but I was wondering if you could offer a few hints on acceptible overlap when grabbing images and what kind of resolution/accuracy your getting. Also, I have the ability to log the roll and pitch attitude of the plane when the images are taken. Is that info needed to get the best results? Is there a way to feed that info to the application or should I pre correct the images using Hugin or some other tool.

    Thanks for sharing!

    Brian
  • @ssk320 The process works with photographs taken in any direction/rotation/skew as long as there is lots of overlap in photographs. What's also nice is that you can use images from multiple cameras, the cameras do not need to be calibrated, and changes in internal geometry don't seem to effect the results.

    If you poke around on my youtube channel you can see the process used on several rock art sites (non-aerial) and get an idea of some of the other possibilities.

    As for types of images that are acceptable, as far as I know, all standard image formats are supported except for RAW. My images were JPEG with minimum compression.

    @Nicholas Budd The downside is that you need a large number of photographs to get a dense mesh. My example used 410 photographs. I haven't tested my UAV yet but I'm guessing the number of photos taken for a similar area are much fewer using a UAV. Great thing about a kite, you can fly it as long as the wind is blowing and get a lot of photos.

    Thanks everyone for the kind compliments. I believe this technique, or some new variation on it, has a lot of potential.

    -Mark
  • As an prehistorian archaeologist myself, I can only say: great job with accessible means. I work in a region for which a sub-1m Lidar DEM is available for day-to-day work, so I know how useful a good DEM is. If I wouldn't have that, I would follow your way of doing, though I would prefer to take my shots from my ArduPilot airframe ;-) Keep going with the good work!
  • T3
    Calls for POST OF THE MONTH AWARD...
This reply was deleted.

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