Mark Willis

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:

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

and

http://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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Steve JoycePermalink Reply by Steve Joyce on February 10, 2010 at 8:20am
Nice work, thanks for sharing this.
I wouldn't say it is off-topic at all, there has been a lot of talk about it lately. The ability to produce georeferenced photo mosaics and even surface height measurements is what turns a bunch of pictures from the air into useful data. Something that has commercial potential and a great application for small UAVs.
/Steve
ssk320Permalink Reply by ssk320 on February 10, 2010 at 8:43am
Does the software only handle nadir-looking (i.e., looking straight down) images? Or can it also handle images taken at an angle? Very cool, nevertheless. What file type of images are acceptable?
Nicholas BuddPermalink Reply by Nicholas Budd on February 10, 2010 at 9:21am
From what I remember of Photosynth, images from any angle should work as long as you have plenty of them and they overlap sufficiently. It basically picks out identifiable points that are common to multiple photos, and then uses the different distances, angles and relative sizes of the points in the images to work out the 3D arrangement of the actual points on the real object, which it generates a point cloud for. From there, it's possible to use the points to generate faces, and a 3D model of some kind. I thought of using Photosynth for that purpose the moment I saw it, but it's definitely way beyond my skills, so I've more or less been waiting for someone else to do it :-P

I have to say, the results are very impressive - the model looks great; a really good resolution. How many photos did it take to get it that good?
Krzysztof BosakPermalink Reply by Krzysztof Bosak on February 10, 2010 at 11:08am
Calls for POST OF THE MONTH AWARD...
RetoPermalink Reply by Reto on February 10, 2010 at 11:18am
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!
Jaime LopezPermalink Reply by Jaime Lopez on February 19, 2011 at 4:48pm

Hi Reto,

 

I will buy an APM soon to install on EasyStar model to capture aerial photos and then create a mosaic from them.

Could you explain me how can I make good DEM this way? (process, tools, etc.)

thanks in advance, Jaime

Mark WillisPermalink Reply by Mark Willis on February 10, 2010 at 1:05pm
@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
Gary MortimerPermalink Reply by Gary Mortimer on February 10, 2010 at 1:27pm
I regularly come home with 600 images from a flight so getting images will not be an issue ;-)

Maybe you will have to plod back and forth over the area for longer.
rogerPermalink Reply by roger on January 9, 2011 at 6:05am
What's the flying time for the 600 photographs? Make and model of camera? Thanks.
Gary MortimerPermalink Reply by Gary Mortimer on February 10, 2010 at 1:56pm
Now your talking that would be flipping handy
Mark WillisPermalink Reply by Mark Willis on February 10, 2010 at 1:57pm
@Michael, I have printed 3D hardcopies of petroglyph models using a commercial service ( 3darttopart.com ) with great success. The 3D model can be exported in a number of formats that should be readable by RepRap (which I'm not that familiar with). Making the model "watertight" might be a little complicated but I'm sure it's doable. Would be fun to see...
Nicholas BuddPermalink Reply by Nicholas Budd on February 10, 2010 at 2:45pm
Actually, there's something I have filed away in my 'Projects' bookmarks folder that might come in handy here. Take a look at this Cambridge University project (the YouTube video in particular) and see what you think. It's a similar kind of thing, and if it's at all possible to get that kind of thing running on something that can be carried by a UAV, or using a video feed beamed down from a UAV, then it would be seriously awesome for realtime environment mapping. It'd also be incredibly useful for generating part models for things like the RepRap.

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