[GRASS-user] Re: [GRASS-dev] DEM creation from stereo pairs

[Michael Barton]

Christoph,

Thanks for the information. I tried out stereo and your tutorial. I  
need to ask something, however. AFAICT, stereo xyz output for only the  
points that you click on in the 2 images. So, after calibration, if  
you click on 6 points, you get xyz coordinates for those 6 points and  
no more. That is, it does not create a regular grid of xyz points to  
use for DEM creation. Is this correct? Or am I missing something?

Note that I could see the xyz point output in the terminal-like  
window, but could not see the rendering because I can't determine how  
what to push on my Mac keyboard that it will recognize as alt-R.

Michael

On Dec 12, 2009, at 4:44 PM, grass-user-request at lists.osgeo.org wrote:

> Date: Sun, 13 Dec 2009 00:39:04 +0100
> From: Christoph H?gl <c.hoegl at gmx.net>
> Subject: Re: [GRASS-user] Re: [GRASS-dev] DEM creation from stereo
>        pairs
> To: grass-user at lists.osgeo.org
> Message-ID: <200912130039.05207.c.hoegl at gmx.net>
> Content-Type: Text/Plain;  charset="iso-8859-1"
>
> Am Samstag, 12. Dezember 2009 22:18:37 schrieb Hamish:
>> Hamish:
>>>> have a look at:
>>>> http://grass.osgeo.org/gdp/stereo-grass/
>>>>
>>>> I'm pretty sure there was a FOSS conference paper on that
>>>> method a few years ago.
>>
>> Michael:
>>> Unfortunately, the requisite "stereo" package not only has
>>> not been maintained since 1997 (according to the stereo-grass
>>> page), but its web page has disappeared too.
>>
>> the question is, how well did it work back then? working code
>> remains working code. :)
>>
>> you might try to look for a copy of the website on archive.org,
>> if not maybe I or one of us has an old copy of the source
>> somewhere in the old dusty attic.
>>
>>
>> shrug
>> Hamish
>>
> Well, the package did work and sometimes still does.
> it's job.
>
> The accuracy of the process after tweaking it a bit for small  
> rounding mishaps
> is about 12 to 14 bits (16bits data input). In comparison a hugin- 
> based
> process achieves a little -literally- 1 bit more of accuracy at  
> about 5 times
> the amount of time for data preparation (human) and 3-fold data  
> processing
> time (cpu-time).
>
> In the old times of stereo I even had to reduce resolution of the  
> source
> images in order to get them processed with 256M of RAM (8 Mega-Pixels
> (3344x2508) down to 4 MP (2364x1772) at that time)
>
> It still provided about pixel accuracy with regards to optical  
> problems
> (mirage/thermal inversion/turbulence of the air caused by heat/cold).
>
> To make it more feasible:
> Using a mounted camera 8 MP (which is common these days in this work  
> field),
> provides at a field of view of 45� a angular distance of about 48  
> arc seconds
> or about 1/75 of a degree which gives a accuracy of about 23cm at a  
> distance
> of 1000 meters. The atmospheric effect at that distance is way  
> beyond (~ 3
> meters)
> If you are after taking photos in less ambitious way, let's say  
> shooting an
> excavation from 100m or less, the accuracy is -> 2.3cm at best,  
> obviously
> better near the center of field (less lens distortion, ...) and  
> nearer to the
> lens (less atmospheric (d)ef(f)ects).
>
> Even if we take a sefety measure of factor 5 for each photo taken  
> using a
> shaky tripod or even factor 10 from a kite mounted camera and  
> multiply all
> worse effects instead of flattening out, the accuracy still exceeds  
> most needs
> at archeological sites (2.54cm = 1 inch) at distances of up to 25 m  
> by a
> factor of 8.
>
> The worst case i had with stereo was a misplacement by 1.5m  at 100m  
> distance
> using scans of ballon-mounted photograph from the late 30ies  
> combined with
> a 2003 photo taken using 4MP camera. (BTW, the 4MP was the culprit  
> in the end)
>
> One of the miracles using stereo was the ability to revisit a place  
> in the
> desert by triangulation using stereo after 8 years with 30cm  
> accuracy and
> pretty unsharp joshua-trees in the background (at distances of ~15m  
> and ~25m
> unsharp, 6 team photos taken in front of a car, taking the rim as  
> rule using a
> 4MP camera). GPS Data was misleading by 4-8 meters.
>
> The main reason for using stereo is it's ability to combine  
> different sorts of
> source materials (plates/film/digital imagery) in pretty short time.  
> (I use it
> still as stated above for small projects, where only a few points in  
> 3d-space
> need to be 'surveyed')
> As stereo takes more than 2 pictures it is even more powerful than  
> most
> standard off-the-shelf commercial solutions.
>
> The main reasons against it's use is the lack of automation in the  
> process,
> the lack of dedicated support (I'm not able to provide it, though  
> stereo would
> really deserve it) as well as the lack of camera/lens adjustment  
> combined with
> a really awful codebase.
>
>
> Hope that helps a bit,
> Christoph