[FWTools] gdalwarp issue?

[Brent Fraser]

Jeff,

   If your aerial photography is at a low altitude, earth curvature 
won't play much of a part.  Then a better transformation might be a 
projective transformation 
(http://www.leptonica.com/affine.html#RELATED-TRANSFORMS).  
Unfortunately GDAL doesn't support the projective transformation...

Best Regards,
Brent Fraser


On 12/16/2010 8:03 AM, jkenneally at cartenav.com wrote:
> Thanks a lot, Frank...that definitely seemed to help.   I've never 
> really encountered 'thin plate splines' before, and I'm going to have 
> to do a bit of reading regarding how they work.  However, in 
> hind-sight I should have realized that I could only do an affine 
> transform with four control points.
>
> I do have just a few more questions about this if you don't mind though...
>
> Regarding the thin plate spline transformation, I do get the general 
> transformed shape I was expecting, but the sides are a little more 
> 'curved' than I was expecting, and I'm not sure how accurate they 
> are.  Is this to be expected?  I am attaching another tiny image to 
> demonstrate the result of my earlier warp with the '-tps'.
>
> Lastly, based on further reading I get the impression that a second 
> order polynomial warp will at least incorporate a better model of 
> earths curvature into the transformation, and that a third order would 
> also help account for the 'perspective' of the camera when the source 
> imagery is not taken from a 'straight down' vantage point.   If this 
> is correct, is it enough to literally just specify a few more gcp's (6 
> total) for a second order, or 10 for the 3rd order?   I assume it 
> would be best to distribute these gcp's fairly evenly over the source 
> image?
>
> Thanks again for your help!
> Jeff
>
> -----Original Message-----
> From: "Frank Warmerdam" <warmerdam at pobox.com>
> Sent: Wednesday, December 15, 2010 6:55pm
> To: jkenneally at cartenav.com
> Cc: fwtools at lists.maptools.org
> Subject: Re: [FWTools] gdalwarp issue?
>
> On 10-12-15 01:49 PM, jkenneally at cartenav.com wrote:
> > Once that is complete I run gdal_warp with the following arguments:
> >
> > gdalwarp.exe -t_srs EPSG:4326 -wt Byte -r bilinear -of GTiff 
> -srcnodata "0 0 0
> > 0" -dstnodata "0 0 0 0" 
> "C:\Projects\example_test_4\test_image_referenced.tif"
> > "C:\Projects\example_test_4\test_image_rectified.tif"
> >
> > I have tried different combinations of input params on both commands 
> with no
> > success so far, but the above are the current 'best guess' at what I 
> should be
> > specifying. I am suspicious that maybe I should be specifying a 
> -a_src param
> > for the translate call?
> >
> > As I mentioned in my previous post, if I load 
> 'test_image_referenced.tif' into
> > Global Mapper after the gdal_translate call it appears as expected, 
> so I don't
> > think it is as simple as my corner coords being wrong. Also, the 
> final warped
> > 'test_image_rectified.tif' seems to be rotated correctly, just not 
> bound within
> > the correct 'footprint' shape.
> ...
> > I am attaching a teeny file comparing the final image loaded into 
> global mapper
> > and OpenEv to show more clearly what I am seeing with the results.
>
> Jeff,
>
> Based on the image you attached I think you want more than a first order
> polynomial transformation. You want, essentially, piecewise warping.
> Four GCPs are only enough to give rotation and shear in a first order
> polynomial. But if you use thin plate splines instead with the GCPs you
> should get a more general transformation.
>
> Try adding "-tps" to the gdalwarp commandline arguments.
>
> Best regards,
> -- 
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up | Frank Warmerdam, 
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush | Geospatial Programmer for Rent
>
>
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