<font face="arial" size="2">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. <br>
<br>
I do have just a few more questions about this if you don't mind though...<br>
<br>
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'.<br>
<br>
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?<br>
<br>
Thanks again for your help!<br>
Jeff<br><br>-----Original Message-----<br>From: "Frank Warmerdam" <warmerdam@pobox.com><br>Sent: Wednesday, December 15, 2010 6:55pm<br>To: jkenneally@cartenav.com<br>Cc: fwtools@lists.maptools.org<br>Subject: Re: [FWTools] gdalwarp issue?<br><br>On 10-12-15 01:49 PM, jkenneally@cartenav.com wrote:<br>> Once that is complete I run gdal_warp with the following arguments:<br>><br>> gdalwarp.exe -t_srs EPSG:4326 -wt Byte -r bilinear -of GTiff -srcnodata "0 0 0<br>> 0" -dstnodata "0 0 0 0" "C:\Projects\example_test_4\test_image_referenced.tif"<br>> "C:\Projects\example_test_4\test_image_rectified.tif"<br>><br>> I have tried different combinations of input params on both commands with no<br>> success so far, but the above are the current 'best guess' at what I should be<br>> specifying. I am suspicious that maybe I should be specifying a -a_src param<br>> for the translate call?<br>><br>> As I mentioned in my previous post, if I load 'test_image_referenced.tif' into<br>> Global Mapper after the gdal_translate call it appears as expected, so I don't<br>> think it is as simple as my corner coords being wrong. Also, the final warped<br>> 'test_image_rectified.tif' seems to be rotated correctly, just not bound within<br>> the correct 'footprint' shape.<br>...<br>> I am attaching a teeny file comparing the final image loaded into global mapper<br>> and OpenEv to show more clearly what I am seeing with the results.<br><br>Jeff,<br><br>Based on the image you attached I think you want more than a first order<br>polynomial transformation. You want, essentially, piecewise warping.<br>Four GCPs are only enough to give rotation and shear in a first order<br>polynomial. But if you use thin plate splines instead with the GCPs you<br>should get a more general transformation.<br><br>Try adding "-tps" to the gdalwarp commandline arguments.<br><br>Best regards,<br>-- <br>---------------------------------------+--------------------------------------<br>I set the clouds in motion - turn up | Frank Warmerdam, warmerdam@pobox.com<br>light and sound - activate the windows | http://pobox.com/~warmerdam<br>and watch the world go round - Rush | Geospatial Programmer for Rent<br><br></font>