[FWTools] gdalwarp issue?

Thu Dec 16 11:26:49 EST 2010

Hi Brent,

That's a really good suggestion, especially as the perspective angle gets more extreme.   

I dont think I can assume low altitude, so I think I will still need to attempt to correct for curvature of the earth, but I'm wonder if I can get away with doing an initial perspective transformation (ie warp it to a 'top down' perspective), then follow up with a second order polynomial gdalwarp for curvature?  

Cheers!

-----Original Message-----
From: "Brent Fraser" <bfraser at geoanalytic.com>
Sent: Thursday, December 16, 2010 11:47am
To: jkenneally at cartenav.com
Cc: "Frank Warmerdam" <warmerdam at pobox.com>, fwtools at lists.maptools.org
Subject: Re: [FWTools] gdalwarp issue?

            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] 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, [mailto:jkenneally at cartenav.com] 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" [mailto:warmerdam at pobox.com] <warmerdam at pobox.com>
        Sent: Wednesday, December 15, 2010 6:55pm
        To: [mailto:jkenneally at cartenav.com] jkenneally at cartenav.com
        Cc: [mailto:fwtools at lists.maptools.org] fwtools at lists.maptools.org
        Subject: Re: [FWTools] gdalwarp issue?

        On 10-12-15 01:49 PM, [mailto:jkenneally at cartenav.com] 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,        [mailto:warmerdam at pobox.com] warmerdam at pobox.com
        light and sound - activate the windows |        [http://pobox.com/%7Ewarmerdam] http://pobox.com/~warmerdam
        and watch the world go round - Rush | Geospatial Programmer for        Rent

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