[FWTools] gdalwarp issue?

[Frank Warmerdam]

On 10-12-16 10:03 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'.

Jeff,

To be honest, I don't fully understand all the properties of thin plate
spline warps, but I'm not surprised by the curvy effects.

> 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?

Yes, providing six distributed GCPs should be sufficient to develop a second
order polynomial which should model the warp you want to do better.  I would
advise against going to higher order polynomials unnecessarily.  They can
quickly get weird.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent