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screenshot from QGIS

1) Is anyone able to explain in simple manner what does mean error exactly mean? I use metric values, so suppose 3.64109 is expression of meters and "e" is mathematical constant: http://en.wikipedia.org/wiki/E_%28mathematical_constant%29 but what's next?

2) Why does image become more brighter after TPS transformation?

I work on archival aerial imagery and I'm testing all of the transformation types and re-sampling methods to find the best one. As you can anticipate, I'm not mathematician and I'd like to understand what's result of my work in fact.

  • 3.64109e-08 is 3.64109*10^-08, i.e. 0.0000000364109. – AndreJ Mar 19 '15 at 12:41
  • TPS aims at warping so that each ground control point stays where it has placed. In ideal case that would mean zero residuals always. Thus this is not a good measure for TPS. – user30184 Mar 19 '15 at 12:49
  • Thanks for answers. So, shouldn't I look for metric expression of residual using TPS? Or 0.0000000364109 is the answer, then what does it express? Meters or any angles? – Adam Mar 19 '15 at 13:11
  • If your data is in meters, the error will be too, unless it is scaled to the map extent. – AndreJ Mar 19 '15 at 13:13
  • Okay, so this transformation type generates very small errors (in comparison with polynomial basing on the same CPs, for example). – Adam Mar 19 '15 at 13:50
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Thin plate spline works perfectly for all GCP you have entered, but possibly not for the rest of the image.

If you use polynomial interpolation, you get residuals for all GCP points.

If one or several of your GCP have a low accuracy, the polynomial interpolation can compensate that, leaving the image as a whole with less distortion.

With TPS and one GCP offset, you get a nasty distortion around it, with no way to eliminate that than deleting the GCP. See this question for an example: Georeferencing an old map in QGIS using Thin Plate Spline

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