[Qgis-user] Fwd: Georeferencer Plugin Help - QGIS

[Greg Troxel]

Alexandre Neto <senhor.neto at gmail.com> writes:

> So, I think you need to sum the squares of each GCP residual, divide it by
> the number of GCP, and then get the Square root of the result.

That makes sense, but there's a deeper question about what the OP is
trying to calculate.  Beware that the following details are out on a
limb:

There are 4 degrees of freedom for georeferencing if one assumes
constant scale of the source image, basically an x/y point, a scale, and
a rotation.  So with 2 GCPs you will get zero residuals.  But that
doesn't mean your image is perfectly aligned.

If you are trying to estimate the RMS error for points that aren't your
GCPs -- which I think you might be, then you might want to subtract from
the GCP number to account for errors absorbed by estimated parameters.

Basically, dividing sum of squares by GCP-2, for four estimated
parameters.

If you have lot of GCPs, this starts to not really matter.  If you
really care about characterizing the error, I'd suggest having at least
10 and maybe 20, without any real basis for those numbers.


Here's an example of language about RMS (search in page)

  https://docs.digital.mass.gov/dataset/massgis-data-usgs-color-ortho-imagery-2019

Note that this is "we checked 132 points", not "we used 132 points to
align", so there one should divide by the GCP number - they really are
measuring errors in the dataset at a number of points, not looking at
residuals.


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