[GRASS-user] Forward and backward RMS error.

[Markus Metz]

On Wed, Feb 27, 2013 at 3:57 PM, Aldo Clerici <aldo.clerici at unipr.it> wrote:
> Dear GRASS users and developers,
>
>
>
> where can I find an explanation about the meaning of forward and backward
>
> RMS errors in i.points? In the 6.4.3 and 6.5 manuals for i.points a unique
> value for RMS error is reported, but the two values are actually computed.
>
> In 7.0 manual the two values are mentioned but no description of their
> meaning is reported.
>
> How can the two values be used to evaluate accuracy in georectification?
> Which is used to rectify the map?

i.points, i.rectify, etc. calculate transformation equations from
ground control points (GCPs). RMS (root mean square) errors are then
determined by transforming each GCP and calculating the root mean
square from the deviations of the transformed coordinates from the GCP
coordinates. Since you can transform coordinates from source to target
(forward) and from target to source (backward), you can also calculate
two RMS errors. The forward RMS error is probably of more interest and
ideally smaller than the target resolution (sub-pixel transformation
accuracy).

HTH,

Markus M

>
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> Many thanks in advance.
>
>
>
>
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> Aldo Clerici
>
>
>
> Earth Sciences Dep.
>
>
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> Parma University
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>
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