Parent Topic: Polynomial Transformations
Determining the Accuracy of the Polynomial Fit
In order to determine the accuracy of the derived coefficients of the
GCPs, examine the results of the least squares regression of the initial
GCPs:
- The GCP scatter plot report, which can be activated by selecting
``GCP Scatter Plot'' from the ``Reports'' menu on the GCP Selection and
Editing panel, shows the X and Y residual errors for each GCP point on a
cross hair graph.
- The RMS error for each GCP is reported in the lists of Accepted and
Check GCPs on the ``GCP Selection and Editing'' panel. A total
RMS error is also reported on this panel. The RMS error is calculated
in pixel units by the following equation:
By computing the RMS errors for all of the GCPs, it is possible to see
which GCPs exhibit the greatest error, and to sum the RMS errors. If
a given set of control points produces a total RMS error which exceeds
your acceptable limit (a limit of less than the dimension of one pixel
is suggested), you should consider deleting those GCPs which have the
greatest errors.
The Image Fit Report is also available to help you assess the fit of
the regression model. This report can be activated by selecting
``Image Fit'' from the ``Reports'' menu on the GCP Selection and
Editing panel. It shows a graphical representation of the outline
of the georeferenced image area, uncorrected image area, mosaic cut line,
and GCP points. The uncorrected image outline is transformed according
to the current GCP model. This preview of how the uncorrected image
would map onto the georeferenced data set with the current set of GCPs allows
the registration to be visually assessed before it is performed.
See Also: Image Fit Report, GCP Scatter Plot Report
Parent Topic: Polynomial Transformations
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