Residuals

## Residuals

The residuals and, if you are using 3D coordinates, the Z deviation values in the
point list window
are your key to the reliability of your rectified image. Large values will result in an
unreliable rectification.
No residuals can be calculated from a rectification that uses only 4 points (the minimum
number) so for confirmation of your measurements you should have at least 5 points for
each image to be rectified, and more is better.

Perfect measurements on a perfect plane with perfect point placement in images
of infinite resolution will show zeros in all columns but in the real world close will have
to do.

If your measurements are bad all you can do is eliminate points or re-measure but if
your measurements are good, high residual values will probably be the result of poor
point placement in the source image.

In this case select the point with the largest residual value and move that point in the
direction opposite the value (for +Y move the point down, for -X move it to the
right, etc.) then rectify again
**(F5)**
to see the changes.
Once all the values are about equal the results will be as good as it is going to get
with the points you have.

The residuals are presented in real world units so the maximum error in the rectified
image should be the same as the largest residual.

For a discussion of the Z deviation see the section on
3D coordinates.

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Inquiries to:
steve@icomos.org

Generated by: CART

(Thu Sep 20 17:56:23 2007
)