the process of making a map's coordinate system correspond to the
Cartesian coordinate system used by AutoCAD Map 3D.
You select specific calibration
and control points for each map that you digitize.
The crosses in this
illustration mark the digitizer calibration points.
Choosing the Number of
Selecting a maximum of
nine points is a good rule; specifying more only increases computation
time with little increase in precision. If the original is not badly
distorted, four corner points and some intermediate points are sufficient.
It is more important to distribute the points evenly around the
whole map than to concentrate more points in an area.
To complete the calibration,
select one of the following transformation options:
- Orthogonal — Needs
only two digitized and specified calibration points to create a
transformation consisting of arbitrary translation, uniform scaling,
and rotation. This method is not recommended unless only two control
points can be found.
- Affine — Requires
three calibration points. Allows a tablet transformation combining
translation, independent X and Y scaling, rotation, and some skewing.
Provides an arbitrary linear transformation in two-dimensional space.
This is the preferred method for registering paper maps drawn in
a Cartesian coordinate system.
- Projective — Needs
at least four calibration points. Makes a transformation equivalent
to a perspective projection of any plane onto another plane. Allows
for some stretching of an original paper drawing by different amounts
along a perspective projection. This is the best option for orthophotos.
The TABLET command calculates
the errors associated with the available transformation options,
depending on how many points you digitized. When the calculations
are complete, or when you cancel the calculations, a table showing
the results of your calibration appears.
There are four possible
responses from the calibration calculations for each transformation
- Exact — Number
of points specified is correct for this type of transformation.
- Success — More
points specified than needed. For a Success, the RMS error and standard
deviation are reported.
- Impossible — Not
enough points specified to define a transformation.
- Failure — Points
specified are colinear or coincidental. The transformation may not
have a valid definition.
If you get either Impossible or Failure,
do not use that transformation. If all the results are Impossible or Failure,
check the calibration on the digitizer, recalibrate, and digitize
the map again.
RMS error, Standard Deviation,
and Residual Values
The RMS error indicates
how uniform or extreme the required transformation is: the lower
the value, the better the calibration.
Standard deviation is
reported at all calibration points. The residual error is the difference
between the actual calibration point and the computed transformation
point. A large difference indicates a procedural error. You may have
entered the wrong coordinates or digitized a wrong point.
If the RMS and standard
deviation values seem high, you might have made an error when entering
location values or the map might have slipped during calibration.
In this case, restart the procedure to calibrate the digitizer.