2011

The crosses in this illustration mark the digitizer calibration points.
Choosing the Number of Calibration Points
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.
Calibration Options
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
134 | Chapter 2 Setting Up