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Depending on the number of points specified, you have a choice of transformation types to use, along with the type recommended for progeCAD.
Transformation refers to the calculation of the points on the screen that correspond to points you digitize on the tablet.
Orthogonal Preserves all angles between lines, preserves all relative distances, and, in general, preserves shapes. If you specify only two points, an orthogonal transformation is automatically generated. The orthogonal transformation type yields the most precise mapping between the digitized points on the tablet and the corresponding points on the screen.
Affine Preserves parallel lines, but not necessarily the angles between intersecting lines. If you specified three points, progeCAD can no longer represent this mapping as an exact orthogonal transformation. Therefore, you have a choice of an exact affine transformation or a "best fit" orthogonal transformation.
Projective Does not preserve parallel lines or angles. If you specify exactly four points, you have a choice of an exact projective transformation or a "best fit" orthogonal or affine transformation.
Number of points specified  Transformation type recommended  "Best fit" (approximate) 

2  Orthogonal  None 
3  Affine  Orthogonal 
4  Projective  Orthogonal, Affine 
510  None  Orthogonal, Affine 
Choose the recommended type unless you know it will not be appropriate for what you are digitizing. The most appropriate type is not always the one with the least error; for example, you might digitize three points and select the orthogonal transformation, even though the affine transformation would yield a closer representation of your calibration entries.
 
Digitizing by selecting points ( 1 , 2 , 3 , and 4 ) on the tablet. 
 Corresponding screen coordinates:

 
Screen result of orthogonal transformation after tracing polyline on tablet. 
 Screen result of affine transformation 
 Screen result of projective transformation after tracing polyline on tablet. 
Specify a point within the progeCAD drawing window to correspond to the point you digitized on the tablet, or enter coordinate values in the command bar. To specify more than the two points required, click a point on the tablet to define as the third calibration point. You can enter up to 10 points.