2011

Table Of Contents

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 type:

■ 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.