Start the tablet calibration wizard by clicking the Draw | Tablet | Tablet Calibration command or clicking the 
button.
Click the Next button four times.
Click the Finish button in the last dialog to accept the existing calibration.
When an existing Didger project .PJT file is opened, the tablet calibration must be reestablished before digitizing additional objects. The tablet cannot be activated for digitizing if it has not first been calibrated.
If you are sure the paper document has not moved on the digitizing tablet, you can follow these steps after opening the project file with File | Open.
Start the tablet calibration wizard by clicking the Draw | Tablet | Tablet Calibration command or clicking the button.
Click the Next button four times.
Click the Finish button in the last dialog to accept the existing calibration.
When you are working on a project, it is possible to inadvertently move the paper document on the tablet. When this occurs, the current calibration is no longer valid and must be reestablished. You can easily reestablish the calibration in Didger.
If the paper document has moved on the digitizing tablet, or if you are not sure if it has moved, you can follow these steps.
Start the tablet calibration wizard by clicking the Draw | Tablet | Tablet Calibration command or clicking the button.
Click Next in the Tablet Calibration Wizard dialog.
In the Create Calibration Points dialog, click on the first record in the calibration points table.
Locate the currently highlighted calibration coordinate on the digitizing tablet and click on that point using the tablet puck. The new tablet coordinates for that point are entered in the appropriate fields and the next record in the Create Calibration Points dialog is automatically selected.
Repeat step 4 for each of the calibration points the calibration points table until all of the calibration points have been updated.
Click Next to move to the RMS Calibration Settings dialog and verify that the RMS error is acceptable.
If the RMS error is not acceptable, click the Back button and repeat steps four and five. If the RMS error is acceptable, click Next twice.
Click Finish and continue with the digitizing project.
See Also
Digitizing Accuracy and Acceptable Error - RMS Values
Introduction to Tablet Calibration