# Prism constant

When you select a Trimble prism in the Prism type field of the Target properties screen, the prism constant is automatically defined. If you select Custom in the Prism type field, you must manually enter the prism constant.

Selecting the correct prism type and entering the correct prism constant ensures that the appropriate correction values are applied to the slope distance and vertical angle for geocentric offset and prism constant. The correction is only significant when observing step vertical angles.

Enter the Prism constant in millimeters (mm). Enter a negative value if the prism constant is to be subtracted from measured distances.

When using a Trimble total station, all corrections are applied in the Trimble Access software.

For some third-party instruments, the Trimble Access software checks to see if a prism constant has been applied by the instrument and the software. When you select Station setup, status line messages show what has or has not been checked.

If the software cannot check the setting on the conventional instrument, but:

- If there is a prism constant set on the instrument, make sure that the prism constant in the software is set to 0.000.
- If there is a prism constant set in the software, make sure that the prism constant in the instrument is set to 0.000.

If required, you can edit prism constant records for observations already stored in the job using Review job or Point manager.