  For proper cave surveying using DistoX, the device needs to be calibrated
with adequate accuracy. Calibrating does not require any tools; but, tools to
make calibration easier have been developed. Theoretical consideration shows
that the use of certain tools enables one to introduce a type of calibration
error that goes undetected by the calibration software. In this study, the
existence of such errors is experimentally confirmed and their magnitude is
estimated. It is demonstrated to be crucial that the DistoX is calibrated and
that the calibration is valid, that is, that the device has not changed since
it was last calibrated. No part of the DistoX must have moved or changed its
magnetization since calibration, not even the battery. The calibration method
used and the quality of the resulting calibration are important too. It is
highly recommended that the DistoX be checked immediately before surveying a
cave and thus avoid the possibility of using an uncalibrated, not validly
calibrated, or poorly calibrated device. To complete the check, a few survey
shots are measured multiple times with the device at different roll angles, and
the back shot of one of the shots is measured. If the device is properly
calibrated, the measurements will agree with each other within the acceptable
measurement error. This is not the case for a device that is not properly
calibrated.
