Larsen & Brusgaard PROTRACK II Manual page 19

Press or to set an altitude offset to match the altitude of the aircraft takeoff
relative to the Landing Zone.
Examples:
1) If the aircraft takeoff is 2000 ft. lower than the Landing Zone, the altitude offset must
be set to "-2000 ft."
If the aircraft takeoff is 1500 ft. higher than the Landing Zone, the altitude offset
must be set to "1500 ft."
2) If the PROTRACK II is switched OFF during climb to altitude. Switch the PROTRACK II ON
and adjust the altitude to the same altitude as shown on the airplane altimeter (AGL).
3) When doing a B.A.S.E. Jump (> 6 sec. freefall)
At the exit point enter the present altitude above the landing area.
4) When climbing to exit altitude in an airplane with a pressurized cabin activated.
When cabin pressure has been released, program the unit to the present altitude or
to a compensated altitude.
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WARNING
Be cautious when operating the Altitude Offset.
If programmed to an altitude different from the present altitude, the pre-selected
warning alarms will sound relative to the new programmed altitude.
NOTE: DO NOT PROGRAM ALTITUDE OFFSET LESS THAN 1 MINUTE PRIOR TO EXIT.
NOTE: When performing manual altitude offset the PROTRACK II enters
Flight Mode and stays in Flight Mode for 14 hours, if no jump is made.
After 14 hours the PROTRACK II recalibrates to the field elevation where it is currently
located and enters Ground Mode. The altitude offset is not retained when the PROTRACK
II is powered OFF.
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