An altimeter is a tool that measures altitude above a reference point. It works by detecting small changes in atmospheric pressure, and bushwalkers can use a hand-held or wrist-mounted device.

Knowing altitude can help to verify the user’s location, particularly on long uphill sections. When used conjunction with a topographic map, a barometric altimeter is often more accurate than a GPS unit, which is susceptible to failure from satellites dropping in an out of availability and being wildly inaccurate when all available satellites are near the horizon.
Since barometric pressure is affected by weather and changes in weather, bushwalkers need to recalibrate the device periodically at a known location (e.g. trail junction, hill).