A barometric altimeter reads air pressure and converts it to elevation. Because weather also changes air pressure, the reading drifts as conditions shift — sometimes by several hundred feet over a day. Calibration is not a repair; it is routine operation.
- 1
Understand the drift
A passing front can change pressure enough to shift an uncalibrated altimeter by 200 to 500 feet over several hours, with you standing still. Stable high pressure produces almost no drift. This is why the same watch seems accurate one weekend and useless the next.
- 2
Calibrate at the trailhead
Nearly every trailhead sign, parking area, or map marks an elevation. Set the watch to that number before you start walking. This single habit fixes the majority of altimeter complaints.
- 3
Recalibrate at known points during the day
Summits, named passes, lake surfaces, and trail junctions are usually marked on topographic maps. Resetting at one or two of these across a long day keeps the reading trustworthy when you actually need it for navigation.
- 4
Recalibrate every morning on multi-day trips
Overnight pressure change is the single biggest source of error. Reset each morning against your known campsite elevation from the map before breaking camp.
ABC sensors explainedWhy altitude and pressure share a sensorRead Guide → - 5
Know when to switch to auto-calibration
GPS watches can correct the barometric altimeter against satellite elevation continuously. That is more convenient and slightly less precise for short-term readings, and it costs battery. On a non-GPS watch like a Rangeman or Pro Trek, manual calibration is the only option.
Calibration Points
| Reference | Reliability | Notes |
|---|---|---|
| Trailhead sign | High | Usually surveyed |
| Topo map summit | High | Check the map datum |
| Lake surface | High | Stable and mapped |
| Trail junction | Medium | Depends on map detail |
| Phone GPS elevation | Medium | GPS altitude is less accurate than position |
| Road pass sign | High | Commonly marked |