ABC Watches Explained: Altimeter, Barometer, Compass

ABC stands for altimeter, barometer, compass — the three sensors that turn a watch into a hiking instrument. The important thing most buyers do not realise is that the first two are the same sensor interpreted two different ways, which explains most of the confusion about accuracy.

1. 1

   The altimeter and barometer are one sensor

   Both read air pressure. Pressure falls predictably with altitude, so the watch converts pressure to elevation. But pressure also changes with weather, so the watch cannot tell a 300-foot climb from an approaching storm. This is why barometric altimeters drift and need calibrating, and it is not a defect.

   Calibrating your altimeterHow and when to reset it

   Read Guide →
2. 2

   What the altimeter is genuinely good for

   Confirming your position against a topographic map. If the map says the trail junction is at 3,200 feet and your watch reads 3,180, you are there. Elevation is the second coordinate that turns a rough position into a confident one, and that is far more useful than total ascent numbers.

3. 3

   What the barometer is genuinely good for

   Trend, not absolute value. A steady fall of a few millibars over three hours means weather is arriving, often before the sky shows it. On an exposed ridge that is actionable. Most ABC watches show a pressure graph for exactly this reason.

4. 4

   What the compass does and does not do

   A digital compass gives you a heading and needs calibrating by rotating the watch through a figure-eight. It is good for general orientation and worse than a baseplate compass for taking precise bearings off a map, because you cannot lay a watch on a map and sight along it.

5. 5

   When ABC beats GPS

   Sensors draw almost no power, so an ABC watch runs on solar or a multi-year battery. That means you get elevation, weather trend, and heading for an entire month-long trip with no charging — which a GPS watch cannot do. The trade is that ABC tells you your altitude and heading, not your position.

What Each Sensor Gives You