What an ABC watch is, what the three sensors actually do, and why one can outlast every GPS watch you will ever own

A, B and C

• Altimeter: your elevation, derived from air pressure against a reference. Casio's Rangeman manual describes readings taken from a pressure sensor and converted to altitude, with a reference altitude you can set.
• Barometer: the air pressure itself, and its trend over hours. A falling trend is the oldest weather forecast there is.
• Compass: magnetic north from a bearing sensor. Casio's manuals describe a bearing sensor calibration and a magnetic declination correction so the watch can show true north; Suunto's Core Alpha page lists a compass with adjustable declination.

Most ABC watches add a thermometer (Casio's "Triple Sensor" is pressure, direction and temperature), sunrise and sunset times, and on the radio models, atomic time calibration.

Three readings from two sensors

Calibrating, from the manuals

1. Altitude: at a trailhead sign, a summit marker or a known contour, set the reference altitude. Casio's manual describes this for the Rangeman; do it again at the next known point on a long day.
2. Compass: run the bearing sensor calibration away from metal, set your local magnetic declination, and keep the watch level when reading. Casio's manuals list all three.
3. Temperature: take the watch off the wrist for a few minutes; the sensor reads the case, which reads you.
4. Sunrise and sunset: on a Casio, set the home city and, where the manual allows, your latitude and longitude; the default is Tokyo, and the times are only as good as the location you gave it.

What an ABC watch cannot do

It cannot tell you where you are. It has no map, no breadcrumb track, no waypoints, no return-to-start. It gives you a bearing and an elevation, and you supply the map. For a hunter who knows the ground and wants a watch that is never dead, that is the right trade; for a hiker on new ground who wants to be walked back to the truck, it is the wrong one. The decision for hunters is worked through on GPS vs ABC watch for hunting.

Why it lasts longer

A GPS chip and a screen are what drain a smartwatch; Garmin rates the Instinct 3 Solar at 40 days in smartwatch mode and hours with GPS on. Casio rates the Rangeman GW-9400, with all three sensors, at about 8 months in the dark on Tough Solar, and indefinitely in light; the Pro Trek PRW-61 at about 6 months; Suunto gives the Core Alpha a replaceable CR2032. The sensors are not what drain a watch; the satellite receiver is. Remove it and the power clock slows by an order of magnitude, and with no firmware to update, the software clock stops. That is why the buy-it-for-life winner is an ABC watch.

The picks

ABC or GPS

• ABC if you carry a map, know the ground, and want a watch that is never charging: Rangeman, PRW-61, Core Alpha.
• GPS if you need a track back or a map on the wrist: Instinct 3 Solar keeps the cell clock slowest; the fenix 8 adds full maps.
• Both is a common answer among the hunters this site is written for: an ABC watch on the wrist and a phone or handheld for the map.

The ranking is best ABC watches; the G-Shock version is best G-Shock with compass.