Recently I bought a new multi-sport watch, the forerunner 255. And although there are many nice functions, the altimeter quite surprised me. I thought, the forerunner 255 was a GPS watch with a barometric altimeter. Which I like, because GPS and barometric altimeter have their own use depending on the activity.
The barometric altimeter, which measures pressure and converting that with a model using standard atmospheric data, measurers the relative (pressure) attitude. This is useful for trail runs, hike, etc. in uneven terrain with relatively small variation in height (let’s say below 100m) in stable atmospheric conditions.
However for long hikes in mountain areas with large ascents and descents (1000m+) a GPS, measuring the geometric altitude, would be more accurate and usually measures the height within 20 m. For this larges ascents the error in the pressure altitude relative to the geometric altitude will become large (up to few hundred meters). This is because the models can’t take into account all the local variation is air pressure relative the “standard” atmosphere used in a model. And the further away from your geometrical reference point, the larger the error. (see for example forums.garmin.com/.../altimeter-accuracy-issue)
When a bought the forerunner, my believe was that when I switched the barometric altimeter to barometer only it would use the GPS altitude instead. This might be naïve, but I thought very reasonable, but when I switched to barometer only it doesn’t measure the altitude at all! While the watch is able to. This I found very disappointing.
But maybe I misunderstood al the settings and therefore hope someone still has a solution to switch to GPS. Otherwise I considering to get rid of the watch already after few weeks.
