Re: [lsm] mounatineering wristwatches...
Short version About the accuracy, one can easily mistake the accuracy of the
display (often referred to as 'Altimeter resolution') as the accuracy of the
altitude estimate, at least I did... Actually, the amount of meters the estimated altitude is off, is not just a single number, but depends on air temperature and altitude.
I have the Suunto Observer, and as an example of it's altitude estimate accuracy: With 0 degrees celsius at sealevel and at 100m (I live in Finland…

) the Observer's estimated altitude would be within about 102 and 110m (or equivalently about 106 +/-4m, see formula below). The accuracy of the
display is 1m though (so the display shows values at 1m steps (...,102,103,104,…))
Longer version About a year ago I was checking a few alternatives for an altimeter watch, and I got in contact with the Suunto tech support, and here's what I learned:
1. 'Altimeter resolution' is not the same as the accuracy of the altitude estimate. 'Altimeter resolution' usually refers to the display. One of the worst examples I've seen is 'Altimeter resolution 1m' whereas the altitude accuracy was later admitted to be +/-50m, which of course was not mentioned on their web site…, and it doesn't make sense anyway, see 2. (don't remember the brand)
2. Air pressure is measured, and altitude is calculated/estimated based on the air pressure. The accuracy of the air pressure measurement in the Observer is in effect about +/-0.5hPa. Since the air gets thinner as you go higher, the absolute error gets bigger in higher altitudes. As already mentioned, the accuracy is also temperature dependent. The Observer actually assumes a certain temperature at a certain altitude, so if the temp is not what it thinks it is, the estimate is off. In general, for the Observer, the altitude estimation error can be calculated like this:
-0.002*(temperature deviation)*(altitude difference) +/- the effect of the air pressure measurement error (which at see level is about +/-4m)
The temperature dependency is explained in the Observer manual (page 24):
http://www.suunto.com/...r_manual_en_e5d3.pdf (1.7MB)
So, the more the temperature deviates from what he Observer assumes and the higher the altitude, the more meters the estimate is off. (20 degrees off, 2000m => 80m off)
The +/-4m comes from Suunto support. Actually, they said that the typical accuracy (excluding effects of temperature deviation and changes in air pressure due to weather changes) is 'within 4m' at sealevel. Since English is not my native language, I'm not sure if this means +/-4m or does it mean +/-2m???
<pimp_my_ride>"I'm very happy with my Observer, and thanks to Suunto, people now respect me for what I have and not for who I am" </pimp_my_ride>
Anyway, I think this kind of accuracy is representative for any high end altimeter manufacturer.
Bernt