The reason you need to calibrate you altimeter every day is that the altimeter actually works on air pressure changes.
Air pressure changes every day which, of course, affects your altimeter.
If you are in rapidly changing weather conditions, it is important to watch your altimeter for weather related changes that don't reflect actual changes in your altitude.
So, let's say you are mountain climbing for several days. When you start, you calibrate your altimeter at a known altitude, climb (or desend), and keep notes of your altitude. That night, when you stop climbing, you have to make a note of your altitude before you hit the sack.
The next morning, you may notice a change in the reading on your altimeter due to atmospheric pressure changes. At that time, you recalibrate your altimeter and head out.
Bear in mind that in the mountains, there is usually a change in atmospheric pressure from early morning to late afternoon as the temperatures change in the valley below and push warmer air upwards.
If you forget to keep track of your altitude and forget to calibrate everyday, you will eventually, over the course of several days, only be able to estimate your elevations. Once you start guestimating, you start losing confidence in your equipment and then you can get into trouble (like getting lost BUT GOOD).
By the way, you can get altitude information from at least two traditional sources:
One is to look for altitude postings on signs at the location. This is common in mountainous areas.
Second is to use topographic maps and locate a known location from which you can get the altitude information such as a lake or point. That is, you may be at "White Eagle Lake" and you can see from your topo map that White Eagle Lake is at 3200 meters. That would be a good time to calibrate your altimeter.
Some atlas books have a section in them with the altitudes of various cities in the world that may be so-so useful.
It will be interesting to see it there is any related web-site as well.
Last edited by mike; 05-14-01 at 11:52 AM.