Altimeter drift – how much can this be??

An error of 70 m is nothing. It's very common for atmospheric pressure to change in a given spot by 30 mm of mercury (1.2 in Hg) over a period of a few days (more if the weather is extreme). This is equivalent to a change of about 330 m (1100 ft) in altitude. It's very important to adjust your altimeter every time you're at a known altitude—multiple times a day, if possible, especially if it's windy or the weather is changing.

Many altimeters are also subject to significant errors due to changing temperature.

The most I've personally noticed with my Casio Pathfinder watch (PAW1300) is 110m overnight. If I haven't calibrated my altimeter, I typically consider it accurate to +/- 100M.

Usually if a significant change in air pressure is occurring, you're going to notice it because the weather will be highly changing too. On a nice blue sunny day, the pressure might fluctuate and throw your altimeter by 10 or 20m without you perceiving a change in the weather. But anytime the pressure has changed by over 30-50m, there's been a pretty obvious change in the weather occurring as well in my experience. So if the weather is staying constant while you hike, I wouldn't bother calibrating it more than once or twice a day. It all depends on how much you're relying on your altimeter though. I'm typically using mine more for interest sake and to know how much vert I've climbed, than as an actual navigational tool…..although I was trying to navigate in a white out while ski touring 2 days ago and at that time I was wishing I'd bothered to calibrate it.

Thanks guys – now i at least know im not crazy or stupid…
I know that "i wish i had calibrated feeling"

Mike

Why use barometric altimeter when most gps have this feature?

My garmin foretrex will give highly accurate altitude readings, and specify its own accuracy within x feet, usually less then 10. granted its a digital device reliant on batteries, and satelite signal but its more accurate then any wristwatch altimeters I've encountered.

If you're somewhere that knowing your altitude at all times could mean life or death, I see the attraction to a barometric model but many of us are already carrying a gps device that can do better.

I concur with other comments: You needn't calibrate until your next trip. It is common for altimeters to vary by a 100 meters, sometimes more, due to changing atmospheric pressure.

You can minimize that if you track it each day. Get to camp – note your elevation. Reset it the next morning to whatever it showed last night to minimize the cummulative error. Also note if you "went uphill" during the night. My mnemonic for it is:

"Going uphill for free is bad." by which I mean if your elevation reading increases without you moving uphill, the pressure has fallen and you'll probably have worse weather.

Conversely, the elevation reading decreasing overnights means higher pressure and likely better, sunny weather.

If you can get a radio signal (or surf the web on your smart phone) and get a local weather report giving a corrected air pressure (they are corrected to sea level) or weather data for an airport (they ALWAYS give air pressure data), you can correct your altimeter to actual atmospheric pressure and improve its accuracy.

I should check out GPSs again. 25 years ago, they weren't great on the vertical.

The combination of GPS and an altimeter gives you some weather forecasting ability and some back up.

Still using my mechanical Thommen – no battery, no satelites, no radio signals, no subscription fee.

I often use altitude to locate my position on a ridge; to know how much more climbing I need to do or help locate the top of a side spur I need to descend. I don't want to run my gps all the time or leave it dangling around my neck so I use a Suunto Vector watch which I can glance at its altitude readout whenever I need to. I do calibrate the watch at the bottom/top of the ridge.

"I should check out GPSs again. 25 years ago, they weren't great on the vertical."

GPS wasn't generally available to civilians 25 years ago. That could be a reason.

Due to the geometries of satellites and GPS receivers, the best case vertical error will be 1.5 times the horizontal error. However, if you are in a deep canyon or a similar bad place, the vertical error can run off the charts to the point where you can't even get a position fix.

–B.G.–

>"GPS wasn't generally available to civilians 25 years ago."

My bad. A typo. 1995-ish. 15 years ago.

3 ways to tell your elevation with a GPS with dead batteries:

1) Drop it onto a surface of known elevation, timing its descent. D = g * t^2 / 2 at first. D = V(terminal) x time at the end, with a complicated curve in between.

2) Mark on a cliff the height of your 4 inch GPS unit. Repeat. Count the marks.

3) Offer a hiker with a mechanical altimeter, "I'll give you this nice GPS unit – it only needs new batteries – if you tell me what elevation we're at."