What is an Altimeter ?

Right back to basics, an altimeter is a device which measures changes in height or as the pro's say - altitude. The simplest way of measuring altitude is to measure the way the pressure of the air around us changes as we ascend or descend, the air gets thinner the higher we go up. Unfortunately this change is very slight (unless you have a jet !) and you need an extremely accurate instrument to measure the change in air pressure experienced by walkers, or even downhill ski racers. Until recently the only way to do this was with a very sensitive mechanical device, too big, delicate and expensive for the average Joe to carry around. Then electronic gauges were made for aircraft - but these were incredibly expensive, very exotic. And then those electronics whizz kids went and miniaturised them, shrunk them until they fit into an ordinary watch !!! Now everyone can have one.

But air pressure changes with the weather too doesn't it ?

Yes it does, but not enough to make the alti-meter too inaccurate and with a little care you can actually read the changes in the air pressure and get an idea of the weather to come. In other words you can have your own weather forecast on your wrist !

But what will I do with an Altimeter-watch ?

It depends upon what you do with your life, but just imagine how many of todays action sports involve going up... and then coming back down. Imagine knowing how far you've climbed from the bottom of the hill, your total height gain for the day on your mountain bike, how far you've ski'd down a hill - and how long you took. You can go back next week and try do more, faster ! There are other things, mountaineers can pin point where they are - if you are on a knife edge ridge, if you know your height you can read off a contour on the map and fix your position. If you're camping in the wilds, you can watch the air pressure overnight and see if tomorrow is going to bring that glorious day you're hoping for! Whatever you do from cycle touring to parachuting, wouldn't it be good to know just that bit more about where you are and what you've done ?

Ok, I've bought one, how do I use it to measure height ?

The basics are dead simple, if accuracy isn't a matter of life or death or you are just interested in the relative change in altitude (relative change is the difference between A & B, not the actual height of point A or B), then you just read your height or change of height, straight from the watch face - nothing to it. Zero it (or set it to the nearest contour on your map) at the bottom and read off your climb at the top, dead easy and with these great watches, accurate to a metre or two. Even if you don't zero it, the altitude reading will be approximately right for any particular place (say within 10m) and the height gain will be accurate to within a metre or two.

But the spec says it measures height accurate to within one metre ?

Well, what they mean is that it will measure the change in height to within one meter, to measure the absolute height to this accuracy you have to callibrate the meter and because the weather affects the air pressure, to maintain accuracy you'll have to re-callibrate whenever you can throughout the day. This sounds like a chore but it isn't. All you do is, every time you reach a point where you know the height, say at the top of a mountain, a lakeside or any other point where you can read the height off a map, you reset the actual height into the watch. This take a couple of button presses and a few seconds. Even when you need the utmost accuracy, say when going into the cloud at 2,000ft on the Cairngorm Plateau, you only need to do this every few hours at most. I've set mine at the sea's edge and been meter perfect on on the top of Ben More Assynt 4 hours later !

And the weather?

Well, set the watch to barometer mode and check the air pressure right now. If its high, 1020Mbar and above it'll likely be dry. 980Mbar and below and it'll probably be grey and wet. I say probably, because it isn't foolproof, but it's a pretty good rough guide. If you watch the weather over time, say overnight you'll get to see the trend (shown on the little greaph on the watch), this shows whether the pressure is rising falling or maybe just staying the same. Generally, if the pressure is falling, it's getting worse, if it's on the up, the sky will probably be clearing soon. It takes a while to get the knack, but pretty soon you'll have a good idea of what's going on around you. When you see that baro at 980 and plummetting, well maybe you'll put the bike back in the shed and meet your mates down the pub to talk about how good it was last week. If you're a canoeist - maybe this weekends going to have some big water !!!

Yeah, but you said the altitude affects the air pressure and the weather affects the air pressure - how do I know which one's affecting the readings ?

Well now, an advanced user!!! You're right, they do affect each other, and the altitude has much more effect than the weather, so if you're going up and down a bit the weather becomes quite hard filter out, but you can do it. The main thing is to keep calibrating it, if the weather or height is important - keep calibrating. Generally you'll calibrate the watch from a map height, this will ensure the altitude is always accurate. But when you calibrate, stop and think a minute, is the watch reading low, or high. If it's reading low, it's good news, since you last calibrated the air pressure has risen - remember - higher pressure means lower altitude - so the watch reads low. If the altitude is reading high, the opposite applies, the pressure has dropped - it could be bad weather ahead. The amount you have to change the watch by also tells you something - a metre or two means the weather is really pretty stable, 10 metres and you'd better keep an eye out. For a rough guide the pressure changes about 1 mbar for every 8m of height, thus at if you are at 800m and the air pressure is 900mbar, at sea level it is 900+800/8=1000mbar. If you're a real weather nut, you can use the table below to estimate the air pressure at sea level from the altitude and barometer readings from your location. Remember:- this is an estimate, many factors affect the actual air pressure - keep calibrating and keep your eyes open!!! (If you really want to know more check the net, there's some really good geeky weather sites out there).

m mbar ft mbar
0 0 0 0
100 11 200 7
200 22 400 13
300 33 600 20
400 43 800 27
500 54 1000 33
600 65 1200 40
700 76 1400 46
800 87 1600 53
900 98 1800 60
1000 109 2000 66
1100 120 2200 73
1200 130 2400 80
1300 141 2600 86
1400 152 2800 93
1500 163 3000 99

And this ones got a compass too!!!

Some of them do, they're not the best military marching compasses, but they do the job. The pointers tend to be small, but with the backlit display you can see enough to check your course and since it's on your wrist it's always instantly available and never gets in they way of your ice axe. Keep your marching compass handy for real navigational decisions, but check your watch often to stay on track you've set.

The Final Picture

At the end of the day, alti-watches are just another tool, they can be just fun, a serious training aid, or really get your ass out of the deep stuff. If you are going to use it seriously you need to practice using it and keep checking it against all the other inputs you have around you. Always keep it visible, in good weather on your wrist, in bad weather put it on over your jacket - it's waterproof! A good navigator uses all his tools together and thus will spot it if one of them is misleading him. Personally my alti-watches (I'm on my second!) have saved me some serious grief and kept me off bad ground more than once, I wear it every day and watch the weather so I know whether to take the bike to work or not!

I can't say you have to get one, but if you spend time outdoors I think you'll use it, and at todays prices why not just give it a try.

Have fun, but keep safe out there!