What The Hell is an Altimeter For Anyway?

I’m taking my GPS, and the spare batteries, out of my pack.  I only use it these days to practice finding my location on a map using UTM coordinates.

These days it seems I’m only using my wrist watch altimeter.  An altimeter measures your elevation, in relation to sea level.

I carry a topo map, and normally, ooh I’d say, maybe, 100% of the time, I stay on the trail.

I have a map, I’m at the trail head, I locate the trail on the map, where I’m standing at the moment…and I head out on the trail.  Later on, the only question remaining is, where am I now on the trail?

To find my location, I take a reading on the altimeter, it says, say, 1200′.  I then look at my map, find the trail, then find where the 1200′ contour line, on my topo map, crosses the trail.  I should be approximately where that contour line crosses the trail.  Pretty simple.

There are few more rules though.  An altimeter measures relative air pressure to indicate elevation, much like a barometer.  Air pressure changes due to ‘weather’.  I need to re-set my altimeter at each known location, to make sure it’s adjusted properly for changes in air pressure.  Example, at the trail head, I check my map and determine the elevation.  I then check my altimeter to see if they match.  If the map says the trail head is 1500′, and my altimeter reads 1200 I re-set my altimeter to read 1500, the elevation of my ‘known location’.

I do this while on the hike as well at other known locations I can pin point on the map, say a trail junction, a stream crossing, a ridge, or a saddle.

Since I’ve been using the altimeter technique, I haven’t pulled out my compass in years.  I did practice with my compass skill in the Colorado Weminuche Wilderness, up on the Continental Divide Trail, with Geardog and Herc.

One more footnote or word of caution, the best topos (topographical maps) are the 7.5 min quad maps available for free download from the USGS web site, however, I’ve learned the hard way that the trail locations are old and not up to date, and the AT has apparently been relocated over the years in many areas.  The most up to date maps, or ‘more’ up to date are the trail maps from the National Geographic Society.  The scales are larger, this means that everything is smaller, so the contours are a little harder to read, but at least the trails are pretty accurate.

An altimeter bonus is using your altimeter in a barometer mode to predict changes in the weather.

As you ascend, go higher, air pressure decreases, and your altimeter reads higher – BUT – if you’re not going up, and you altimeter thinks you are going up, i.e. the air pressure is decreasing, or – barometer is dropping, this could be a signal of  bad weather (rain) moving in.  Conversely, a rising barometer (dropping altimeter) could mean better weather.  Confused, we’ll, that’s for another time.

Later, kiddoe’s!  This uncle professor phil, signing off.

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