Deleted
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Post by Deleted on Apr 26, 2016 10:01:09 GMT -8
So I recently bought a small mechanical altimeter. It is currently 1014 hPA (about 0.299 inHg ) where I am. When I dial that into the altimeter, it says that I am at a little over 100 feet. My GPS configured to WGS-84 however, says that I'm at 88 - 112 m (289 - 357 ft).
Government topographical maps say that I should be about 85m (278 ft), which is close enough to the GPS values (I'm writing this from the second story of my house, so there is bound to be a bit of error there).
I'm curious why there is so much of an error between my handheld altimeter and those other two sources. I feel like there is some math that I'm missing, I'm just not sure what. I wrote to Sun (the manufacturer) about this as well. They didn't post any accuracy/error rates with their device, but a 20+/- ft error rate is pretty extreme.
Thoughts?
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Post by Deleted on Apr 26, 2016 11:03:48 GMT -8
I have not used a mechanical alltimeter before. The hysteristic properties of the device could give errors.
Also, is the device handheled or worn on the wrist? Our altimeter has an internal heater but when worn against the body the heater gets out of wack. This makes the reading to be a bit off.
My wife wears a (cheap) altimeter around her neck, it can be up to 200 feet off as we go trapising about the hills. After we make camp and settle in for a few hours, the altimeter settles in to being closer to the correct altitude (based on topo maps).
We adjust our altimeter according to the altitude of the local firestation. Ask the station what the altltude is, they should have it recorded and then find the benchmark, place the meter on the benchmark and adjust the meter to the altitude.
Also, inside your residence, the HVAC cycles can change the airpressue of the inside of the residence, which will change the altitude.
A +/- 100 foot error rate is reasonable. We just look at the map and know we cannot be 'there'.
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Post by High Sierra Fan on Apr 26, 2016 14:24:19 GMT -8
What's the source of that atmospheric pressure you dialed in?
Analog altimeters depend on that number. I go in the other direction when calibrating mine (a Thommens I've used for a long time): I dial in my position's known elevation, benchmark or specific position datum and go from there. On the trail I'll have marked my map with some waypoint elevations for easily identified points: bridges, trailheads, passes, to check/recalibrate as that keeps my altimeter current to the ambient pressure as time goes by.
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gabby
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Post by gabby on Apr 26, 2016 14:35:40 GMT -8
I dial in my position's known elevation, benchmark or specific position datum and go from there. On the trail I'll have marked my map with some waypoint elevations for easily identified points: bridges, trailheads, passes, to check/recalibrate as that keeps my altimeter current to the ambient pressure as time goes by. And that is the story on that. I have an old analog altimeter/barometer, and that is the way I was told you have to go. Waypoints along a trail with known elevations are used to calibrate the altimeter. If the weather is clearly changing, these recalibration points may need to be somewhat more often than otherwise, however, given that altitude is one way to verify your location along a trail ( between known elevations), it's good to pay attention to maintaining the relationship at all times, since air pressure is always changing to a greater or lesser degree. |
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Post by Lonewolf on Apr 26, 2016 16:29:09 GMT -8
That had better be 29.9"hg or you're in the upper atmosphere, nearing a vacuum. All mechanical altimeters work off barometric pressure. The displayed altitude will depend on current weather conditions and must be set/calibrated for current conditions and will only be accurate until the weather changes. |
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BigLoad
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Post by BigLoad on Apr 26, 2016 19:03:11 GMT -8
What's the source of that atmospheric pressure you dialed in? Analog altimeters depend on that number. I go in the other direction when calibrating mine (a Thommens I've used for a long time): I dial in my position's known elevation, benchmark or specific position datum and go from there. On the trail I'll have marked my map with some waypoint elevations for easily identified points: bridges, trailheads, passes, to check/recalibrate as that keeps my altimeter current to the ambient pressure as time goes by. And when you stop for the day, write down the elevation and reset to that value in the morning before setting out. |
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Post by Deleted on Apr 27, 2016 8:56:49 GMT -8
Wow, this is really great advice guys... thanks!
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