I tried a barometric altimeter, seems to work well

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Chip Down
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I tried a barometric altimeter, seems to work well

Post by Chip Down » May 3rd, 2018, 7:03 pm

I use my GPS almost exclusively for elevation. It's miserable. Takes too long to find satelites. Gives wildly variable readings so I have to do a statistical analysis of 20 readings. It's got too many bells and whistles, and isn't user-friendly. It's a battery hog. Maybe there are better GPSs out there, but I decided to get a barometric altimeter. Perhaps you've wondered how well they work.

Here are a few observations on trips where I calibrated my altimeter at the trailhead, generally in the neighborhood of 100'-400':

Hamilton Mountain should be 2400'-2445', depending on source. Altimeter read 2400'.

Augspurger Mountain should be 3667. Altimeter read 3700.

Westway/PCT junction should be 1980. Altimeter read 1960.

Looks pretty good, right? But other measurements were discouraging. Let's face it though, a lot of published elevations are suspect, so it's hard to be sure.

One good measure is this: how far does the altimeter drift through the day? If I calibrate it at the TH, does it show that same elevation when I return? I'm finding it varies about 100-150' from start to end of a fairly long hike (let's say about 10-12 hrs). Of course, that drift will be greater on some days. I suspect the observed drift has nothing to do with the instrument, but reflects real pressure changes.

In conclusion, I do intend to keep using my barometric altimeter. It's fast, simple, and it doesn't vary as wildly as my GPS. Over a short time period, it seems to be remarkably accurate. If I'm on a route with known elevation points, I'll want to remember to calibrate my altimeter along the way.

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texasbb
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Re: I tried a barometric altimeter, seems to work well

Post by texasbb » May 3rd, 2018, 7:06 pm

Yep, yep, yep.

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aiwetir
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Re: I tried a barometric altimeter, seems to work well

Post by aiwetir » May 4th, 2018, 1:50 pm

The lidar digital elevation model gives an elevation of 2412 ft for Hamilton Mtn BTW. I'm not sure what resolution it is, but it seems to be less than 1 foot, so it's within a foot of 2412 if we can trust technology
- Michael

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kepPNW
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Re: I tried a barometric altimeter, seems to work well

Post by kepPNW » May 5th, 2018, 6:31 am

Curious what you're using as a GPS?

Most dedicated units have built-in barometers to dampen the elevation swings from satellite, and keep things pretty much drift-free over the course of a day. Big exception if the unit's exposed to wicked winds, causing the apparent pressure to swing wildly. I'm often within a few feet of published elevations, it seems, using either my Oregon 450 or 64s. Could be that, on some units, this is a toggle that needs to be turned on?

Most phone GPS apps, however, don't use the phone's built-in barometer to do what the dedicated units do. Only one I've found that does is AlpineQuest.
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retired jerry
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Re: I tried a barometric altimeter, seems to work well

Post by retired jerry » May 8th, 2018, 12:23 pm

My Garmin 60 has a barometric altitude measurement which I use all the time

It can be off 100 feet or so during the day, a few 100s overnight. Or it can be within 10 feet. Just depends on what weather is happening.

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kepPNW
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Re: I tried a barometric altimeter, seems to work well

Post by kepPNW » May 8th, 2018, 12:56 pm

Do you calibrate the altimeter at the start of a hike?

Definitely have Barometer Mode set to "Variable Elevation" right?

Do you have Auto Calibration turned on?
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retired jerry
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Re: I tried a barometric altimeter, seems to work well

Post by retired jerry » May 8th, 2018, 6:39 pm

I calibrate altimeter at the start of hike, either based on it's elevation that is known to me, or from the topographic lines based on latitude and longitude and USGS map data. Then I just let the barometric pressure be used to determine altitude. I know there's some way to use the satellites to calculate altitude on Garmin 60s but since the barometric pressure is more accurate I just use that.

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kepPNW
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Re: I tried a barometric altimeter, seems to work well

Post by kepPNW » May 9th, 2018, 5:43 am

Yeah, Garmins use the barometer in conjunction with satellite data. Tracking pressure changes, they can dampen the crazy swings GPS alone provides. (Why phone apps show such nutso EG numbers.) You might try experimenting with those other settings, if yours isn't staying relatively accurate over a day. Especially one without really nasty passing fronts.
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pcg
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Re: I tried a barometric altimeter, seems to work well

Post by pcg » May 9th, 2018, 6:23 am

Because barometric pressure is constantly changing I calibrate at the trailhead and then every chance I get, at least hourly, using elevation data I've gleaned from topo maps. This isn't necessary for casual hikes, but is sometimes mandatory for more serious bushwhacking, especially in winter, and it has become a habit for me so I do it constantly.

The reading on a barometric altimeter changes due to changes in three things - altitude, temperature, and local pressure changes (weather). They are temperature compensated, but I'm not sure how accurate and responsive the compensation is, particularly if you are moving the device back and forth between a warm environment (jacket pocket) and a cold environment. The local barometric pressure changes during the day because weather happens, even when it appears nothing is changing.

Walter Bonatti's, "The Mountains of My Life", is a great read. What he accomplished as a solo climber in sometimes terrible winter conditions is absolutely amazing and he relied heavily on a barometer to stay alive. If he saw the barometer falling more than it should due to elevation gain, or falling while he was in camp, he knew he was in for a storm and took appropriate action.

I don't use a GPS (although I carry an inReach Explorer) or smart phone and navigate principally with a barometric altimeter and a map, rarely using a compass. I used to use an analog Thommen and continued to use it after digital altimeters became available because I didn't want to find myself on a mountain with an altimeter with a dead battery. However, it is heavy and not as accurate, and I finally got a Suunto watch that has a user-replaceable battery and I carry a spare battery.

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Bosterson
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Re: I tried a barometric altimeter, seems to work well

Post by Bosterson » May 9th, 2018, 1:34 pm

pcg wrote:
May 9th, 2018, 6:23 am
However, it is heavy and not as accurate, and I finally got a Suunto watch that has a user-replaceable battery and I carry a spare battery.
I also have a Suunto watch, and my understanding is that the watch will display a barometric trend or the current altitude, but not both at once. It's a shame, because I use the altimeter function to assess my location on a map on ridges and such, and it would be nice to also be able to check for pressure changes indicating oncoming storms. I think the Suunto even has a setting where it will detect falling pressure and warn you, but I've never tried it cause I always have the watch set to show the altimeter.
Will hike off trail for fun.

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