PDA

View Full Version : Altimeter accuracy q



ncmtns
05-03-2005, 20:55
Weather and elevation the same, which would give a more accurate elevation reading? A GPS or a barometric pressure reading?

dougmeredith
05-03-2005, 20:59
Weather and elevation the same, which would give a more accurate elevation reading? A GPS or a barometric pressure reading?

I don't have a definitive answer on this, but some higher-end GPS receivers include a barometric pressure altimiter. One of the benefits is that you can get altitude even in conditions where you might only be tracking three satalites. Would the GPSR beat the altimiter in an open field? I have no idea.

Doug

Ridge
05-04-2005, 02:05
If the GPS can be setup to use WAAS (wide area augmentation system) and you are able to receive signals from the experimental satellite that transmits this signal, it would be within 9 vertical feet accurate. A good barometric meter will probably be the best indicator. A Garmin Vista, with built in barometer, and a calibrated altimeter gives a fairly accurate elevation and is more convenient than a barometer and charts. The best will be when the FAA gets the WAAS fully functional and more satellites in space to accommodate everyone. The FAA is developing this system so as to become the primary nav system for pilots.
BTW, I got this info from my husband, he's on someones unlimited airtime cell phone out in CA. He and a cousin are doing the PCT and he's the GPS/Nav expert. hikerwife

SnakebiteSurvivor
05-04-2005, 04:53
I've carried both a GPS (Garmin Etrex) and a barometric altimeter watch (Suunto Vector). In my experience, the watch is accurate within about 20 feet for a couple of hours once it has been calibrated at a known point (getting the elevation from guidebook, topo map, etc.). The GPS altitude can be accurate to 9 feet, but for that to happen, you have to have a lot of satellites captured (like about 7). The most satellites I ever had was 9, on top of Gregory Bald in the Smokies once, and sure enough, it got the altitude right on the button (within 10 feet). But most places on the AT, you're lucky to get 3 or 4 good satellites in the "green tunnel", and the altitude reading on the GPS isn't accurate enough to use (and if you only get 3 satellites, you don't get any altitude reading at all).

SGT Rock
05-04-2005, 10:17
Accurate to within 10 meters should be close enough for just about anything don't you think? Seems what would be most cost effective should be the consideration.

Footslogger
05-04-2005, 10:26
Weather and elevation the same, which would give a more accurate elevation reading? A GPS or a barometric pressure reading?=================================
Unless you're willing to reset the "reference elevation" value of your barometric altimiter on a regular basis the GPS is going to yield a more consistently accurate indication (provided you're not under the canopy and unable to get a good lock on the satellites).

'Slogger
AT 2003

rickb
05-04-2005, 11:26
I have an old GPS which will give altimeter readings which are practically useless. On my watch the readings are good.

I'm still trying to give away my working Casio Altimeter Watch (the wrist strap cant attach to one side due to wear but would be perfect if you want to attach to pack as a fob), as per my offer in another thread, BTW. If anyone wants it, just let me know and I'll e-mail you an address where you can send me a SASE.

Rick B

rickb
05-04-2005, 13:17
Watch is gone. I'll send the one and only person who responded a PM.

RockyTrail
05-04-2005, 16:44
The problem with a barometic pressure altimeter is that it must be calibrated frequently because the ambient air pressure changes daily with the weather. The GPS does not have this issue, but then it has the satellite issues mentioned above.

You can calibrate your barometric altimeter in two ways:

1. Stand on a known altitude (for example a known peak) and set the dial to read that known altitude.

2. Set the altimeter's calibration window dial (if it has one) to the local reported sea-level pressure, usually in reported in inches of mercury, for example 29.92 inches of mercury.


Aviation altimeters use method No. 2 and depend on getting an accurate altimeter setting every couple hundred miles or so during a flight. This can be done in the air or on the ground via an air-band receiver that allows you to pick up automated real-time weather broadcasts from local airports (called AWOS systems) or from air traffic controllers. You could try to hike with one of these radios but reception is likely to be difficult in most places.

Something to think about: a 1 inch of mercury variation in pressure equates to approximately 1000 feet of altitude. It's not unusual to see that kind of shift in a day's time if weather fronts are passing through, and even on calm days the pressure is usually drifting high or low, it is rarely stable.
So if you can get a GPS to guarantee within 10 feet I'd say that is pretty darn good!:sun

SGT Rock
05-04-2005, 18:46
With a GPS you get three dimensions of coordinates, so it gives you a couple of references in addition to altitude to determine location. They don't weigh much and don't cost that much these days. I bought a Magellan Sport Track about 2 years ago and it only weighs about 5.8 ounces with lithium batteries and seems to run forever on them. I carry it in my LBE at work and just pull it out every once and a while and check it to make sure where I am like you would check your watch to see what time it is. It can use WAAS so you can be within 7' laterally and 9' elevation. Close enough for just about anything you need to do on foot.

Footslogger
05-05-2005, 08:57
I carry it in my LBE at work and just pull it out every once and a while and check it to make sure where I am like you would check your watch to see what time it is. ==========================================
"LBE" ...now there's something I haven't seen or heard in several decades.

'Slogger
AT 2003

Footslogger
05-05-2005, 12:52
Hey Rock ...do they still call it TA50-901 ??

'Slogger

SGT Rock
05-05-2005, 13:58
CTA 500 Clothing Table of Allowances 500

Footslogger
05-05-2005, 14:22
CTA 500 Clothing Table of Allowances 500=========================
Guess I'm dating myself there eh ??

'Slogger (US Army '68-'71)

SGT Rock
05-05-2005, 15:55
Roger

Rock US Army 1985-????