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How Is Elevation Determined?


Nathannah
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That is my question...How is elevation determined? :huh: Does it use satellite data or is it barometric pressure? It's something I never thought about until someone just asked me. Is it different for different models? I have an etrex Venture.

 

I did a search and didn't find the exact answer, but I did see mention of people reading pressure with their GPSr. Can I do that with mine???

 

Thanks for the help!

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It depends on your GPSr. If you have one of the sensor types with built in altimeter, altitude is barametric. If not, then it is determined using the GPS data string, just like determining location. Actually, the GPS calculates the spherical location, and bases the Lat, Long and altitude based on the datum.

 

Cache Well

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Generally speaking, yes they are. They combine the altimeter with an electronic compass, and that adds between $75 and $100 to the price.

 

Elevation is a weak point in GPS. You need 4 satellites to get an 3-dimensional reading (location with elevation). Depending on a number of factors, the 3-d reading can be off considerably. If you only have a lock on your position with 3 satellites, then any elevation information given is not very reliable.

 

In those circumstances, if you really want or need to know a fairly accurate altitude, then sensing models give a more reliable altitude reading. If your non-sensing GPSr is getting a good signal from 5 or 6 satellites, then the altitude is generally pretty accurate.

 

Also, airliner cabins are pressurized so that you can breathe without oxygen masks. If you take a barometric reading inside the airplane at 40,000 feet, the GPSr will only show about 5,000 feet. A non-sensing GPSr will give you a much more accurate reading up there.

Edited by Neo_Geo
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as was said gps can detrermine elevation by either using an altimeter or interpolating from the sats info.

 

using an altimeter would be resonably accurate in my estimation provided atmospheric conditions are such that the computer algorithym is 'usable'. this may or not be the case and recalibration and adjustment may be necessary due to the conditions, since i dont have an altimeter equiped gps i cant comment on these processes. on the other hand the problem with using the sat info is that the elevation is based on the geoid from which the datum is derived, that is the elevation is only as reliable as the geoid modeling of the earths surface which is an estimate in itself.

 

in any case elevation is little better than an estimate and may not be reliable depending on the conditions or the location.

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I have a Venture. The elevation is as acurate as the coordinates. I was standing still on top of a hill (about 100 ft higher than surroundings) with no trees on it. I watched the elevation swing 10 ft either way. It was right near a fault, but I don't think it was that active!

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Elevation is a weak point in GPS. You need 4 satellites to get an 3-dimensional reading (location with elevation). Depending on a number of factors, the 3-d reading can be off considerably. If you only have a lock on your position with 3 satellites, then any elevation information given is not very reliable.

 

Another problem is that satellite configurations that give good 2D coordinates, are poor for determining elevation, and vice-versa.

 

Also, airliner cabins are pressurized so that you can breathe without oxygen masks.  If you take a barometric reading inside the airplane at 40,000 feet, the GPSr will only show about 5,000 feet.  A non-sensing GPSr will give you a much more accurate reading up there.

 

The next generation of airliners are supposed to have a considerably higher cabin pressure.

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I've been using Avocet wrist altimeters for several years (I've been through four of them so far!) so let me add a few more comments about barometric vs. satellite elevations:

 

Properly adjusted barometric altimeters can measure altitudes to within 10m (35') which makes them powerful navigation tools. They are also exquisitely sensitive and readings don't bounce around as GPS readings do sometimes. Granted, they don't give accurate readings on airplanes (as GPS altimeters do) , but this, in my opinion, is more than compensated for by the fact that they don't need a clear view of the sky (as GPS altimeters do).

 

Prior to the advent of cheap, portable consumer GPSrs, some people even argued that barometric altimeters were even more useful than a compass, since knowing one's elevation and the fact that one is on a given river, creek, trail or ridge allows one to quite accurately fix one's position on a map. And knowing one's elevation is more useful when trying to reach the top of a mountain than, say, a compass bearing.

 

However, barometric altimeters meaure elevation by measuring air pressure. And both altitude and local weather systems affect air pressure. This can be a good thing or a bad thing.

 

On the one hand, one can use an stationary altimeter (or GPS w/ barometric altimeter) to alert one to weather changes: if the measured altitude rises, a low pressure weather system is moving in (with the attendant risk of bad weather). Climbers aware of this trick will note the elevation given by their altimeter at night and then check it again in the morning. If the apparent campsite elevation has risen, it's probably not a good day to climb.

 

On the other hand, barometric altimeters that haven't been calibrated (by checking their elev. reading with a point of known elevation) can generate significantly mistaken readings.

 

While on an expedition in the Arctic a couple of years ago, I asked my partner for his altimeter reading so that I could check mine. He told me that his altimeter was not calibrated and was typically out by 600 m (2000') - which renders the device nothing more than a heavy and expensive wrist watch.

 

GPS elevations, on the other hand, don't suffer from this error. But because of the way that GPSrs calculate coorodinates, the elevation error is typically 1.5 times the horizontal positional error (which is about 15 m or 50')- so the vertical error will be about 22m or 75' without WAAS, most of the time. Other times it may be worse, just as GPS horizontal co-ordinates can be wildly out.

 

Most topo maps (in Canada at least) have contour lines at 20 to 40 m intervals (66' to 130') so this sort of accuracy is adequate for most map use. And if one's interest is just in measuring elevation gain or loss, (say, while climbing a mountain or while making ski runs), the error is less important.

 

Bottom line? For the most precise elevations, use the barometric altimeter - so long as you have frequent opportunities to recalibrate. But for general use and ease, I'd trust the GPS altitude.

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This is my offering from a thread a while back:

 

Thanks a lot - very clear explanation of a complicated subject.

 

Since my posting I got bits and pieces from the usegroups. A couple of interesting tidbits: almost all of the altitude measurements displayed in the 60CS's fields are readings from the altimeter. The only GPS altimeter field seems to be the one accessible from the satellite page (which I had not discovered before).

 

The logic of the altimeter/GPS combination is as follows. GPS elevation data is less accurate than your long/lat position. It is also much more subject to jumpiness because of the nature of GPS reception, changes in satellite geometry, etc.

 

The elevation data from the altimeter is generally more accurate than the GPS measurement: 1) if properly calibrated; and 2) assuming there is not much weather-related change (i.e. non-altitude connected change) in the barometeric pressure. It also gives much steadier readings than the GPS-derived figures.

 

My understanding of the interplay is that the primary driver of the altitude measurement is the barometer, and this is what is reported in the tracklog and I presume the datastream. However, the GPS-derived reading will be used to correct the barometric reading if there is a large disparity. This will happen if a weather front comes in quickly.

 

Before doing any calibration, I was consistently getting altitude readings of -100 feet at my home (real elevation +17 feet). I will experiment and see how it goes. It's fun learning about this stuff.

 

I wish the user could choose whether the altimeter is on or not. Sometimes rough numbers are enough, or you don't have a source for calibration.

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Not to be picky or tedious here but I can clarify this issue a little more..

 

The elevation data from the altimeter is generally more accurate than the GPS measurement: 1) if properly calibrated
This is where the picky part comes in. Yes, if properly calibrated and, just as important, RECENTLY calibrated. As noted, atmospheric pressure changes over time, sometimes rapidly. I suppose if I was hiking, and started at a known elevation, setting the elevation in the GPS then the smart thing to do would be to note the elevation at camp, and the change the next day. Then, reset the altimiter to the last know good reading from the night before as a starting point that next morning.

 

Pilots set their altimeters to a barometric pressure setting, adjusted for sea level, that is reported constantly on recorded weather broadcasts and airport information frequencies. Above 18,000 feet, all altimeters are requred to be set to a "standard" setting of 29.92" but that is another subject.

 

The alternative is to set the altimeter on the ground to a known ground level elevation. If you were hiking near an airport, had an aviation radio receiver, and knew the frequncy for the ATIS (Air terminal information services) for that airport, you would always have up-to-date pressure readings, adjusted for sea level, to set in your altimeter. Barring that, you could do the same with a weather radio, I suppose, if it were really important to you.

 

My understanding of the interplay is that the primary driver of the altitude measurement is the barometer, and this is what is reported in the tracklog and I presume the datastream. However, the GPS-derived reading will be used to correct the barometric reading if there is a large disparity. This will happen if a weather front comes in quickly.

 

This is the best of both worlds. My Meridian Platinum has a barometer, but it is not tied to an altimeter or used for altitude calculations. It is really just a silly feature but I suppose it is usefull for predicting weather. It also has a thermometer but the sensor is inside the unit so it shows the temperature in there as it warms up through use. I don't know what they were thinking but I'm sure the marketing people like it. The magnetic compass is a great feature. But again, I digress.

 

The most accurate GPS elevation readings, excluding WAAS, are when at least one of the sats used for data is low on the horizon. GPS altitude readings are based on a geodatic datum but reported as feet (meters, etc) above sea level, same as an altimeter.

 

David

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The alternative is to set the altimeter on the ground to a known ground level elevation. If you were hiking near an airport, had an aviation radio receiver, and knew the frequncy for the ATIS (Air terminal information services) for that airport, you would always have up-to-date pressure readings, adjusted for sea level, to set in your altimeter. Barring that, you could do the same with a weather radio, I suppose, if it were really important to you.

The easiest thing to do is find your local benchmark. I guarantee you that almost everyone lives near a geodetic benchmark disc. I walk over about 25 of them going between work and home (about 1.5 km).

 

Look it up and find it's elevation. Then, you've always got a precise location for setting your altimeter.

 

That being said, I don't calibrate mine everytime I use it, and the altitude readings are never that bad. I believe it is roughly corrected by the GPS position.

 

Most times, you don't care if you're at 750 m or 760 m. You're more concerned with getting to the top of the hill!

 

Regards,

Anthony

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Unless you have a port you notice in your GPS you probably have altitude per triangulation. I have found this to be much more accurate than barometric. Barometric is very dependent on air pressure which is dependent on air temp and the fluctuations of weather. Start a hike on a clear cool morning and you will not be very accurate once it starts to rain, or the air heats up as the day progresses, etc.

I hike for caches in Colorado with a topo map and altitude is helpful sometimes. I am using a Magellan 330. Compared to a barometric altimiter it is much more accurate.

A GPS will measure the altitude of a plane in the plane at altitude and not the cabin pressure equivalent of altitude.

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Before doing any calibration, I was consistently getting altitude readings of -100 feet at my home (real elevation +17 feet). I will experiment and see how it goes. It's fun learning about this stuff.

This comment makes me nervous. Yes, your GPSr may be incorrect and you're not really 100 below sea level. But what if it's accurate? If I were you, I'd buy a sump pump (grin).

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Before doing any calibration, I was consistently getting altitude readings of -100 feet at my home (real elevation +17 feet). I will experiment and see how it goes. It's fun learning about this stuff.

This comment makes me nervous. Yes, your GPSr may be incorrect and you're not really 100 below sea level. But what if it's accurate? If I were you, I'd buy a sump pump (grin).

I noticed the same thing while traveling across Puget Sound on one of Washington State Ferries. My Garmin V said I was at -127' and the 60C (not CS) said -121; yet I could see the surface of the water.... below me (Thank god!)

Edited by Right Wing Wacko
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