Calibrating Elevation

[+Doubledareya]

I want to calibrate the elevation for my GPS. At work--eight miles from where I live--there is a construction site for a new building they have just finished and there is a stake driven in the ground there saying the elevation is 21.93 (22) feet. My question is this: If I use that site to calibrate my GPS, do I add an additional 4 feet while entering the elevation? I am 5'8" tall and (when standing) the unit would be at chest level, or 4 feet off the ground. I know that in the whole scheme of things, four feet either way is pretty insignificant. But I am curious: Is the elevation determined by the Earth's surface or by the height of the unit above that surface? TIA.

[Northwoods Tom]

Not an expert, but I believe its the elevation of the ground at that spot. Construction site-elevation of various points at the site. Why not just place the GPS unit on the ground while entering the information? Airports, especially rural, are good areas to check elevation. Very often they even have it posted somewhere.

[+Doubledareya]

Not an expert, but I believe its the elevation of the ground at that spot. Construction site-elevation of various points at the site. Why not just place the GPS unit on the ground while entering the information? Airports, especially rural, are good areas to check elevation. Very often they even have it posted somewhere.

 

Thank you for answering my question, Tom; I really appreciate it. And it would appear that you are right! I just checked the Google Map elevation of where my house is and they say it's 32 feet right where my parking pad is located. While standing out on the parking pad a few minutes ago I got an elevation reading of 36 feet while holding and reading my GPS at chest level, 4 feet above ground. From my parking pad I climbed the adjoining four steps to my porch (2 feet higher than my parking pad) and got a reading of 38 feet while holding the unit at chest level. Ergo: It would appear from this empirical data that you are correct. Thanks again.

[+John in Valley Forge]

I think my GPSr tries to caclulate the elevation of the handheld unit not the ground you are standing on. I was on a cruise this summer and it would give me readings well above 0ft depending upon which deck I was on. The ocean should be pretty close to 0.

 

It might be different for different brands/models. What you could try is to take a reading with it at the highest window in your home then again on the ground directly under that window. See if it changes.

[+Doubledareya]

I think my GPSr tries to caclulate the elevation of the handheld unit not the ground you are standing on. I was on a cruise this summer and it would give me readings well above 0ft depending upon which deck I was on. The ocean should be pretty close to 0.

 

It might be different for different brands/models. What you could try is to take a reading with it at the highest window in your home then again on the ground directly under that window. See if it changes.

 

Mmmmm, John. Interesting. I wish I had waited another five minutes until after you posted. Also, another "highly scientific" experiment I did two days ago was at work. We have a large 228,000-gallon storage tank at work with stars leading to to the very top; the top stair platform is exactly the height of the outside top of the storage tank. I know (from specs) that the tank is 24 feet high. The other day, as I recall, I took an elevation reading while standing at the base of the tank while holding my GPS at chest level. I then climbed the stairs and went to the top. As I recall, the difference (at chest-level readings) was right at 24 feet. I will check again tomorrow. My GPS, BTW is a Garmin etrex vista cx. So you may be right...different units may read differently, but to me that really doesn't make sense. Anyway, thanks for the input.

[+webscouter.]

To be "accurate" you want the GPS to indicate the level of your feet, thus the ability to "calibrate" it.

 

Think about it, if an airplane calibrated to the height of its GPS you would be attempting to land 30 feet lower than the deck resulting in a really hard landing When you are calibrating an aircraft GPS you dial in the number ASL that is listed at the airport. This is done every time you preflight to account for barometric differences at the time of flight.

[+dfx]

the elevation isn't elevation above ground level, it's elevation above sea level. you're calibrating a barometric altimeter, which (as the name indicates) relies and depends on air pressure, which will vary from one day to another.

[Northwoods Tom]

the elevation isn't elevation above ground level, it's elevation above sea level. you're calibrating a barometric altimeter, which (as the name indicates) relies and depends on air pressure, which will vary from one day to another.

 

Excellent point to include.

[+John in Valley Forge]

Oh yeah. I had my gps with me on a couple airline flights. Reception was lousy but it said I was 30,000 ft up and moving at 500 mph.

[+Doubledareya]

the elevation isn't elevation above ground level, it's elevation above sea level. you're calibrating a barometric altimeter, which (as the name indicates) relies and depends on air pressure, which will vary from one day to another.

 

Excellent point to include.

 

Very excellent point!

 

C&A (Clarification and Amplification)...In the above post re the 24-foot-high storage tank: Before climbing the stairway to the top of the tank, I took (at chest level) an altimeter reading (GPS held four feet off the ground) and got a reading of 22 feet. This is the number of feet--according to the previously mentioned benchmark stake--that the base of the tank is above sea level. When I climbed to the top of the tank I got a reading of 46 feet. Subtracting 22 feet from 46 feet gave me a reminder of 24 feet. I know (from blueprints and specs on file) that the tank is 24 feet high. So everything is as it should be, right? (I will double-check all this as described tomorrow when I go back to work just to make sure I did it right.) Thanks, one and all!

[+NYPaddleCacher]

Oh yeah. I had my gps with me on a couple airline flights. Reception was lousy but it said I was 30,000 ft up and moving at 500 mph.

 

That's probably fairly typical. The "behind the seat" screen on my flights to/from China (as well as pretty much intercontinental flight I've been on) has a real time flight info channel. It sequences through three resolutions of satellite maps with flight tracking information. It also shows screens with air temperature (it got as low as -65F), distance/time to the destination airport, current time at the destination and originating location, as well as the speed and altitude. For a good part of the flight it was going around 600MPH at 37,000 feet.

[+John in Valley Forge]

Oh yeah. I had my gps with me on a couple airline flights. Reception was lousy but it said I was 30,000 ft up and moving at 500 mph.

 

That's probably fairly typical. The "behind the seat" screen on my flights to/from China (as well as pretty much intercontinental flight I've been on) has a real time flight info channel. It sequences through three resolutions of satellite maps with flight tracking information. It also shows screens with air temperature (it got as low as -65F), distance/time to the destination airport, current time at the destination and originating location, as well as the speed and altitude. For a good part of the flight it was going around 600MPH at 37,000 feet.

 

It was my car GPS, and it did some strange things.

Satellite reception is terrible in an airliner. Probably has something to do with the aluminum fuselage thing.

I have no "off-road" setting, and even on pedestrian mode it kept trying to put me on whatever road was under us. Because we weren't flying on a road, this confused it to no end.

For a while we were flying parallel with an interstate, so it figured I was driving on it. Unfortunately it also thought I was driving 500 mph OVER the speed limit and it started beeping and the little voice called out Caution, Caution, Caution. Not a good thing in an airplane. Luckily I hit the mute button really fast and no one noticed.

[+apollosmith]

GPS receiver elevation calibration is about worthless. While you can calibrate them to increase accuracy quite significantly, the calibration is only good for a very short period of time and only within close proximity of the calibration point. If you turn the receiver off, move it more than a short distance, or wait more than a couple hours, it will be no more accurate than without calibration.

 

The only time I calibrate is while benchmark hunting. I calibrate to an adjusted elevation point and then measure accuracy of the unit between benchmarks. It's quite good, but only under the parameters listed above.

[7rxc]

GPS receiver elevation calibration is about worthless. While you can calibrate them to increase accuracy quite significantly, the calibration is only good for a very short period of time and only within close proximity of the calibration point. If you turn the receiver off, move it more than a short distance, or wait more than a couple hours, it will be no more accurate than without calibration.

 

 

Yep... there are two types of elevations available these days... the old style 'calculated' form, which has always been close but not quite... type of reading... As you say, it can be fine tuned on many rigs... and it is related to the session at hand... but it is nothing more than a correction factor applied to the calculation... move much and it loses its effect because you change what the correction was being added to... but it can help for a while, as you said.

 

Then the newer devices often have electronic 'barometric' altimeters, which should give much better results once they are adjusted for '0'. Even aircraft altimeters have two forms of adjustment... below x thousand feet, closer to the ground the adjustment is for local barometric pressure as measured close by... more useful for terrain avoidance and landing procedures... the other is for above x thousand feet so that all aircraft are talking the same measurement... and uses standard pressure for MSL... ( I'm rusty, 29.92?).

 

OP didn't mention which he was talking, but it sounds like it was barometric.

Calculated doesn't give (or didn't) very good precision either.

Myself I like a dedicated separate altimeter that doesn't rely on batteries, and that is only used for side hill gougeing at times, only for maintaining an elevation, not measuring it.

I use a separate compass for bearings etc. as well.

 

Doug 7rxc

[+Student Camper]

My GPSr has an altitude feature but it is not barometric, it is done using the same triangulation technology used for locating the coords and is subject to the same errors.

I live at sealevel and my numbers vary 10 to 30 feet, the same as the coords I'm looking for.

It is not likely that you need to calibrate your GPSr

Edited November 8, 2010 by student camper