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GPS barometric altimeters - a treatise

#1 User is offline   apollosmith 

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Posted 03 January 2010 - 05:03 PM

I started compiling a response to another post and it turned into a full-fledged overview of GPSr barometric altimeters, so here it is. If there are inaccuracies or errors, please let me know. This was mostly an effort for me to learn what they heck these things are and how to properly calibrate them.

Barometric Pressure and Altimeters

Modern GPS receivers (GPSr) often include a barometric altimeter. Barometric pressure is essentially a measurement of the weight of the air above a given point. When a high pressure weather system is in the area, barometric goes up because the air is more dense or heavier - this is what pushes the rain clouds away. Low barometric pressure usually means more clouds. Barometric pressure is typically reported in inches of Mercury (e.g., 29.92 inHg) or in millibars (e.g., 1013.25 millibars).

A barometric altimeter is tool that measures the amount of air pressure at that location. A GPSr with a barometric altimeter can provide more accurate elevation data (sometimes within 10 feet or so) than it can obtain from using the GPS satellites alone (sometimes within 100 feet or so - yes, elevation accuracy from satellites kinda sucks, and it gets worse as your elevation increases because you're closer to the satellites making it harder to determine your distance from them).

A GPSr with a barometric altimeter knows that if the pressure decreases, that there is less air above it. Thus one of two things has occurred - either the GPSr has moved to a higher elevation OR the natural barometric pressure for that location has decreased due to weather changes. The problem is that the GPSr doesn't know which has occurred.

Altimeter Calibration

To get accurate elevation readings, the GPSr must be calibrated so it can equate a pressure reading to an elevation.

There are four ways to calibrate the GPSr barometric altimeter:
1. Enter the KNOWN elevation when your barometric pressure is unknown.
2. Use the GPS-calculated elevation when your barometric pressure is unknown.
3. Enter the ADJUSTED barometric pressure when your elevation is unknown.
4. Let the GPS-calculated elevation help auto-calibrate the barometric altimeter over time.

Method #1 tells the GPSr that the currently measured barometric pressure in the GPSr is what should be expected for that exact elevation.

Method #2 does the same thing, except that it uses the rather inaccurate (+/- a couple hundred feet) GPS-calculated elevation.

Method #3 allows the GPSr to determine the current, accurate elevation by determining the difference between the measured pressure in the unit and the sea-level adjusted pressure you provide.

Once the GPSr has a good idea of what the accurate elevation is for the internally measured pressure, changes in pressure can more accurately be represented as increases or decreases in elevation. For example, a pressure change of .01 inch of mercury as measured by the internal barometer equates to ~10 feet of elevation change.

But, your GPSr assumes that the the only thing that changes pressure is it moving higher or lower - it ignores the fact that weather also affects pressure. Thus, if the atmospheric pressure around you changes, your elevation accuracy will suddenly be out of whack. This means you should only calibrate your altimeter using pressure or elevation if you want increased accuracy over short periods of time (shorter if the weather/pressure changes) and if you'll remain within a small geographic area (because changing locations is more likely to result in an atmospheric pressure change).

So which calibration method is best?

Methods #1 (known elevation) or #3 (known pressure) arguably provide the same level of accuracy, though using a known elevation is typically better because it is a finer value than the measures used for barometric pressure. Either way, the calibration values should only be entered outdoors, out of the wind (which can arguably affect barometer readings), and once your GPSr has been on, immobile, and well established for some time. All GPS altimeters require good GPS reception AND accurate pressure readings. Using the GPS altimeter in your car or indoors will not result in high accuracy - and could result in VERY poor accuracy (e.g., 1000's of feet off).

Letting the GPS auto-calibrate the altimeter is BY FAR the easiest - and by far the most accurate over long periods of time or distance or weather. This method uses the GPS-computed elevation to hone in on a 'best-guess' elevation and then uses the altimeter to help maintain accuracy and consistency of the displayed elevation over time. Most units recalibrate every 15 minutes using this method. Once your GPS location is well established, the accuracy of auto-calibrated altimeter readings are only slightly less accurate than manually calibrated readings. The advantage of auto-calibration is that you can be assured that natural pressure changes are not distorting elevation readings over time.

In short, there really are very few advantages to manual calibration over auto calibration. Perhaps the only notable advantages are increased accuracy within a short period of time after proper calibration and that most manually calibrated GPSr units can provide high accuracy almost immediately after turning them on - you don't have to wait for the unit to establish your position before getting a highly accurate elevation reading (e.g., the unit can read the barometric pressure much faster than it can triangulate your position).

If you calibrate your GPS altimeter with known pressure or known elevation, you must turn off "Auto-calibrate" function in your GPS otherwise it will ditch your entered value and go back to the best-guess GPS elevation in a matter of minutes. Most units prompt you to turn this off after manual calibration. But be sure to turn this function back on later otherwise the reported elevations will likely be WAY off because the pressure will likely have changed.

Some tips on using pressure calibration

If you choose to calibrate using a known pressure value, be sure to use sea-level adjusted pressure readings (sometimes referred to as ASL, MSL, or elevation adjusted). You can get these from local weather reports and from airport METAR reports. METAR reports for your local airport are available here - just find the numbers after the A and put a decimal point in the middle. For example, my local airport METAR contains A3035, so my current sea-level adjusted pressure is 30.35 - or 30.35 inches of mercury. METAR and weather station pressure values are typically accurate for perhaps 100 miles from the reporting station/airport (naturally less if the weather is changing).

Your GPSr expects an elevation adjusted pressure. The pressure can typically be entered in inches of Mercury (inHg) or in millibars.

If you're using a home weather kit, barometer, or get the pressure from another GPS system or weather station data feed, these will typically NOT report elevation or sea-level adjusted pressures. Using these values will cause great inaccuracies - higher inaccuracies the higher your elevation. Because one inch of change in mercury represents ~1000 feet of elevation, if you live at 5000 feet elevation, your elevation adjusted pressure might be 30.10 inches, but a barometer would probably show a local (unadjusted) pressure of 25.10 inches. If you enter 25.10 inches into your GPS, elevations shown on your GPS will be off by 5000 feet!!!

Some tips on using elevation calibration

The optimal method for calibrating using a known elevation is to use an elevation benchmark. Go to http://www.geocaching.com/mark/ and enter your zip code and try to find a benchmark you could use (U.S. only). Be sure to look for one that has recently been found in good shape (has a smiley face icon) and that has an adjusted (e.g., very accurate) elevation (check the description for "Altitude is ADJUSTED"). Benchmark elevations are VERY accurate - usually within a few 1/10s of an inch - pretty remarkable considering most were placed in the 20's and 30's.

Because the GPS unit itself is only accurate to within 10 or so feet of elevation at very best, you may be just as well off using a good topographic map or even Google Earth to determine your location's elevation for calibration. One good method is to use a benchmark initially then use that to determine your home's elevation - then use this elevation to calibrate your unit each time you leave home. Be sure to measure an elevation outdoors - taking it inside or calibrating inside will ruin your accuracy.

GPS altimeters and aircraft altimeters

It is important to note that airplane altimeters and GPS altimeters may vary a lot except when on the ground. One reason is that the airplane altimeter uses the pressure at some nearby airport to provide a basis by which the elevation is determined. This is why pilots are constantly updating the pressure setting on their altimeter - not only to ensure accuracy for when they land or fly over local terrain, but also so that their pressure setting is the same as every other plane's in the immediate vicinity.

When you're flying at 15,000 feet, the pressure will be significantly lower than at the airport below because there is less air above you than the airport (just like the local, unadjusted pressure at Denver is much lower - ~5 inches of mercury lower - than at sea level). But you want all airplanes in the immediate area to have one uniform pressure setting in their altimeters - otherwise you could risk collisions. One plane flying at an indicated altitude of 15,000 feet with a pressure setting of 29.75 and another plane with an indicated altitude of 14,000 feet but a pressure setting of 30.52 will be MUCH closer in ACTUAL altitude than 1000 feet.

Planes flying above 18,000 feet tend to fly faster and thus use a standard pressure setting of 29.92 so that all planes up there are reporting the same indicated altitude and don't have to update their pressure settings every few minutes as the pressure at the ground changes below them. Because of this standard setting, a plane flying at 35,000 feet with a standard pressure setting of 29.92 might actually be flying much higher or much lower than 35,000 feet above sea level. All that pilots care about is that what they think is a certain altitude is the same as what everyone around them thinks. They only care about really accurate altitude when landing.

If you take a GPS on your flight (make sure it's OK with your airline before using it in-flight!), you'll notice that the GPS elevation and the reported airplane altitude may vary a lot. Also note that GPS elevation accuracies decrease slightly as you gain elevation. Oh, and be sure to disable your GPSr barometric altimeter on a pressurized plane or the results will be, well, WAY off.

This post has been edited by apollosmith: 03 January 2010 - 05:27 PM


#2 User is offline   mtbikernate 

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Posted 03 January 2010 - 08:33 PM

You forgot to mention that in an airplane, the cabin is pressurized (usually equivalent to about 8000ft), and that is the primary reason your barometric altimeter differs. It is because it measures the pressure inside the plane.

#3 User is offline   Team CowboyPapa 

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Posted 03 January 2010 - 08:49 PM

This is what I use:
http://www.digitaldutch.com/atmoscalc/

#4 User is offline   Suscrofa 

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Posted 04 January 2010 - 09:52 AM

Who still uses inch of mercury ? :anibad:

This post has been edited by Suscrofa: 04 January 2010 - 09:54 AM


#5 User is offline   apollosmith 

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Posted 04 January 2010 - 09:55 AM

View PostSuscrofa, on Jan 4 2010, 10:52 AM, said:

Who still use inch of mercury ?


All US and Canadian airports, air traffic control, and weather services.

#6 User is offline   Chrysalides 

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Posted 04 January 2010 - 10:16 AM

Thank you very much for a very well written explanation. I've been curious about the altimeter, but never curious enough to do all the research you did.

#7 User is offline   Team CowboyPapa 

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Posted 04 January 2010 - 10:31 AM

View Postapollosmith, on Jan 4 2010, 09:55 AM, said:

View PostSuscrofa, on Jan 4 2010, 10:52 AM, said:

Who still use inch of mercury ?


All US and Canadian airports, air traffic control, and weather services.

Roger that.

And all the launch vehicle engineers that work to put the GPS satellites in orbit use pounds (force) per square inch. :anibad:

#8 User is offline   madman1892 

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Posted 04 January 2010 - 10:57 AM

Actually if you have a Garmin gps with a barometric altimeter, and you use the auto-calibrate, the situation is more complex.

Garmin uses the GPS elevation in order to calibrate the barometric altimeter. Using this method,the barometric altimeter attempts to take out changes due to weather. It looks at the gps elevation to determine if a change in air pressure is due to a change in actual elevation or weather. I've seen some reports (but can't find them right now), that suggest that using this method provides better elevation data than either a gps or barometric altimeter can individually.

My suggestion is to calibrate the altimeter using a known elevation or barometric pressure, and then let the auto-calibrate mode take over.

#9 User is offline   Chrysalides 

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Posted 04 January 2010 - 11:49 AM

View PostTeam CowboyPapa, on Jan 4 2010, 10:31 AM, said:

And all the launch vehicle engineers that work to put the GPS satellites in orbit use pounds (force) per square inch. :anibad:

*cough* Mars Climate Orbiter *cough*

I think Imperial units are silly as well, but one uses whatever the locals use and don't complain (too much) about it :anibad:

#10 User is offline   user13371 

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Posted 04 January 2010 - 01:32 PM

View PostChrysalides, on Jan 4 2010, 11:49 AM, said:

...but one uses whatever the locals use and don't complain (too much) about it :anibad:
Unless you're on a crusade to convert the world -- and feel a need to make fun of anyone from unenlightened realms who mentions one of those archaic units.

Nobody around here would do that though :anibad:

#11 User is offline   Team CowboyPapa 

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Posted 04 January 2010 - 01:47 PM

View PostChrysalides, on Jan 4 2010, 11:49 AM, said:

View PostTeam CowboyPapa, on Jan 4 2010, 10:31 AM, said:

And all the launch vehicle engineers that work to put the GPS satellites in orbit use pounds (force) per square inch. :anibad:

*cough* Mars Climate Orbiter *cough*

I think Imperial units are silly as well, but one uses whatever the locals use and don't complain (too much) about it :anibad:

Exactly, the poster child for: "If it ain't broke, fix it until it is."

#12 User is offline   39_Steps 

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Posted 04 January 2010 - 06:33 PM

View PostTeam CowboyPapa, on Jan 4 2010, 01:47 PM, said:

Exactly, the poster child for: "If it ain't broke, fix it until it is."

In that case, you might take a look at your digitaldutch calculator. It doesn't seem to handle min and max elevations correctly in all cases without erroring out with those units the rocket scientists use.

#13 User is offline   fizzymagic 

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Posted 04 January 2010 - 08:09 PM

While what you have written is very nice, it leaves out at least one very important concern for handheld GPS users: the time response of the barometric altimeter. Since handheld GPS units tend to be rated as waterproof for short durations, the holes that equalize the pressure inside and outside the case are very small. WIth my PN-40, I observe a lag of over a minute between the GPS altimeter reading and the barometric altimeter reading.

#14 User is offline   Team CowboyPapa 

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Posted 04 January 2010 - 08:40 PM

View Post39_Steps, on Jan 4 2010, 06:33 PM, said:

View PostTeam CowboyPapa, on Jan 4 2010, 01:47 PM, said:

Exactly, the poster child for: "If it ain't broke, fix it until it is."

In that case, you might take a look at your digitaldutch calculator. It doesn't seem to handle min and max elevations correctly in all cases without erroring out with those units the rocket scientists use.

For example.......

#15 User is offline   Team CowboyPapa 

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Posted 04 January 2010 - 09:15 PM

View Post39_Steps, on Jan 4 2010, 06:33 PM, said:

View PostTeam CowboyPapa, on Jan 4 2010, 01:47 PM, said:

Exactly, the poster child for: "If it ain't broke, fix it until it is."

In that case, you might take a look at your digitaldutch calculator. It doesn't seem to handle min and max elevations correctly in all cases without erroring out with those units the rocket scientists use.

Hey, I was offering that as help to some who want to work with their barometric altimeters.
Is there anything wrong with that?
Are you capable of offering any help yourself?
Or, is criticizing the help of others the extent of your capabilities?

#16 User is offline   Suscrofa 

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Posted 05 January 2010 - 12:11 AM

View PostTeam CowboyPapa, on Jan 4 2010, 09:15 PM, said:

View Post39_Steps, on Jan 4 2010, 06:33 PM, said:

View PostTeam CowboyPapa, on Jan 4 2010, 01:47 PM, said:

Exactly, the poster child for: "If it ain't broke, fix it until it is."

In that case, you might take a look at your digitaldutch calculator. It doesn't seem to handle min and max elevations correctly in all cases without erroring out with those units the rocket scientists use.

Hey, I was offering that as help to some who want to work with their barometric altimeters.
Is there anything wrong with that?
Are you capable of offering any help yourself?
Or, is criticizing the help of others the extent of your capabilities?


Don't you think that part of the inability of Americans to deal with these basic things is due to some kind of
"mathematical illiteracy" which is one of the reason of the failure to adopt the metric standard ?
On the other hand, the confusion of these imperial units lead to confusion in understanding the basics of physics for ex..

Soldiers lost in Vietnam because they couldn't read map, compute distance etc...

And we can see that it is still the case on this forum !

When I was in the wood of Maine, I could use the map better than the locals, because 1cm = 240m on the 24k Topo map !

Besides, in this global economy, good luck to export products that are non standard !

See:
http://redtape.msnbc.com/2009/12/when-i-pu...e-fraud-al.html

#17 User is offline   39_Steps 

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Posted 05 January 2010 - 04:50 AM

View PostTeam CowboyPapa, on Jan 4 2010, 09:15 PM, said:

Hey, I was offering that as help to some who want to work with their barometric altimeters.
Is there anything wrong with that?
Are you capable of offering any help yourself?
Or, is criticizing the help of others the extent of your capabilities?

TeamCowboyPapa-
I'm in way over my head, but just wanted to point out what appears to be a bug in the link above. :unsure:

Suscrofa-
When it comes to 'easy learning', I concur with your premise that students in the USA are disadvantaged by not having Metric units as the standard. On the other hand, The mental gymnastics those of us in the USA must go through to understand the system may be beneficial by forcing us to use a few more brain cells. :D

As to the Vietnam mapping problems, was that due only to the use of the metric system, or to a combination of the metric system plus maps which were created using Paris, not Greenwich, as the global reference point?

#18 User is offline   robertlipe 

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Posted 05 January 2010 - 06:42 AM

Moderator's Note This thread is about altimeters. If you want to lobby for your favorite standard of measuring systems, that might be OK in the Off Topic forum. If you want to snipe at each other about illiteracy killing soldiers and such, please find some other forum that allows such discussions.

#19 User is offline   poor_PDOP 

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Posted 08 January 2010 - 12:50 AM

All I want is the ellipsoidal elevation derived from the GPS observations. I can apply the corrections myself. Is a GPSr able to show that ellipsoidal elevation value on the screen?
People who fly may require altimeters to determine height above ground.
You have to be carefull when talking about Geoids & Ellipsiods. The seperations can be quite large on some continents. And there is the added difference that some geoids models will differ from the Reduced levelling adjustment models (usually within a hand held GPSr accuracy), i.e. values shown on Govt survey mark databases.

#20 User is offline   Walts Hunting 

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Posted 08 January 2010 - 06:12 AM

Quote

Moderator's Note This thread is about altimeters. If you want to lobby for your favorite standard of measuring systems, that might be OK in the Off Topic forum. If you want to snipe at each other about illiteracy killing soldiers and such, please find some other forum that allows such discussions.


Intersting a moderators note posted by someone who is not. Maybe he wants to be. If a diversion from original intent in a forum topic is officially discouraged they they need to a better job of policing. Look at soome that are 50 pages long and see where they go.,

The conversations range far and wide taking off in different directions when a post sparks that diversion. Frankly I enjoy the diversions. They help make my morning coffee interesting. Much like sitting in a coffee shop and Berkeley and saying "I really liked the Bush years" as a conversation starter.

Interesting to see where this topic goes from here. I suppose I should get my nomex suit out.

#21 User is offline   Chrysalides 

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Posted 08 January 2010 - 12:00 PM

View PostWalts Hunting, on Jan 8 2010, 06:12 AM, said:

Intersting a moderators note posted by someone who is not. Maybe he wants to be.

Erm, check http://forums.Groundspeak.com/GC/

Look up the GPS and Technology section.

Forum Led by: Hemlock, Keystone, robertlipe

I don't know why he doesn't have a mod badge. But in any case, I guess you like living dangerously, from the rest of your post :P

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