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garmin 60csx - altimeter vs. gps elevation


polarisprox

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i have the garmin 60csx. I see the altimeter screen...it shows the elevation.

then, under the "satellites" page, i select menu, and then gps elevation, to get another elevation reading...

 

what is the difference? specifically, i took the gps on the airplane with me today, and it was cool. all speed readings and gps coordinates were awesome to see during flight. However, the altimeter reading was around 6500 feet for most of the flight. when i looked at the satelling-menu-gps elevation, it was the reading i was expecting, i.e. 38000 feet (or so) during flight.

 

any ideas/thoughts? maybe i dont know what the altimeter is really used for...????

 

thanks!

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The altimeter is actually a barometric altimeter. It calculates altitude based on barometric pressure. You calibrate it for a known altitude and it will keep track of any ups and downs.

 

In a pressurized cabin of a jetliner, the readings will be off. :laughing:

 

Reverting to gps elevation allowed you to see your elevation calculated based upon triangulation from the various satellite signals you were receiving, so cabin pressure will not mess with your readings.

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i have the garmin 60csx. I see the altimeter screen...it shows the elevation.

then, under the "satellites" page, i select menu, and then gps elevation, to get another elevation reading...

 

what is the difference?

 

your altimeter screen is using the 60csx internal barometric altimeter, and due to cabin pressure in the plane was off. The gps elevation on the satellite screen is figured out from gps signals, and is usually accurate +/-100ft.

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6500 feet

Well that's interesting. I've wondered where they keep the pressure at on board a plane. If that corresponds to actual air density that would make sense. At 6500' most people coming up from sea level are still comfortable. At 8000' some will start getting sick and many will feel the effects. At 10,000', many will feel some sickness and most will feel dizzy or weak. From there up you can have really medical problems pop up.

 

I have a barometric altimeter on my watch. You have to calibrate it and it can still be of by changing weather conditions. I trust my GPS elevation more. Off by 100' at the most, most often less, never affected by the weather. Good enough for me. I have a 60cx.

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Yea, I've got a 60CSx, and took it with me on a plane once too. Altimetry was all kinds of screwed up due to pressurized cabins.

 

Thing about altitude is GPS is quite poor for it, 2-3 times worse than in the horizontal, so your accuracy might be 50m rather than 15m. However, it will always be "right," with some large random error.

 

Barometric altimeters are quite a bit more precise, so you'll be able to resolve smaller changes in altitude more accurately with a barometic altimeter than with the GPS altitude. Change your alt by 5m, and it'll be lost in the noise of the GPS error, but your barometer will know.

 

Now, if your barometer is miscalibrated, it will still be very precise, but it will always be wrong. And if you're in an environment where the pressure is being altered like a plane cabin, well it's really useless.

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The reason for having the barometric altimeter on the GPSr is so that if/when you lose reception of GPS signals, you can still have reliable altitude information if that information is critical (think search & rescue and mountain climbing). The barometric altimeter needs to be calibrated for each use, and sometimes more often than that. That makes it more of a pain for me. When I grab my GPSr, I just wanna turn it on and go! I don't wanna have to mess around with the thing every time I turn it on.

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hey all, great info - thanks very much. makes perfect sense.

 

last question - calibrating the altimeter...whats the best way? i presume its just inputting a "known" altitude where our at, and it goes from there...right?

 

If you have a barometer you can enter the barometric pressure. You could also enter a published barometric pressure from a news source, but that may not be as accurate. Calibrating the GPSr is not critical if you only want to know how much your elevation changed. Its only important if you want to know your present altitude as accurately as possible.

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6500 feet

Well that's interesting. I've wondered where they keep the pressure at on board a plane. If that corresponds to actual air density that would make sense. At 6500' most people coming up from sea level are still comfortable. At 8000' some will start getting sick and many will feel the effects. At 10,000', many will feel some sickness and most will feel dizzy or weak. From there up you can have really medical problems pop up.

I've found the pressure to vary quite a bit on different flights. The requirement is that the pressure should never be lower than an 8000' equivalent to avoid health problems with the passengers. But some flights I've been on have pushed it right up to that limit (i.e. having a pressure altitude of 7900' or more) while others have kept it down below 6000'.

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The actual altitude an airplane flys at will have a big effect on the cabin altitude. (inside pressure) The pressurization system in designed to only allow a certian maxim pressure differental between inside and outside the airplane. This is to pervent structural damage to the airplane. (hatches and window blowing out and rivits shooting off the plane wound be bad.) I don't know what the Max Differential is on a commercial plane is, but on a C-130 it's 15 inHg. I don't have access right now to the charts that will tell how that relates to altitude.

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The actual altitude an airplane flys at will have a big effect on the cabin altitude. (inside pressure) The pressurization system in designed to only allow a certian maxim pressure differental between inside and outside the airplane. This is to pervent structural damage to the airplane. (hatches and window blowing out and rivits shooting off the plane wound be bad.) I don't know what the Max Differential is on a commercial plane is, but on a C-130 it's 15 inHg. I don't have access right now to the charts that will tell how that relates to altitude.

 

When you are flying above 18,000 ft. the altimeter pressure will always be set at 29.92. (Sea Level Pressure.) When descending below that altitude the pressure is set at the nearest reporting station pressure.

 

Altitude sickness does not occur at 10,000 ft. When flying a Pilot must use oxygen above 12,000 ft. (maybe 12,500) and made avaliable to passengers above 14,000 ft.

 

In a pressurized aircraft set your barometric pressure to 29.92 and the cabin altitude will be closer to actual.

:unsure:

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Altitude sickness does not occur at 10,000 ft.

 

Altitude sickness can occur at 8000'. I have seen it happen there twice. Generally 10,000' and above is where you may have real problems, especially over 12,000'. At very high altitudes strokes can be a problem in normal healthy people, or your lung filling up with fluid. One guy on Everest who had had laser eye surgery, even had that fail on him.

 

From Princeton:

"What is High Altitude?

Altitude is defined on the following scale High (8,000 - 12,000 feet [2,438 - 3,658 meters]), Very High (12,000 - 18,000 feet [3,658 - 5,487 meters]), and Extremely High (18,000+ feet [5,500+ meters]). Since few people have been to such altitudes, it is hard to know who may be affected. There are no specific factors such as age, sex, or physical condition that correlate with susceptibility to altitude sickness. Some people get it and some people don't, and some people are more susceptible than others. Most people can go up to 8,000 feet (2,438 meters) with minimal effect. If you haven't been to high altitude before, it's important to be cautious. If you have been at that altitude before with no problem, you can probably return to that altitude without problems as long as you are properly acclimatized.

 

What Causes Altitude Illnesses

The concentration of oxygen at sea level is about 21% and the barometric pressure averages 760 mmHg. As altitude increases, the concentration remains the same but the number of oxygen molecules per breath is reduced. At 12,000 feet (3,658 meters) the barometric pressure is only 483 mmHg, so there are roughly 40% fewer oxygen molecules per breath. In order to properly oxygenate the body, your breathing rate (even while at rest) has to increase. This extra ventilation increases the oxygen content in the blood, but not to sea level concentrations. Since the amount of oxygen required for activity is the same, the body must adjust to having less oxygen. In addition, for reasons not entirely understood, high altitude and lower air pressure causes fluid to leak from the capillaries which can cause fluid build-up in both the lungs and the brain. Continuing to higher altitudes without proper acclimatization can lead to potentially serious, even life-threatening illnesses. "

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You can set the GPS to use automatic calibration of the barometric altimeter.

If you do, it will use the GPS elevation to set the barometric elevation, when you turn on the unit. Then, the influence from the GPS elevation calculation will decay exponentially, but still have some impact, to correct for weather related pressure changes.

In reality, you get the sensitivity to small elevation changes from the barometric altimeter and the automatic function from the GPS elevation combined.

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OK - We now know the difference between the tow differen elevation/altitudes in the 60CSx. What I would love (Tell me I can do it and haven't figured it out yet) is to be able to add GPS elevation to one of my data fields so when I'm flying I can also see the altitude. It dodn't seem to be an option. Or, can I switch the unit between GPS elevation and pressure based elevation? Its a pain to go to the sat screen, press menu and select GOS elevation when I want to know how high I'm flying.

 

Maybe this should go into the what do you want to see from Garmin thread.

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OK - We now know the difference between the tow differen elevation/altitudes in the 60CSx. What I would love (Tell me I can do it and haven't figured it out yet) is to be able to add GPS elevation to one of my data fields so when I'm flying I can also see the altitude. It dodn't seem to be an option. Or, can I switch the unit between GPS elevation and pressure based elevation? Its a pain to go to the sat screen, press menu and select GOS elevation when I want to know how high I'm flying.

 

Maybe this should go into the what do you want to see from Garmin thread.

It has been an often requested feature for the Garmin models that have barometric sensors, but so far Garmin has not been willing to change the firmware to allow the GPS elevation to be shown continuously as a data field or for it to be used optionally as the altitude data in the recorded tracklog.

 

In reference to the comment above that:

The actual altitude an airplane flys at will have a big effect on the cabin altitude. (inside pressure) The pressurization system in designed to only allow a certian maxim pressure differental between inside and outside the airplane. This is to pervent structural damage to the airplane. (hatches and window blowing out and rivits shooting off the plane wound be bad.) I don't know what the Max Differential is on a commercial plane is, but on a C-130 it's 15 inHg.

While there will be some structural limit to pressure differences, all commercial airliners are able to maintain a pressure altitude well below 8000' while at their maximum cruising altitude and therefore this is not a limitation in practice. The pressure altitude inside the cabin is adjustable by the crew but should never exceed 8000' since that is the official limit based on health considerations for the passengers.

 

Note that the stated maximum differential of 15 "Hg would allow a sea level pressure altitude to be maintained inside even if the aircraft could somehow be taken into the vacuum of space since nominal atmospheric pressure at sea level is only 14.7 "Hg.

Edited by peter
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OK - We now know the difference between the tow differen elevation/altitudes in the 60CSx. What I would love (Tell me I can do it and haven't figured it out yet) is to be able to add GPS elevation to one of my data fields so when I'm flying I can also see the altitude. It dodn't seem to be an option. Or, can I switch the unit between GPS elevation and pressure based elevation? Its a pain to go to the sat screen, press menu and select GOS elevation when I want to know how high I'm flying.

 

Maybe this should go into the what do you want to see from Garmin thread.

As another poster has replied, this is an often requested feature that Garmin has turned a deaf ear to; conceptually, it should be such a simple change too!

 

As to logging the GPS elevation, there is one work-around, but it would be a big hassel: If you don't mind flying with your GPSr connected to a laptop, then you can log the tracks with nRoute; nRoute logs the GPS elevation in place of the barometric.

 

Another quick way of seeing the GPS altitude is to mark a waypoint, then average it; the initial mark of a waypoint records the barometric altitude, but when averaging it uses the GPS altitude. A nice side effect of this is you would then have a record of the location the time of the elevation reading.

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What about the active tracklog on the new "x" series that can be logged on the mem card? Wouldn't it be the GPS elevation that gets logged and not the Barometric elevation? The "non S" versions log elevation data as part of the active tracklog also.

 

I haven't done anything to prove it , but I "think"that the barometric elevation is just for "looking at" on the GPSr unit itself.

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I've complained to garmin a couple of times about the inability to ignore the baro. They keep saying they'll send it on to the programmers and occasionally they use user input. Whatever.

 

Anyways, the NMEA output seems to be GPS altitude (right now my baro's saying 740m, the gps is saying 675. Actual seems to be about 665 here - what's more accurate?). No way to display it on the unit, but at least it is output somewhere....

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What about the active tracklog on the new "x" series that can be logged on the mem card? Wouldn't it be the GPS elevation that gets logged and not the Barometric elevation? The "non S" versions log elevation data as part of the active tracklog also.

 

I haven't done anything to prove it , but I "think"that the barometric elevation is just for "looking at" on the GPSr unit itself.

 

No, the tracklog uses the baro altimeter. This actually isn't stupid because if you really care, you can go and shift it after the fact, but you can't really smooth it out after the fact.

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What about the active tracklog on the new "x" series that can be logged on the mem card? Wouldn't it be the GPS elevation that gets logged and not the Barometric elevation? The "non S" versions log elevation data as part of the active tracklog also.

 

I haven't done anything to prove it , but I "think"that the barometric elevation is just for "looking at" on the GPSr unit itself.

 

No, the tracklog uses the baro altimeter. This actually isn't stupid because if you really care, you can go and shift it after the fact, but you can't really smooth it out after the fact.

 

If that's true .......then what elevation is it that is included in the active tracklog from "Non S" units that don't even have a barometer/altimeter? With no sensors, it has to be GPS elevation.

 

And according to Garmin, in the "S" units, the barometric elevation is the one that gets "Corrected" by the GPS, not the other way around. (not GPS elevation corrected by baro)

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Note that the stated maximum differential of 15 "Hg would allow a sea level pressure altitude to be maintained inside even if the aircraft could somehow be taken into the vacuum of space since nominal atmospheric pressure at sea level is only 14.7 "Hg.

 

Technically, only half-way to space.

 

Standard atmospheric pressure = 14.7 psi = 760 mm Hg = 29.92 in Hg

 

Therefore, a 15 in Hg differential would represent half the mercury column or half the height of the atmosphere.

 

I'm not sure the point of this, but there it is.

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If that's true .......then what elevation is it that is included in the active tracklog from "Non S" units that don't even have a barometer/altimeter? With no sensors, it has to be GPS elevation.

 

And according to Garmin, in the "S" units, the barometric elevation is the one that gets "Corrected" by the GPS, not the other way around. (not GPS elevation corrected by baro)

 

It is true, the S units record barometric elevation in the track log, which is why I like having the barometric altimeter. If the barometer is properly calibrated, it provides a smoother and more realistic representation of elevation changes over the length of the track.

 

Other units (including my ol' GPS V) record GPS elevation as that is the only elevation data available. GPS elevation data is typically very "noisy." As Hurley point out, it is fairly easy to shift elevation after the fact, but correcting for a noisy signal is more challenging.

 

Not sure what you mean by "the barometric elevation is the one that gets "Corrected" by the GPS." Unless I have the unit set to auto calibrate as Apersson suggests, the elevation data is not corrected - it is what the barometric altimeter records.

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I've complained to garmin a couple of times about the inability to ignore the baro. They keep saying they'll send it on to the programmers and occasionally they use user input. Whatever.

 

Anyways, the NMEA output seems to be GPS altitude (right now my baro's saying 740m, the gps is saying 675. Actual seems to be about 665 here - what's more accurate?). No way to display it on the unit, but at least it is output somewhere....

 

If you want to ignore the barometer, why don't you just set your unit on "Fixed Elevation"? That way it forces the unit to use (record) the GPS elevation. At least that is my interpretation, based on my manual which says "Trip data is not recorded when in Fixed Elevation Mode". That's just another use for that setting, as a "disconnect" between GPS and Altimeter so that the Altimeter can function just as a Barometer.

 

"Not sure what you mean by "the barometric elevation is the one that gets "Corrected" by the GPS." Unless I have the unit set to auto calibrate as Apersson suggests, the elevation data is not corrected - it is what the barometric altimeter records.

 

My manual says"When On (Auto Calibration), elevation is corrected by GPS. " Personally, I manually calibrate the altimeter before leaving home (known elevation) or at some known elevation and always leave unit set on auto calibration to compensate for pressure fluctuations due to changing weather.

Edited by Grasscatcher
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From some of the conversations above out Altitude Illnesses makes me wonder if what the levels of oxygen are in an airplane. I have seen my GPS barometric altimeter report as high as about 8500 feet on an airplane but i have a feeling that the oxygen levels inside the plan are higher than they would be at 8500 on land. Any one know this?

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If you want to ignore the barometer, why don't you just set your unit on "Fixed Elevation"? That way it forces the unit to use (record) the GPS elevation. At least that is my interpretation, based on my manual which says "Trip data is not recorded when in Fixed Elevation Mode". That's just another use for that setting, as a "disconnect" between GPS and Altimeter so that the Altimeter can function just as a Barometer.

 

Nope. Even on "fixed elevation", the altitude reported on, say the trip odometer screen or wherever you've decided you want to see it is still the one reported by the barometric altimeter.

 

The ONLY way to get real time GPS calculated altitude out of Garmin units with barometric altimeters is by reading it off of the external data ports (and I can only say that NMEA has it, not sure about text or garmin proprietary formats).

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From some of the conversations above out Altitude Illnesses makes me wonder if what the levels of oxygen are in an airplane. I have seen my GPS barometric altimeter report as high as about 8500 feet on an airplane but i have a feeling that the oxygen levels inside the plan are higher than they would be at 8500 on land. Any one know this?

 

Not likely. Air is a mixture of about 21% oxygen, 78% nitrogen, the balance argon, CO2, organics, and traces of neon, krypton, xenon. The oxygen concentration remains essentially fixed thoughout the troposphere and stratosphere.

 

The only way the oxygen concentration could be higher is if the airline "goosed" it. They don't offer peanuts anymore, good luck with added oxygen.

 

I suspect that the airlines will allow the internal pressure of the cabin to drop because 1) it burns fuel to boost the pressure and 2) lower pressures will help make customers lethargic and a little sleepy. I doubt they will let the pressure drop to a point that it puts ANY customer's health in danger, the liability is just too great these days.

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Personally, I manually calibrate the altimeter before leaving home (known elevation) or at some known elevation and always leave unit set on auto calibration to compensate for pressure fluctuations due to changing weather.

 

Your post got me thinking.

 

Why leave the altimeter on auto-cal when you have a known elevation to calibrate to? By leaving in auto-cal you will effectively overwrite the known elevation with a “corrected” GPS elevation.

 

But then I started thinking about the effect of changes in barometric pressure on the reported elevation from the altimeter.

 

At elevations below 5,000 ft, each 100 ft increase in elevation is roughly equivalent to a 0.1 in/Hg drop in barometric pressure. Average ranges in barometric pressure are about 0.25 in/Hg vs standard atmospheric pressure of 29.92 in/Hg (with the extremes at +/- 0.5 in Hg). Therefore, if you calibrated your altimeter when the barometric pressure was at standard, and during the course of the day it increased to the top of the typical range, the altimeter would be reading “too low” by 250 ft.

 

What about the GPS? I have read that reported GPS uncertainty in the vertical can be 10X the uncertainty in the horizontal plane. In the bush, the typical EPE of my 60CSx is +/- 18 ft. Sometimes it is better, sometimes even worse. That implies a vertical uncertainty of roughly 180 ft too high or too low.

 

Good news for those in the Mile High city of Denver – at elevations over 5,000 ft each 0.1 in/Hg change corresponds to a smaller elevation change, so barometric pressure changes will have a smaller effect on the altimeter’s reported elevation.

 

My conclusions:

 

1. If you want accurate elevation data in your tracks you must calibrate the altimeter every time you use the GPS to account for changes in barometric pressure. If you don’t like this hassle, get the Cx rather than the CSx.

2. If you have a known elevation to calibrate to, use it. Leaving the auto-cal on is a toss up between accepting bias as the weather changes or accepting uncertainty by incorporating the GPS signal.

3. A corollary to the point above would be, on “steady” weather days, leave auto-call off, on “changing” days leave it on.

4. If you don’t have a known location to calibrate to, the auto-cal will give you reasonably reliable elevation data for most situations.

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A further complication. GPS altitude is expressed as elevation above the spheroid not above sea level. The spheroid is a theoretical approximation of the shape of the earth that is contained in the receiver and is used by the software to calculate great circle routes and other things. Since it is impractical to use an actual mathematical model of the Earth, geodesists have created a reasonable approximation of the earth called a standard spheroid. The shape of the standard spheroid is refined from time to time as more data becomes available. In some locations, the actual earth and the standard spheroid earth are spot on but in others it is not. Likewise, at some locations, the GPS elevation corresponds to altitude but at others it does not. If your altitude is important to you, you should calibrate the altimeter. This will give you the most accurate altitude possible but, as every pilot knows, accuracy can be a fleeting thing. The accuracy will deteriorate as soon as you enter it and will degrade with time and movement away from the calibration location. That’s why pilots re-calibrate their altimeters each time they move to a new location lest the ground rise up and smite them.

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Nicely put (last post).

 

geoid2.gif

 

The accuracy of GPS height measurements depends on several factors but the most crucial one is the "imperfection" of the earth's shape. Height can be measured in two ways. The GPS uses height (h) above the reference ellipsoid that approximates the earth's surface. The traditional, orthometric height (H) is the height above an imaginary surface called the geoid, which is determined by the earth's gravity and approximated by MSL. The signed difference between the two heights—the difference between the ellipsoid and geoid—is the geoid height (N). The figure above shows the relationships between the different models and explains the reasons why the two hardly ever match spatially.

 

http://www.esri.com/news/arcuser/0703/geoid1of3.html

Edited by EraSeek
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"Precise measurements taken from space are applied to GPS readings. These measurements are based on an ellipsoid surface that is a mathematically generated model of the earth obtained from a three-dimensional Cartesian coordinate system. The GPS receivers can provide only the ellipsoid (geometric) height.

 

However, most users expect accurate elevation readings that are related to MSL. Consequently, newer GPS devices output orthometric (geoid) height measurements as a product of "behind the scenes" calculations based on a combination of formulas, tables, and matrices that use geographic coordinates as inputs. The appropriate height for the geographic location taken from a coarse or fine DEM matrix is provided instead of a real measurement of the z value (or height). Some receivers use approximations of the geoid height to estimate the orthometric height from the ellipsoid height. Still other units, based on an older technology, provide direct readings of the z value based on the ellipsoid."

 

So I wonder which units use this calculated MSL and which ones use the (gps) elipsiod height?

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Another source. So I think we can assume that our GPS's think in ellipsoid, but then calculate locally and display in orthometric (MSL). So what we see SHOULD be the same as posted topo elevations.

 

"A minimum of four GPS NAVSTAR satellites (more can be used in overdetermined PVT solutions) are used to determine three position coordinates and time. Position coordinates are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ) coordinates. Most receivers compute (and store) positions in geodetic coordinates (datum WGS-84) latitude, longitude and height above the ellipsoid (HAE). Note - height calculations are independent of any user selected horizontal datum.

 

The height displayed on most consumer handheld GPS receivers is orthometric height, the height above mean sea level (MSL). It is straight forward to approximate MSL world wide by interpolation of the GEOID model (table) and making the simple calculation: "

 

http://www.edu-observatory.org/gps/height.html

Edited by EraSeek
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So I wonder which units use this calculated MSL and which ones use the (gps) elipsiod height?

AFAIK, all current Garmin, Magellan, and Lowrance models report height above the geoid which is the surface that closely approximates MSL. The old Lowrance GM100 didn't include the ellipsoid vs. geoid correction but I believe their newer models do so. There were also issues with units that used the early SiRF chips (I and II) which didn't properly correct for the geoid.

 

If you set your receiver to output NMEA sentences and look at the data stream with hyperterminal or other programs you should see periodic output of the $GPGGA sentence. The 8th data field in this sentence indicates the difference between geoid and ellipsoid height being applied by your receiver at the current location.

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Thank you Peter for confirming that. I think many people in the community have heard about ellipsoid height and think their display is showing that, and thus consider their elevation "off" from traditional elevation because of this. I always love to learn new things and I have learned some more here.

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