Difference in GPS altitude and density altitude?

bflynn

bflynn

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Brian Flynn
last flight, there was a big difference (over 500’) between the altimeter altitude and the GPS altitude. The altimeter as adjusted to the current barometer and close to known altitude on the ground.

Anyone know why?p the big difference? Is GPS altitude just wrong?
 
GPS and pressure altitude are measuring different things. GPS is measuring a geometrically determined altitude which is accurate to 20 meters or so depending on received satellite geometry. (It could be a LOT worse depending on satellite geometry.) The pressure altitude reported by your altimeter assumes a standard atmosphere (temperature and pressure lapse rate) based on 29.92" Hg at MSL. If the atmosphere is warmer or colder, or has a temperature/pressure gradient that is non-standard, the two altitudes will not agree, which is much of the time. For traffic separation, it doesn't matter if pressure altitude is perfectly accurate as long as everyone in the area is using the same setting. The RELATIVE accuracy of pressure altitude between different aircraft will be extremely close, on the order of the allowed altimeter calibration tolerance, which is on the order of what, 10-20 feet for a given pressure?
 
So GOS altitude can be off by several hundred feet? I think I saw 700 on a recent flight? Could be bad memory, it was significantly different.
 
Depends on the datum plane and the height units. Are you talking about GPS altitude in Mean Sea Level? Or is it showing Height Above Ellipsoid (HAE)? Is it using the WGS84 or WGS96 datum? Multiple factors that could all give an entirely different displayed number derived from a GPS solution, all totally valid. Just depend on your settings. Also, not at all related to a barometric alt. Differences can easily be several hundred feet off from each other, for reasons others have mentioned.

None of those numbers are wrong. Just depends on what answer you're looking for. I realize that all sounds like quibbling and wishy washy answering, but there are so many variables, different device settings, and different GPS constellation solutions it's tough to give a straight answer.

Short answer? I almost always use baro altitude as that's what other aircraft, ATC, and charts are all using. GPS alt is interesting, but rarely relevant.
 
Schokie said:
Short answer? I almost always use baro altitude as that's what other aircraft, ATC, and charts are all using. GPS alt is interesting, but rarely relevant.
Click to expand...

I’ve never thought about this before, but I always assumed the VFR chart contour maps are actual surveyed elevations, that would match any topo hiking map you might buy at REI. Which I would also imagine would match closely with GPS readings.

Is that incorrect? If so, how are VFR chart elevations worked out?
 
What you’re forgetting is that the altimeter does not measure altitude at all. It measures air pressure at the static port on the side of your airplane and then presents that air pressure as an indicated altitude. The Kollsman window allows you to correct for the nonstandard pressure at sea level from which the altimeter will measure. But the device cannot measure anything but air pressure, so the indicated altitude it presents will be affected by nonstandard lapse rate/temperature. (Note that it will never display density altitude, and will only display pressure altitude if you set it to 29.92.)

Meanwhile, the GPS calculates your true altitude based on data encoded in radio signals from satellites and calculations that it does based on the time delay in receiving that data. I have no reason to doubt chemgeek’s information about GPS having a 20 m (66 ft) precision in altitude.

I broke out my E6-B for this next part. Let’s assume I am cruising over the top of my house at an ATC-assigned altitude of 6,000 feet, which is precisely where my altimeter’s needles are pointing. The forecast soundings for where I am show a temperature of 22C at 6,000 feet. The METAR altimeter setting is 29.74. The E6-B inputs to find approximately true altitude are pressure altitude, temperature, and indicated altitude. From my altimeter setting and indicated altitude, I get a pressure altitude of approximately 6,180 feet. From these inputs, the E6-B says that my true altitude is approximately 6,600 feet. The GPS would try to calculate my true altitude, and end up showing a figure about 600 feet above my indicated altitude, plus or minus the GPS error. (This has nothing to do with the density altitude, which would be about 8,000 given those conditions.)

In other words, your GPS is not “off by several hundred feet.” It is just measuring something totally different from your altimeter and calculating a different measurement of your altitude. The GPS calculates true altitude from the difference between the transit times of data signals from several satellites. The altimeter calculates indicated altitude from the difference between the static air pressure and the sea level pressure dialed into the Kollsman window. Between the two, the GPS is closer to true altitude. But the altimeter is what you should be looking at because, when ATC and pilots say “altitude,” they mean indicated altitude unless they specify otherwise.

Indicated altitude is what we use for ATC altitude assignments, traffic separation, the east/west rule for VFR and IFR cruising altitudes, airspace floors and ceilings (other than the class G airspace ceilings based on AGL), and most other things we do. The exceptions that come to mind are terrain (use true altitude to go over mountains), aircraft performance (density altitude), and oxygen requirements (cabin pressure altitude).

If you haven’t read it, you should download the PDF of the Pilot’s Handbook of Aeronautical Knowledge from the FAA. My knowledge of how altimeters work comes from Chapter 8 of the PHAK and was tested on the private pilot knowledge test. So far, I have not found myself crashing a plane for want of knowledge of how altimeters work, so the PHAK seems to be a good enough source.
 
chemgeek said:
GPS and pressure altitude are measuring different things. GPS is measuring a geometrically determined altitude which is accurate to 20 meters or so depending on received satellite geometry. (It could be a LOT worse depending on satellite geometry.) The pressure altitude reported by your altimeter assumes a standard atmosphere (temperature and pressure lapse rate) based on 29.92" Hg at MSL. If the atmosphere is warmer or colder, or has a temperature/pressure gradient that is non-standard, the two altitudes will not agree, which is much of the time. For traffic separation, it doesn't matter if pressure altitude is perfectly accurate as long as everyone in the area is using the same setting. The RELATIVE accuracy of pressure altitude between different aircraft will be extremely close, on the order of the allowed altimeter calibration tolerance, which is on the order of what, 10-20 feet for a given pressure?
Click to expand...

BS. The above response is nothing but fluff and fill. See post #2 for the correct answer. ;)
 
woodchucker said:
I’ve never thought about this before, but I always assumed the VFR chart contour maps are actual surveyed elevations, that would match any topo hiking map you might buy at REI. Which I would also imagine would match closely with GPS readings.

Is that incorrect? If so, how are VFR chart elevations worked out?
Click to expand...
The ground elevations on charts are true altitude, or at least as close to true altitude as the surveyor could get with whatever equipment he had at the time. If you read the chart’s explanation of how the MEF numbers are determined, though, you will find that the charts have a huge fudge factor built in to keep you from hitting anything even when your altimeter is indicating much higher than true altitude.
 
Rgbeard said:
I haven't seen anyone discuss temperature here, yet. Isn't that one of the #1 variables for density altitude?
Click to expand...
The title says "density altitude" and one would definitely expect a difference between that and GPS altitude (I said "huh? to myself when I first read it). But the post itself only asks about the difference between GPS altitude and indicated altitude.
 
bflynn said:
So GOS altitude can be off by several hundred feet? I think I saw 700 on a recent flight? Could be bad memory, it was significantly different.
Click to expand...
Actually, indicated altitude can be off by several hundred feet. That's one of the reasons for "Cold Temperature Restricted Airports." You know, the old "high to low, look out below" stuff.
 
Last edited:
GPS, due to the satellite geometry, is inherently less accurate for altitude than for horizontal position.

Here is an altitude track recorded by a GPS with pressure sensor:
upload_2019-9-16_6-32-37.png

And here is a [different] flight, recorded by a GPS with no pressure sensor:
upload_2019-9-16_6-33-43.png
 
I took this pic of the altimeter and screenshot recently to show exactly this difference. Notice the indicated altitude is 19,000, while the GPS altitude is 20,328 - a 1,328 ft difference!

This has nothing to do with GPS accuracy or altimeter accuracy. It has everything to do with all the assumptions we make regarding the atmosphere. We assume standard temperatures, standard lapse rates, etc., when the atmosphere is rarely standard. And the error increases with altitude above the reporting station. But as long as we're all using the same altimeter setting, it doesn't matter.

Now, this was in Class A, so my altimeter was set to 29.92. However, the actual altimeter setting was not far off from that - 29.94 if I remember correctly.

2019-08-09_15-05-58.png
 
RussR said:
I took this pic of the altimeter and screenshot recently to show exactly this difference. Notice the indicated altitude is 19,000, while the GPS altitude is 20,328 - a 1,328 ft difference!

This has nothing to do with GPS accuracy or altimeter accuracy. It has everything to do with all the assumptions we make regarding the atmosphere. We assume standard temperatures, standard lapse rates, etc., when the atmosphere is rarely standard. And the error increases with altitude above the reporting station. But as long as we're all using the same altimeter setting, it doesn't matter.

Now, this was in Class A, so my altimeter was set to 29.92. However, the actual altimeter setting was not far off from that - 29.94 if I remember correctly.

View attachment 77828
Click to expand...

Texas in August in the afternoon. Looks about right
 
bflynn said:
last flight, there was a big difference (over 500’) between the altimeter altitude and the GPS altitude. The altimeter as adjusted to the current barometer and close to known altitude on the ground.

Anyone know why?p the big difference? Is GPS altitude just wrong?
Click to expand...

Chemgeek nailed it. GPS altitude is measured above a mathematically derived surface that you have no instrumentation for. It is not above sea level or above ground level. Do not compare it to anything terrestrial.

Bob Gardner
 
Until the GTX-345 was installed in my plane with the OAT sensor option, I focused on density alt for both the departing and arriving airport and not much in route as conditions change.

A recent flight in the Joshua Tree area of California show GPS alt 11,600; Pressure Alt 11,300; Density Alt 13,800. Nice seeing all three in realtime.
 
There are four things at play here.

First, your GPS unit is reporting height above the ellipsoid most likely. In order to have an accurate geoid requires more memory than most GPS units had.
Second, GPS positions are less accurate in the vertical plane than horizontally because of the way the geometry of the visible satellites is.
Third, your itself altimeter is providing only an approximation of the geoid altitude based on assumptions about standard pressure lapses and what it's been set to.
Fourth, density altitude involves things not reflected by either your altimeter itself nor the GPS. It relies on not only pressure but temperature (and to a small extent) humidity.
 
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