Altitude Discrepancies

[Michael]

yesterday morning on my way back from So Cal, while using Flight following, I noticed my altimeter was showing 9500 give or take a foot or two but my GPS showed 10004 or something close to that. I always reset the altimeter when ATC gives new settings and figured they would say something if I were 500' off altitude, but they never did..also when I landed the altimeter was right on the money (I forgot to check the GPS after landing). I guess my question would be...what would cause the GPS to show an altitude off by 500', and if it can give that error, what's to stop it from moving me 500' to the west etc.. the model in question was a handheld, not for IFR use. But I was always under the impression that a GPS would be more accurate than the altimeter, because of the difference between settings from point a to b.
If the GPS grabs its info from 3 or more satellites, you would think that data would be pretty accurate. if this were just a few feet in question I wouldn't think twice about it, but 500 feet, makes me wonder now which one was lying to me.

Michael

 

[poadeleted3]

Someone else will have to post the math for you, and I'm sure they will, but gps has never been extremely accurate for altitude use. That's why WAAS had to be invented.

 

[Jeff Oslick]

Someone else will have to post the math for you, and I'm sure they will, but gps has never been extremely accurate for altitude use. That's why WAAS had to be invented.

Joe is correct. GPS altitude information, unless adjusted using WAAS or another technology to supplement the vertical accuracy, is not very reliable, no matter how many satellites your receiver is picking up. This is a good example of precision vs. accuracy. The GPS knew you were at "precisely" 10,004 feet, and it was completely inaccurate at the same time.

Jeff

 

[ErikU]

The resolution for GPS altitude isn't very good, though I would expect something much better than 500ft. Also, if flying VFR, they usually don't say anything to me if I am off altitude. Though you are supposed to report your altitude, unless it was assigned by them, they don't seem to care that much.

If it happens again, ask them what they see for your mode C, but keep in mind that doesn't mean much either because the mode C is derived from the same source as your altimeter.

-Erik

 

[SkyHog]

maybe I'm way off here - but the GPSes I have been using have an altimeter setting too - and if its not set right, the altitude doesn't display correctly.

These are older Garmins tho...

 

[grattonja]

maybe I'm way off here - but the GPSes I have been using have an altimeter setting too - and if its not set right, the altitude doesn't display correctly.

These are older Garmins tho...

The KLN 94 IFR approach certified GPS that I fly has that same altimeter setting control, so it is not just older GPS units that you can set altitude into.

Jim G

 

[mikea]

1) GPS altitude is TRUE altitude, not barometric altitude, above sea level in reference to the center of the satellite's orbit, which should be the center of the earth, but it isn't (see #2 below),

2) The idea of TRUE is in reference to a few different models of the "geoid," The models have to fudge an average to get an idea of where "ground" is. It is NOT the height above the surface of the earth - including sea level. The geoid is is not a sphere. The geoid is not exactly where sea level is. Where the real sea level is determined by gravitational abnomalities.

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

BUT to some extent what we call MSL now, the barometric altitude is based on the same inaccurate models with more fudging. We already know that the altimeter setting where my plane is is going to be slightly off from what the ATC is telling us to use - which was taken at a point in time earlier at a place not where we are.

3) GPS receivers need WAAS to get good accuracy in the Z-axis. They are much more accurate in the other two axi.

As was mentioned, some GPS receivers have also barometric altitude capability so they they can reference the number that you and ATC expect.

You wanted an easy explanation?

 

[ejensen]

Unless the atmosphere is absolutely standard in all aspects; temp, humidity & pressure, the altimeter not be accurate the minute you leave the ground where you set it. Might be quite a bit less than true MSL depending on conditions.

My observations of GPS altitude and known MSL points is within 200' usually less than 100'

A combination of both error sources can easily explain 500' difference.

Eric

 

[gismo]

The KLN 94 IFR approach certified GPS that I fly has that same altimeter setting control, so it is not just older GPS units that you can set altitude into.

IFR GPS units are required to input the pressure altitude from your transponder encoder and compensate for SLP (via user entered SLP) to supplement the GPS position accuracy (knowing the altitude allow the GPS receiver to reject some incorrect solutions). This is what most receivers display for altitude and in many cases will only show altitude in 100 ft increments. In addition the GPS receiver can calculate a GPS derived altitude but as others have said, the accuracy is poor without WAAS. The first should be labled barometric altitude and the latter GPS altitude on the display (or at least in the manual).