AGL question

[Bdflies]

On a recent flight, I noted that the "AGL" was indicating 12,700 while the baro altimeter indicated 12,000. Am I right in assuming that AGL is GPS derived and accounting for topography? If so, then my altimeter is off by nearlly 1,000' at that altitude?
If I'm not understanding this correctly, then school me please. If I'm understanding this correctly, then I need to take a close look at the static system on my RV14, right??

Bill

[Jonathon]

Am I right in assuming that AGL is GPS derived and accounting for topography?

That is correct.

If so, then my altimeter is off by nearlly 1,000' at that altitude?

This is not a necessary conclusion of the data. Not sure why you would assume a GPS derived altitude source to be more authoritative than a precision barometric altimeter. The AGL reading on our display is supplementary information for pilot awareness.

If I'm not understanding this correctly, then school me please.

While an error of this magnitude does sound excessive, consider all the possibilites for measurement errors in both of these systems:

- errors in synthetic vision terrain height owing to the interpolation between the sampling points of the digital elevation model. This error will be especially pronounced in rugged terrain.
- errors in GPS altitude due to integrity of GPS fix
- errors in baro altitude due to pressure gradient from where last entered baro setting was taken ("From a high to a low, look out below!")
- errors in baro altitude due to static system or ADAHRS pressure sensor errors

If multiple such errors exist, and all go the wrong way, a descrepancy on this order is possible. While it certainly may be the case, the data is not there to conclude that the fourth category must be what is going on here.

If I'm understanding this correctly, then I need to take a close look at the static system on my RV14, right??

Have you passed a recent IFR cert? Does your altimeter read field elevation on the ground and with your baro set to the ATIS report for that field?

[Bdflies]

Yes, the altimeters do reflect field elevation, on the ground. I should have mentioned that I have the AFS ADHARS and the G5 (independent ADHARS), that display similar baro altitudes, both seeing the same static source. The issue really never occurred to me, until this high flight. Looking at photos of previous flights, the 'high AGL' is a constant, amplitude depending on altitude. Higher = more error. I’m flying over S. Louisiana, so no mountainous topography.
I’ll have an IFR cert done next week.

"The AGL reading on our display is supplementary information for pilot awareness."

Understand. Please bear with me, in that an instrument displaying 1,000’ error is cause for investigation. Over Death Valley, I could be higher AGL than MSL, but not over Louisiana.

"Not sure why you would assume a GPS derived altitude source to be more authoritative than a precision barometric altimeter."

I’m thinking that way, because the precision barometric altimeter is totally dependent upon the 'static pressure' source. I fully accept the accuracy of the instruments. I'm questioning the static source. Also, my daily experience with GPS (I'll allow RTK enhanced) is demonstrated accuracy under 6”. Well, it’s closer to 2", actually.

I'll do the IFR cert and report back.

Thanks, Jonathon, for your attentiveness!

Bill

[Jonathon]

Forgot to include temperature as another source of errors in baro altitude readings. In a nonstandard atmosphere the indicated altitude will be off, you will be higher than indicated in a hotter than standard atmosphere, lower than indicated in a colder than standard atmosphere. Never been, but I imagine Louisiana must be rather hot this time of year.

Since we enter altimeter settings based on ground level readings adjusted to sea level pressure, this error will grow as you get higher and thus further away from where the reading was directly applicable. Above 17,999 is where we officially no longer care about this error and just set altimeter to 29.92 so all class A traffic is at least on the same page. But above 10,000 or so, the errors are still there, and noticeable, even as we continue to go through the motions of adjusting our altimeters to the nearest station readings.

[Bdflies]

To add information; I've had the IFR staticcheck done. The altimeters are no more than 10’ off. I’ve flown the 8 leg test patterns and determined that my static system is not a problem.
The most interesting thing I’ve learned is that the phenomenon of "higher AGL then MSL' is not limited to AFS equipment. Discussions with folks flying Garmin equipment revealed that the phenomenon is identical. The closer to ground level you are, the more accurate the AGL number becomes. Higher up, the error is pronounced.
I guess the good thing, is that I don’t have a problem. The troublesome thing is that something is not quite right and it’s very consistent.

[Jonathon]

Had occurred to me that a much better way to think of this than appealing to what is and is not "authoritative", as I wrote earlier, is instead the old precision vs. accuracy yarn.

GPS altitude is accurate, but not precise, at least not as precise as an IFR certified barometer.
Baro altitude is precise, but not necessarily accurate. It will unfortunately never be economically viable to release as many weather balloons as we would need to to have timely, direct readings for the entire column of air, everywhere there are ground stations.

Precision is what keeps airplanes from smashing into each other, accuracy is what keeps airplanes from smashing into the ground. So if you're ATC trying to keep two airplanes from smashing into each other, precision matters a great deal. Since smashing into the ground is also bad, accuracy with respect to MSL elevation is important, but it is of diminishing importance as you get higher. Again, why altimeters get set to 29.92 above 17,999. At this point you are so far above all CONUS and nearly all North American terrain that having an accurate altimeter value is just not important anymore, while having a precise altimeter value emerges as more important than ever.