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07-10-2013, 07:37 AM
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Join Date: Nov 2007
Location: NJ
Posts: 1,747
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GPS altitude
I've noticed a difference between my altimeter and my GPS altitude. I use a portable Garmin Aera 560 that is not wired to my encoder or anything like that. I never really give it much thought because I only fly VFR. I've been thinking perhaps I should clarify that difference.
I would like to discuss the following scenario in an effort to fully understand what my little box can and can't do.
You are flying VFR over the top. Beautiful up there and only another 40 miles til clear skies. Your trusty engine stops... You troubleshoot as much as you can but you cannot restart. You are sinking into the clouds below. Reported ceilings are less than 500'. You know you will be making an off field landing. You are confident you can hold the aircraft straight and level as you are descending at best glide through the clouds. As if you aren't rattled enough, the GPS starts screaming "OBSTACLE" or "TERRAIN". You were savvy enough to switch to the "Profile" view of your GPS but since the GPS altitude is different than your actual altitude, you're not sure what to trust. How do you decide which way to make corrections to give you the best odds of survival?
(This same discussion could be used if you got caught scud running, I don't want to discuss the decision to go VFR on top or VFR in low weather conditions. I want to discuss options and ways to use the equipment/information that is available should we ever find ourselves in such a dangerous situation)
I know these situations should be avoided at all costs but obviously they DO HAPPEN. I think it's better discussed now than to try and figure it out under duress.
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My ATC opinion is NOT an official FAA recognized opinion, so any advice you get from me is ONLY my opinion.
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07-10-2013, 07:55 AM
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Join Date: Apr 2005
Location: KSLC
Posts: 4,021
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Since most of my flying was in mountain regions, and the fact that I'm a GPS fanatic, I did a lot of experimentation with GPS altitude versus the altimeter. At times, there could be several hundred feet difference. To avoid a solid rock, I'll take the GPS altitude......as long as WAAS is included.
I used XM Satellite weather, which provides altimeter settings as you fly cross country. Their a bit of a compromise between lows & highs, but still a good indication for a current reset of the alimeter. When changing the altimeter to the new setting, I could watch my Garmin six pac altimeter, and the planes altimeter exactly follow each other for a few minutes, then they slowly drift apart. Get a current altimeter setting, and the same happens again. Over & over. I also had a lot of time to compare GPS altitude readouts, to those of various mountain ranges. Given the fact, that barametric pressure is usually on the change, I'll trust the GPS (with WAAS)if needing to make a choice.
L.Adamson
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07-10-2013, 08:00 AM
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Join Date: Mar 2006
Location: Newport, TN
Posts: 7,496
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In the situation you mention, I would use a WAAS GPS and Synthetic Vision any day over the altimeter!
Last edited by Brantel : 07-10-2013 at 08:45 AM.
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07-10-2013, 08:12 AM
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Join Date: Jan 2005
Location: Richmond, VA
Posts: 696
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TK, Two lines of reasoning come to mind but they lead to similar places. Which altimeter to trust seems to me a pointless exercise.
First, in my experience the altimeter reading from a sensitive altimeter is usually pretty accurate if you have a reasonably close airport to set it to. I pay attention when setting this at my home field and I'd guess a 10-15' error is typical.
The altimeter reading from a GPS seems to vary a lot, both in precision and accuracy. I'm using a Garmin Aera 510 and an iFly 720, both WAAS enabled and I figure I'm usually within 100'.
So, for absolute altitude, I put more trust the sensitive altimeter, but for terrain, I'd be looking at the GPS to lead me to the lowest terrain. The bottom line is knowing your absolute altitude is a lot less important than knowing where the mountain is and where the terrain falls away. The terrain info might save your life. All I can imagine knowing which one is closer to telling the truth will do will let you calculate how long until you'll be on the ground (should I say a short prayer or go for a longer one?).
The second line of reasoning assumes you don't have a decent setting for your altimeter. In this case, don't trust either source of altitude information. There's no need to trust anything - all they're telling you is when they think you'll encounter terra firma. The fact is you'll encounter it when you arrive and since you have no power, you've trimmed for best glide, there's no way to delay that encounter any further. So who cares which one is right (or more accurately, closer - since neither one is likely right)? At this point I'd follow the GPS terrain data to find the lowest terrain until I broke out of the clouds. At that point I'd shift gears and use the remaining altitude to find the most suitable landing spot.
....finally, I hope you get the discussion your seeking and avoid the comments you're seeking to avoid. There's a lot to learn here and I'm looking forward to seeing other opinions.
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Don Alexander
Virginia
RV-9A 257SW Purchase Flying - O-320, Dynon D100
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07-10-2013, 08:15 AM
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fugio ergo sum
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Join Date: Apr 2006
Location: Carlsbad, NM
Posts: 1,912
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Quote:
Originally Posted by tkatc
I've noticed a difference between my altimeter and my GPS altitude...
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The two devices are measuring entirely different things. The altimeter is displaying a number based on a model of the atmosphere that is unlikely to correspond to the actual atmosphere very often, if ever. The altimeter should show close to actual elevation when it is at ground level at the spot the altimeter setting is for. The GPS uses geometry to calculate altitude. Inherent GPS errors limit the accuracy of that method. The altimeter altitude is likely to get further and further off true altitude the higher one goes. I suspect this is why IFR terrain clearance rules are different in "mountainous" terrain.
Quote:
Originally Posted by tkatc
GPS altitude is different than your actual altitude, you're not sure what to trust. How do you decide which way to make corrections to give you the best odds of survival?
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Altimeter altitude is the one to use for air traffic separation and MAY be more accurate at low altitudes right by the airport the altimeter setting is for but in general for any kind of terrain or obstacle clearance I would rely on the GPS altitude
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Larry Pardue
Carlsbad, NM
RV-6 N441LP Flying
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07-10-2013, 08:27 AM
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Join Date: Aug 2010
Location: SoCal
Posts: 36
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AIM 1-1-19: "GPS altitude should not be relied upon to determine aircraft altitude since the vertical error can be quite large and no integrity is provided.?
Actually, GPS altitude is fairly accurate. True, it is the least accurate of the measurements provided by GPS. However, it is still very accurate in relation to what it actually measures. It measures altitude above the theoretical center of the earth as provided by the algorithm that is used to determine such things. It is not exact, but as it is not influenced by the greatly varying temperature and pressure, it is actually MORE accurate when used for terrain clearance than a standard altimeter (provided an up to date database). It should not be used for maintaining assigned altitude by ATC, as they are using a pressure based system, but should the GPS freak out and say you are about to hit terrain ahead, you would be wise to heed the warning even if your pressure based altimeter is showing you several hundred feet above said obstacle.
Pressure altitude is often not accurate due to several factors, including temperatures that are different than standard temperature and distance from the reporting station (vertically and horizontally), but it tends to be precise enough for our purposes.
This means that by flying by the indicated numbers on your altimeter, everyone may not be flying at the true altitude, but we are all flying at the wrong altitude together.
Hopefully, no one uses TAWS to push the envelope.
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07-10-2013, 08:32 AM
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Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
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A non-aviation GPS accuracy description...
http://gpsinformation.net/main/altitude.htm
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
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La Cholla Airpark (57AZ) Tucson AZ
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07-10-2013, 08:32 AM
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Join Date: Nov 2007
Location: Lincoln, CA
Posts: 138
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GPS altitude is the geoid height above a mathematical ellipsoid that approximates the earth's surface but usually does not match the actual sea level elevation.
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07-10-2013, 09:00 AM
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Join Date: Nov 2005
Location: KRTS
Posts: 1,798
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Quote:
Originally Posted by aerovin
GPS altitude is the geoid height above a mathematical ellipsoid that approximates the earth's surface but usually does not match the actual sea level elevation.
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This. The earth is not perfectly round, it's oblong. However the GPS system has to use a perfect sphere for calculating GPS altitude, also known as "Height Above Elipsoid."
So you can have MSL, AGL, and now HAE however HAE is just your height above the GPS "center of the earth/sphere."
In some places on the planet it's not that big of a difference, in other places it can be a couple hundred feet. It's a big mission planning factor in GPS guided weapons.
As far as synthetic vision/HAE/MSL/AGL... I believe the terrain data is referenced off of HAE, so you SHOULD be ok using it to avoid cumulous granite, obviously the closer you get the more likely CFIT is. It's a strategic tool, not a tactical one. Where's our resident G3X programmer?
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07-10-2013, 09:23 AM
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Senior Curmudgeon
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 15,420
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Quote:
Originally Posted by Don
At this point I'd follow the GPS terrain data to find the lowest terrain until I broke out of the clouds. At that point I'd shift gears and use the remaining altitude to find the most suitable landing spot.
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This is what I would do for sure.
When I got my first GPS, a Garmin 196, I tested the altitude mode by setting it on a porch railing and checking the altitude------it was roaming around a couple hundred feet up and down just sitting there over a period of 10 minutes or so.
I have no idea if the new GPS units are better than my old 196, but who knows??? Just something to consider.
Quote:
Originally Posted by Don
....finally, I hope you get the discussion your seeking and avoid the comments you're seeking to avoid. There's a lot to learn here and I'm looking forward to seeing other opinions.
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Lot to hope for  thread drift has a pretty solid history here.
__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011 
Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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