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#1
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How come GPS reads different
How come the GPS reads a different altitude than the Altimeter?
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#2
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Aluckyguess wrote:
How come the GPS reads a different altitude than the Altimeter? For many reasons. The main reason is that they derive altitude by two completely different methods and this inevitably introduces errors. Then each method has its own inherent errors. Even two altimeters side by side won't read the same altitude other than by accident. Matt |
#3
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I am getting over 500ft. difference and more.
"Matt Whiting" wrote in message ... Aluckyguess wrote: How come the GPS reads a different altitude than the Altimeter? For many reasons. The main reason is that they derive altitude by two completely different methods and this inevitably introduces errors. Then each method has its own inherent errors. Even two altimeters side by side won't read the same altitude other than by accident. Matt |
#4
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"Aluckyguess" wrote in message ... I am getting over 500ft. difference and more. "Matt Whiting" wrote in message ... Aluckyguess wrote: How come the GPS reads a different altitude than the Altimeter? For many reasons. The main reason is that they derive altitude by two completely different methods and this inevitably introduces errors. Then each method has its own inherent errors. Even two altimeters side by side won't read the same altitude other than by accident. Matt The number of sattelites being received is one of the variables -- how many are you registering on the GPS? What brand/model is the GPS. Is the GPS certified for navigation? Does the GPS have a Kohlsman equivalent input? What are the altitude specifications of the GPS? Are you comparing it to an altimeter in an airplane? Is the calibration data on the altimeter valid? Have you compared the altimeter to the GPS while parked at a known reference (i.e., the benchmark of your local airport)? My Magellan handheld takes about five mintues of averaging to get a reasonably close altitude correlation -- how much time are you giving yours? Bottom line, forget the GPS altitude reading, dial in the local pressure data into the airplane's altimeter and trust it. |
#5
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Its a 296. It seems the like the higher I go the farther off it is.
I am one to think the GPS would be more accurate than the altimeter. I was thinking that we get incorrect altimeter settings "Casey Wilson" N2310D @ gmail.com wrote in message news:x1Pqe.2896$9a1.185@trnddc01... "Aluckyguess" wrote in message ... I am getting over 500ft. difference and more. "Matt Whiting" wrote in message ... Aluckyguess wrote: How come the GPS reads a different altitude than the Altimeter? For many reasons. The main reason is that they derive altitude by two completely different methods and this inevitably introduces errors. Then each method has its own inherent errors. Even two altimeters side by side won't read the same altitude other than by accident. Matt The number of sattelites being received is one of the variables -- how many are you registering on the GPS? What brand/model is the GPS. Is the GPS certified for navigation? Does the GPS have a Kohlsman equivalent input? What are the altitude specifications of the GPS? Are you comparing it to an altimeter in an airplane? Is the calibration data on the altimeter valid? Have you compared the altimeter to the GPS while parked at a known reference (i.e., the benchmark of your local airport)? My Magellan handheld takes about five mintues of averaging to get a reasonably close altitude correlation -- how much time are you giving yours? Bottom line, forget the GPS altitude reading, dial in the local pressure data into the airplane's altimeter and trust it. |
#6
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Aluckyguess wrote:
Its a 296. It seems the like the higher I go the farther off it is. I am one to think the GPS would be more accurate than the altimeter. I was thinking that we get incorrect altimeter settings The altimeter is only "accurate" when the temperature profile of the airmass is equal to the ISA standard atmosphere. The rest of the time, there will be some error which increases with altitude (wrt wherever the source for the altimeter setting is located). This isn't a concern for en-route operations since everybody's altimeter has the same "error". It can be a concern for terrain clearance in the mountains or in some cases instrument approaches in cold weather since the altimeter reads higher than the plane actually is. |
#7
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On Sat, 11 Jun 2005 20:54:01 -0700, "Aluckyguess"
wrote: I am getting over 500ft. difference and more. Oh,. good grief! I'm sure I've never had as much as 100 ft of variance, and more often it's 20 feet or so. (Garmin 296) -- all the best, Dan Ford email (put Cubdriver in subject line) Warbird's Forum: www.warbirdforum.com Piper Cub Forum: www.pipercubforum.com the blog: www.danford.net In Search of Lost Time: www.readingproust.com |
#8
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On Sun, 12 Jun 2005 06:27:43 -0400, Cub Driver wrote:
On Sat, 11 Jun 2005 20:54:01 -0700, "Aluckyguess" wrote: I am getting over 500ft. difference and more. Oh,. good grief! I'm sure I've never had as much as 100 ft of variance, and more often it's 20 feet or so. (Garmin 296) I frequently drive by 2 signs marking the "eastern continental divide" with the elevation. With my WAAS equipped, non-aviation GPSR (Meridian Gold), I habitually flip to a screen with elevation displayed and have yet to differ by more than 40 ft. As Dan says, I am usually within 15 ft. I took two trips across parallel bridges in eastern Virginia 3 days apart; the reading was stable and within 5 ft of what I visually estimated it should be. It differed by 2 ft betweent he 2 days; my eyes told me the bridge heights did also ... I would trust a WAAS equipped GPS, tracking 5 or more birds in a clear environment, to be more ACCURATE than a barometric altimeter. I would USE my barometric altimeter if it were working as other aircraft in the area are, adjusted to local barometer per information gained by radio from official sources. |
#9
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GeorgeB wrote:
On Sun, 12 Jun 2005 06:27:43 -0400, Cub Driver wrote: On Sat, 11 Jun 2005 20:54:01 -0700, "Aluckyguess" wrote: I am getting over 500ft. difference and more. Oh,. good grief! I'm sure I've never had as much as 100 ft of variance, and more often it's 20 feet or so. (Garmin 296) I frequently drive by 2 signs marking the "eastern continental divide" with the elevation. With my WAAS equipped, non-aviation GPSR (Meridian Gold), I habitually flip to a screen with elevation displayed and have yet to differ by more than 40 ft. As Dan says, I am usually within 15 ft. But sometime and someplace you may not be: http://gpsinformation.net/main/altitude.htm I took two trips across parallel bridges in eastern Virginia 3 days apart; the reading was stable and within 5 ft of what I visually estimated it should be. It differed by 2 ft betweent he 2 days; my eyes told me the bridge heights did also ... I would trust a WAAS equipped GPS, tracking 5 or more birds in a clear environment, to be more ACCURATE than a barometric altimeter. I would USE my barometric altimeter if it were working as other aircraft in the area are, adjusted to local barometer per information gained by radio from official sources. Yes, augmented systems are a different matter. Matt |
#10
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Aluckyguess wrote:
I am getting over 500ft. difference and more. "Matt Whiting" wrote in message ... Aluckyguess wrote: How come the GPS reads a different altitude than the Altimeter? For many reasons. The main reason is that they derive altitude by two completely different methods and this inevitably introduces errors. Then each method has its own inherent errors. Even two altimeters side by side won't read the same altitude other than by accident. Not that unusual. GPS isn't very accurate vertically. Most GPS manuals tell you not to rely on the altitude in any serious way. Matt |
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