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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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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 |
#6
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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 |
#7
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"Matt Whiting" wrote in message ... 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 Unless the GPS is WAAS capable? |
#8
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"Matt Whiting" wrote: Not that unusual. GPS isn't very accurate vertically. Most GPS manuals tell you not to rely on the altitude in any serious way. It's usually more accurate than the altimeter, which is not corrected for non-standard temperature. -- Dan C172RG at BFM |
#9
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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. |
#10
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Aluckyguess,
How come the GPS reads a different altitude than the Altimeter? Main reason: Altimeter indication is corrected for barometric pressure. First order of business before comparing is to set the altimeter to 29.92. Even then, 500 feet off is not unusual IF the GPS is not WAAS capable. -- Thomas Borchert (EDDH) |
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