Elevation Calculation
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    17 years
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I flew to Florida last week and I took my NUVI2555LMT on the plane with me.
First off, I was pleased to see that I was able to get satellite lock, for a reasonable portion of the trip, once we got to the planes normal altitude.
I always assumed that the elevation display on the unit was from a database, in the maps, so I was somewhat surprised that after the plane leveled off, that read about 11,700 meters (38,400 feet).
Since the plane is pressurized, I had to assume that the determination is based on some form of triangulation with the satellites.
I contacted Garmin and here is what I got back.
"It mainly uses the triangulation from the satellites to determine elevation, but since those units do not carry a barometric pressure sensor, which works a lot better to determine elevation, the elevation can be off by as much as 150 feet in some places, but yes the satellite triangulation would be how it works.
Our handheld units also have a barometric pressure sensor in the unit as well, so the elevation on those is much more accurate than our auto devices. "
So now you know what I know.
Elevation
I was standing at the sea edge in Key West and my nuvi 855 showed an elevation of -11 feet.
It's interesting to see how sea level is defined. Well worth goggling.
nuvi 855. Life is not fair. I don't care who told you it is.
Goggling? LOL!
My GPSmap 62CSx is horrible with elevation. I use my nuvi 500 to initially calibrate the unit, and even then the altitude could still wander as much as 75 feet up or down. I have been hiking in hills on Long Island and the unit tells me my elevation is -40 feet... I don't think so!
Striving to make the NYC Metro area project the best.
In a canoe
I find it amusing to look at my trip log after a canoe trip on the Kickapoo River. We average 2 mph with bursts up to 43 mph. We go mostly downhill with a few uphill sections.
1490LMT 1450LMT 295w
WGS84--
"Surface" altitude is usually determined with respect to a geoid representing the shape of the Earth (the reference ellipsoid); this geoid is part of the WGS84 definition that GPS uses.
It's one of those things that for the most part, it works. Plus or minus a few meters is really close, considering everything else that goes into it!
Wikipedia has a good introduction if you're interested, and pointers to the more detailed, if you're fanatic...
Nuvi 2460, 680, DATUM Tymserve 2100, Trimble Thunderbolt, Ham radio, Macintosh, Linux, Windows
Kickapoo River
I find it amusing to look at my trip log after a canoe trip on the Kickapoo River. We average 2 mph with bursts up to 43 mph. We go mostly downhill with a few uphill sections.
Never heard of a river flowing uphill before
nuvi 855. Life is not fair. I don't care who told you it is.
It's called a tidal bore, I believe
Happens on occasion when the ocean tides backflow rivers.
http://en.wikipedia.org/wiki/Tidal_bore
Striving to make the NYC Metro area project the best.
I like its french sound
In our neck of the woods, we call that a "MASCARET"
Pronounce "mass-karey"
And we have a few places where people actually ride that wave on surfboards.
Ain't nuthin' never just right to do the things you wanna do when you wanna do them, so you best just go ahead and do them anyway ! (Rancid Crabtree, from Pat F McManus fame)
not bad
When we went through the Eisenhower Tunnel in Colorado my 855 read less then 20 feet of what the sign said, so I didn't think that was too bad.
Anytime you have a 50-50 chance of getting something right, there's a 90% probability you'll get it wrong.
flight level versus altitude
I flew to Florida last week and I took my NUVI2555LMT on the plane with me.
I was somewhat surprised that after the plane leveled off, that read about 11,700 meters (38,400 feet)
If you compare the altitude shown by your GPS receiver at cruising flight to comments made from the flight deck over the PA system, you will soon notice that they often differ by much more than the error in either one should be.
The reason is that when your captain says that you are traveling at 38,000 feet, he really means that you are traveling at flight level 380. That means that the airplane is being controlled to fly at an altitude which would be 38,000 feet if the current ground-level atmospheric pressure equivalent were actually 29.92 inches of mercury AND the pressure-loss profile up to your altitude matched the International Standard Atmosphere model.
Neither is likely actually to be true at any given moment, but because all aircraft operating anywhere near you above the transition altitude are making the same assumption in translating assigned flight levels to physical reality, they should not run into each other. At altitudes below the transition altitude for the given jurisdiction, they instead adjust their altimeters to read with respect to local reported barometric pressure, as it becomes more important to actually miss things like mountaintops, which stay where they are and don't adjust up and down to meet conventions.
personal GPS user since 1992
Different situation.
Happens on occasion when the ocean tides backflow rivers.
http://en.wikipedia.org/wiki/Tidal_bore
No, the water is still flowing downhill. The sea is higher than the river.
nuvi 855. Life is not fair. I don't care who told you it is.
Maybe
Maybe it would have been better to say down stream and upstream.
Nuvi 350, 760, 1695LM, 3790LMT, 2460LMT, 3597LMTHD, DriveLuxe 50LMTHD, DriveSmart 61, Garmin Drive 52, Garmin Backup Camera 40 and TomTom XXL540s.