How Accurate Is The Elevation On Your Garmin GPS?

 

When my Garmin DriveLuxe 51 LMT-S was returned recently, I brought it out to my pickup truck to compare setting with my Garmin Nuvi 3597. I noticed the at the Elevation displayed on each unit was not the same.

Has anybody else here compared the Elevation displayed with more than one device? Were the results similar or did they vary from one another quite a bit?

Elevation Not Good

Elevation readings are not that good. Latitude and longitude are good, pretty accurate.

--
GPSMAP64s, iPhone XR w/Garmin North America, Yaesu VX-8R w/GPS.

elevation

Garmin automotive devices use the satellite signals to calculate an estimated elevation as well as position. This system is optimized for lateral position on the earth surface more than it is for elevation above sea level. So while you can usually get positional accuracy within 10 to 30 feet, sometimes the calculated elevation can be off by 100 feet or more. However, my experience is that the displayed elevation is usually within 20 to 40 feet of being correct.

My Oregon handheld GPS has a barometric altimeter, and it CAN be quite accurate IF you calibrate it correctly and frequently and IF there are no rapid weather changes that affect barometric pressure.

--
Alan - Android Auto, DriveLuxe 51LMT-S, DriveLuxe 50LMTHD, Nuvi 3597LMTHD, Oregon 550T, Nuvi 855, Nuvi 755T, Lowrance Endura Sierra, Bosch Nyon

Depends

I have found on my many GPS that it is fairly close, similar to what alandb mentioned above, provided the GPS has a quality signal. If the signal isn't very good, the accuracy can exceed 100 feet or more.

Often times while traveling we will see elevations posted along the roadway. The GPS is usually close to those elevations but never exactly the same.

3597 pretty close

My 3597 is pretty close and steady. On the other hand, my Nüvi 660 bobbels up and down even when sitting on a solid deck rail outside. It shows the altitude at +/- 50 ft.

--
Metricman Nuvi 660, GTM-20 Traffic Receiver Nuvi 3597 GTM-60 Traffic Receiver Williamsburg, VA

Four satellites needed for elevation...

A GPS receiver needs to triangulate signals from three or more satellites to pinpoint its exact lateral position. Signals from at least four satellites are essential to calculate the elevation and the best accuracy is obtained when one of those satellites is directly overhead with respect to the receiver.

--
John from PA

Confidence Interval?

I’ve never really understood GPS accuracy. For lat/lon when it says +/- 10 feet what does that mean? Accuracy comes with a confidence interval. Does the +/- 10 have a confidence interval of .9, .95, or what?

I’ve read (somewhere, and I’d like to be corrected) that height above sea level’s accuracy is three times worse than lat/lon.

A Fairly Complicated Issue

A simplified explanation of the issue is available here for anyone interested:

https://weather.gladstonefamily.net/gps_elevation.html

EPE

This link below might explain some of smoke and mirrors behind the calculations. As others have noted, the horizontal measurements are the ones of most concerned so that we aren't told to turn right into someone's driveway vs the street 50' beyond. I remember a term often used on one of my early StreetPilots from long ago called EPE, or Estimated Position Error. If it said 10 feet, you could assume you were 50% certain you were somewhere in a circle with a radius of 10 feet. Usually that was correct if there were no natural or man made canyons blocking some and reflecting other of the satellite signals.

As for altitude, I think I see about 50' accuracy in an area with an unobstructed view of the sky. Traveling on gently rolling terrain, I can watch the altimeter and it will go up and down in sync when I'm ascending or descending. In my opinion, the signs on the side of the highway posting the elevation are not all that accurate. When I've been nearly level with an airport runway with an accurately measured altitude, I find the Garmin to be fairly close.

https://www.gps.gov/systems/gps/performance/accuracy/

--
"There's no substitute for local knowledge" nüvi 750, nüvi 3597

GPS accuracy

GPS' are great for horizontal positions, but bad for elevation measurements.

That's why surveyors still use transits.

--
nüvi 3790T | nüvi 775T | Those who make peaceful revolution impossible, will make violent revolution inevitable ~ JFK

My old Nuvi 350

I once took my old NUVI 350 on a plane with me. (JetBlue) I had a window seat.
I asked the captain if I could use it. He said OK but he didn't think it would work.
It worked perfect showingb air speed and elevation. (it holds the readings until you clear them)
As I was getting off, he saked me if it worked.
When I showed him, the speed wasright on but the elevation was off by 10 ft.
He figured it was the position of the GPS compared to the location of the planes sensors.
Haven't tried my other ones as I try for aisle seats now.

--
Don't sweat the petty things and don't pet the sweaty things!

better in back

Timantide wrote:

I once took my old NUVI 350 on a plane with me. (JetBlue) I had a window seat.
I asked the captain if I could use it. He said OK but he didn't think it would work.

Often GPS units have better luck getting a signal in the passenger cabin than the flight deck. Conductive coatings in the flight deck windows can attenuate signals at the GPS frequency.

Two key things: get the antenna as near a window as possible. Be patient waiting for first fix.

I've used GPS of several models on hundreds of flights.

By the way, planes get their clearances and assignments by pressure altitude so if the GPS is getting height perfectly, it won't match the assigned altitude except by chance. Down low, they adjust to local barometric pressure, so should match closely, but above transition altitude they set a standard number.

So don't expect to see 36,000 feet on your GPS when the captain announces that as your cruise altitude, not because GPS can't get height right, but because the airplane is not flying by height.

Transition altitude in 18,000 feet in North America.

For more details than you are likely to want:

https://www.flyingmag.com/everything-explained-altimetry-aro...

--
personal GPS user since 1992

Elevation not that accurate on my 2689

Driving along the coast.. Mine (2689) shows me driving under water lots of times. Or, while driving on a beach shows significant elevation. One would think that at least there would be logic to NOT show negative numbers.

--
Lives in Edmonton AB A volunteer driver for Drive Happiness.ca and uses a 2689 to find my way.

How About Death Valley

Ralph6410 wrote:

Driving along the coast.. Mine (2689) shows me driving under water lots of times. Or, while driving on a beach shows significant elevation. One would think that at least there would be logic to NOT show negative numbers.

So, how's that going to work out in Death Valley?
-282.2′

--
Frank DriveSmart55 37.322760, -79.511267

Use Elevation at Risk

I have a Garmin GPSMAP 64s and it has a barometric sensor, so I effectively have two elevation sensors, the baro and the GPS.

I see 6' elevation when sitting on the beach, which makes sense. I also see negative elevations when going through a tunnel under a river. Makes sense.

But the baro sensor is affected by air pressure in the car which is affected by the open/closed windows, the air fan, car speed, etc. I can see wild fluctuations in the indicated baro elevation with changes in window openings, air recirculation setting, etc.

--
GPSMAP64s, iPhone XR w/Garmin North America, Yaesu VX-8R w/GPS.

Lots of places

phranc wrote:

So, how's that going to work out in Death Valley?
-282.2′

As you drive the road from the coast toward Tiberias, there is a sign by the road to mark sea level. That is a long way above the shore line of the Sea of Galilee (as many of us call it), or Kinneret (local name).

So all the towns around the lake are far beow sea level, including Tiberias and Capernaum. Of course the whole of the Jordan Valley heading south to the Dead Sea has lots of land below sea level, including major places such as Beit She'an, Jericho and Ein Gedi. The surface of the Dead Sea is currently below -1400 feet and dropping.

Death Valley is a bit of a curiousity, but in the lower Jordan Valley below sea level is normal for lots of people.

--
personal GPS user since 1992

I had to look up my Naval references

I left the Navy in about 1978 so it took me awhile to find my navigation reference books. The Sea of Galilee is almost 700 feet below what is called "nautical" sea level.

The surface of the ocean is used as a reference for measuring sea level, but of course it varies with tidal fluctuations. Hence the term "local sea level" which according to my Bowditch is determined by taking hourly measurements of sea levels over a period of 19 years at various locations, and then averaging all of the measurements. At least that is the way the USA does it or did it back in the 1970's. Why 19 years, it allows scientists to account for all the variations of the moon's orbit.

What a job these scientists have!

--
John from PA

Interesting topic

Several years ago I carried my iQue on a plane flight and found it to be very close to altitude reported by the on board altimeter.
This post prompted me to try the same by comparing elevation at a local small airport between both my 3597s and the iQue. iQue was accurate to within 6 ft. and 3597s were 10 ft and 12 ft.
YMMV

--
2017 VW CC w Discover Media, nüvi 3597LTMHD x 2, 1450, 205w retired, iQue first and possibly the best

I’ve checked my 3597 a few times on the WV Turnpike

There is one mountain down there that has the altitude posted. My 3597 is always off by 40-50 feet. Sometimes right at 40 feet; other times by 50 feet or more. It has never shown the same altitude as the sign.

--
It is impossible to rightly govern a nation without God and the Bible. ----George Washington

When I cross Monument Hill...

maddog67 wrote:

There is one mountain down there that has the altitude posted. My 3597 is always off by 40-50 feet. Sometimes right at 40 feet; other times by 50 feet or more. It has never shown the same altitude as the sign.

When I cross Monument Hill
( https://en.wikipedia.org/wiki/Monument_Hill_(Colorado) )
my GPSs all read ~50 feet higher than the sign.

On a petty note, my geriatric memory tells me that the sign there says 7350 feet and the sign pictured in the wiki enter says 7352. If there is anything that my memory is good for it is remembering unimportant numbers.

WAAS

I know I had an old Garmin -- don't remember if it was a Nuvi 360, Nuvi 765, or even an etrex Legend -- that could pick up the Wide Area Augmentation System (WAAS) satellites. You could tell because the WAAS birds showed up on the satellite information page with higher numbers than the normal GPS satellites -- above 40, if memory serves.

I won't bore with the details, but WAAS is a system developed by the FAA that uses several GEO satellites (completely separate from the 20-odd MEO GPS satellites) to broadcast correction data to equipped GPS receivers. The main benefits of WAAS are significantly improved horizontal and vertical (altitude) accuracy. Just going by the satellite information page, my Nuvi 3597 does not use WAAS since no satellite numbers above 40 appear. Seems a little strange since consumer-grade receivers had it years ago, and it did seem to improve altitude accuracy.

Very close

metricman wrote:

My 3597 is pretty close and steady. On the other hand, my Nüvi 660 bobbels up and down even when sitting on a solid deck rail outside. It shows the altitude at +/- 50 ft.

I've found the elevation was very close. Been awhile since I drove through West Virginia & Tennessee, but would compare when we came unto an elevation marker. Always amazingly close on my NUVI 2555.

--
NUVI2555LMT, NUVI350

Better In Back

Actually, In was on JetBlue, and they have a live flight map that shows the flight speed and altitude. My Gps was very close to everything on the screen I also locked onto the satellites faster than normal, that surprised me. As I said, when leaving the plane, the captain was surprised that it qorked, and was very close.
One of the things on my GPS is state lines and my house in Florida.
Every time we crossed a state line it would ping.

--
Don't sweat the petty things and don't pet the sweaty things!

Elevation accuracy LOL

My Garmin Zumo says I'm 75 feet above sea level when I'm in front of my home. I had a surveyor that was checking my level for flood insurance and he said it was 15 feet above sea level and he had a lot of fancy equipment. I don't think the gps is accurate at all.

Orthometric Height

Benoit69 wrote:

My Garmin Zumo says I'm 75 feet above sea level when I'm in front of my home. I had a surveyor that was checking my level for flood insurance and he said it was 15 feet above sea level and he had a lot of fancy equipment. I don't think the gps is accurate at all.

Your GPS reads orthometric height or the height above the ellipsoid ( the "smoothed out" shape of the earth), not the height above sea level. The difference between sea level and the ellipsoid can vary considerably depending on location.

https://weather.gladstonefamily.net/gps_elevation.html

How Accurate Is The Elevation On Your Garmin GPS?

I went out to my pickup truck last night. I powered up the DriveLuxe 51 and also brought out my 3597 to compare. At first they both displayed Elevation readings pretty close to one another. The DriveLuxe 51 remained at about 992 feet and then the 3597 went up to 1046 feet.

That made me wonder what my actual elevation is. The website What Is My Elevation shows my at 988 feet.

I also tried the smartphone app My Elevation. It shows me at 987 ft.

Then I powered us my old Garmin Nuvi 50LM and Nuvi 500. The Nuvi 50 displays Elevation 990ft. The Nuvi 500 displays Elevation 996ft.

For whatever reason(s) the 3597 seems to be the furthest off. I could go look for a way to reset it, but this is not mission critical for me. It is just for fun. It is kind of interest, though.

https://whatismyelevation.com/

Galileo

Jim1348 wrote:

I went out to my pickup truck last night. I powered up the DriveLuxe 51 and also brought out my 3597 to compare. At first they both displayed Elevation readings pretty close to one another. The DriveLuxe 51 remained at about 992 feet and then the 3597 went up to 1046 feet. That made me wonder what my actual elevation is. The website What Is My Elevation shows my at 988 feet....For whatever reason(s) the 3597 seems to be the furthest off.

The DriveLuxe 51 uses both US GPS satellites and Galileo satellites which maybe improves the positioning compared to the 3597's use of US satellites only...just a guess.

I tried the Garmin 1490

I tried the Garmin 1490 during floods a while back and over 100 miles of highway it was barely in the ballpark. I have all the geodesic elevations along the highway and off side roads and Garmin was off by as much as 10 meters in elevation, and very erratic. Lon & Lat were pretty accurate, but elevation is very undependable, and Garmin does not hide this fact.

I don't care how or where Garmin measures elevation; if it does not match geodesic data then it is undependable for this purpose, especially for critical purposes such as monitoring for overland flooding.

I'm pretty sure that Garmin

I'm pretty sure that Garmin has never claimed that any of their automotive units are suitable for "critical purposes" involving elevation. Most of the units discussed in this thread are made to navigate existing streets, roads, and highways. You really don't need elevation data at all to accomplish that. Anyone that needs highly accurate elevation information for their job won't be using an automotive GPS unit.

Altitude

bdhsfz6 wrote:
Benoit69 wrote:

My Garmin Zumo says I'm 75 feet above sea level when I'm in front of my home. I had a surveyor that was checking my level for flood insurance and he said it was 15 feet above sea level and he had a lot of fancy equipment. I don't think the gps is accurate at all.

Your GPS reads orthometric height or the height above the ellipsoid ( the "smoothed out" shape of the earth), not the height above sea level. The difference between sea level and the ellipsoid can vary considerably depending on location.

https://weather.gladstonefamily.net/gps_elevation.html

The reference for altitude (ellipsoid or MSL) is dependent upon the particular GPS receiver. The NMEA $GPGGA message specifies height above MSL (and also contains height of the geoid (MSL) above the WGS84 ellipsoid so you can convert between the two references), however there are receivers which use height above ellipsoid in the $GPGGA message rather than height above MSL. See reference below:
http://www.gpsinformation.org/dale/nmea.htm
Proprietary GPS messages will often contain one or the other or both. Safe bet to assume that GPS receivers with a display could be displaying either. Time to read the manual.
Mark

Not sure how accurate the

Not sure how accurate the Garmin GPS measurement is...

--
Garmin DriveSmart 55 with Traffic

My garmin nuvi 2559 is

My garmin nuvi 2559 is accurate to about 50'. I compared to an elevation road sign. I do not know how accurate the elevation road sign was.
I thought I would tease the tech support people at Garmin by asking different reps how the elevation was calculated. One rep told me that the elevations were built into the maps. Another rep told me that the units had a built in altimeter. I told the rep that the altimeter would also require a barometer. The rep told me that the unit had a barometer also. When I worked tech support I was required to get the correct answer to a question by asking as many people as necessary. If I made stuff up I would have been fired on the spot.

Looney Tunes :-)

John from PA wrote:

I left the Navy in about 1978 so it took me awhile to find my navigation reference books. The Sea of Galilee is almost 700 feet below what is called "nautical" sea level.

The surface of the ocean is used as a reference for measuring sea level, but of course it varies with tidal fluctuations. Hence the term "local sea level" which according to my Bowditch is determined by taking hourly measurements of sea levels over a period of 19 years at various locations, and then averaging all of the measurements. At least that is the way the USA does it or did it back in the 1970's. Why 19 years, it allows scientists to account for all the variations of the moon's orbit.

Due to the interaction of the sun, the moon, and the Earth, the location of the rise and set of the new moon (historically used to tell time) throughout the year varies from year to year over a period of approximately 18.6 years, although even that period has a bit of fluctuation in it. Similarly, the tides vary over a long term cycle.

(I knew your life would not be complete without that piece of trivia!)

- Tom -

--
XXL540, GO LIVE 1535, GO 620

Elevation works, but inaccurate while moving

My experience with elevation from my Garmin Nuvi 1490 unit was hit and miss while driving. The elevation fluctuates all over the map while in motion. It gets far better standing still and the GPS unit can properly calculate the elevation information while in a static position.

Let me illustrate. One trip to the top of Pikes Peak, my Garmin reported 14,103 feet (officially the elevation is 14,110 feet) after standing still for a couple minutes. Conversely, Bad Water Basin, Death Valley NP, CA is officially 282 feet below sea level, my Garmin reported 272 feet below sea level in the parking area that was elevated from the basin floor. Both instances provided decent results when the GPS unit was not in motion.

A good friend of mine was a private pilot and had an older Garmin aircraft unit in his airplane. That GPS was spot on all the time. If not, Garmin would be out of business when pilots rely on absolutely accurate information and the liability to Garmin is virtually limitless if they screw it up. $$$ is a powerful motivator for quality control and accuracy!

perfect horizon

The_WB wrote:

... A good friend of mine was a private pilot and had an older Garmin aircraft unit in his airplane. That GPS was spot on all the time. If not, Garmin would be out of business when pilots rely on absolutely accurate information and the liability to Garmin is virtually limitless if they screw it up. $$$ is a powerful motivator for quality control and accuracy!

Depending on antenna placement you'd think that a plane could always have a perfect horizon.

Auto vs Aircraft GPS Altitude

minke wrote:

Depending on antenna placement you'd think that a plane could always have a perfect horizon.

Actually, the Garmin unit was mounted to the yoke. The fuselage is sheet metal, so being more open is not the case. The antennas were below the dashboard height. If anything, your dash auto Garmin had better line of sight with the horizon that in an airplane.

Garmin must use a different GPS unit module inside aircraft GPS units than automobile units. Since I never opened one up, auto or aircraft, I cannot state that for sure. All I do know is altitude accuracy is far better on an aircraft model that a automobile model. Mountains kinds hurt running into them if you are too low and the GPS fails you accurate information.

more likely satellite views than vehicle motion

The_WB wrote:

Both instances provided decent results when the GPS unit was not in motion.

Your GPS unit is always in motion relative to the satellites at speeds huge compared to your vehicle motion. Orbital velocity is a bit over 2 miles per second, and in receiving the signal from any given satellite you receiver has to figure out the component of that velocity along the line of sight to you, in order to figure out the right Doppler and timing adjustments, just to track the signal.

On the other hand, driving around in a car, one often has blocked signal to several satellites by the car structure, not to mention trees and buildings, and one also has a problem from mis-reception of reflected signals as though they had arrived by the direct path (this is called the multipath problem).

A decent aircraft installation puts the antenna where it is not blocked by anything during normal flight, and the airplane is usually flying well away from temporary signal blockage by trees, tall buildings, and the like.

Way back when, I had a Magellan GPS receiver that could only shift reception of the single receiver hardware among four candidate birds at a time. So every time it gave up on one (because it could not see through my car in the direction I was currently pointed, usually) there was a one minute wait to try another. This sort of thing gave early users a concrete appreciation for how compromised reception at a GPS receiver sitting on a car dashboard is. I think it is a miracle that the modern receivers (with admittedly a much larger constellation of active satellites) do as well as they do for us.

--
personal GPS user since 1992

signal/Noise ratio

archae86 wrote:
The_WB wrote:

... I think it is a miracle that the modern receivers (with admittedly a much larger constellation of active satellites) do as well as they do for us.

I can't easily find a reference to the required signal/noise ratio of consumer grade GPSs but I certainly agree that it seems miraculous.

I was in a ham radio club in the '80s that had a bunch of PhD. EE members. When the first digital signal processing chip became available to mere mortals one of them bought one and wrote the code to hear (and maybe decipher?) Morse code thru noise. In a demo it could reproduce Morse code when my (then competent) ears could hear absolutely nothing but noise.