Legend elevation way out, is this common?

[+tirediron]

I have been doing some caches around San Diego during the last week and noticed that the elevation that my eTrex Legend is displaying is way out, eg -25' when I am in fact at least ten feet above, 0' when I am closer to 100' and such. I know it's not supposed to be bang on, but has anyone else noticed this much discrepancy?

[+TotemLake]

I've read in these forums to expect estimated position error (EPE) times 2.5 for elevation position error.

 

My experience is this is a good formula to work with. Given enough time and if your GPS will average, you will gain an estimate closer to the real elevation.

 

My MeriPlat takes about 20 minutes with excellent sat lock to gain an accuracy to within 5 meters or less.

 

Cheers!

TL

[+Tahoe Skier5000]

I agree with totem. Give the GPS some time.

 

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N38 49.027 W120 01.064

Garmin V

[+Rubberhead]

I've taken an eTrex Legend and an eTrex Camo up on the Blueridge Parkway. The parkway has elevation markers and both eTrex units are usually very close (well within the EPE).

 

Elevations on the Parkway are between about 2,500 ft amsl to 6,054 ft amsl.

 

Ducks - Flying, great tasting, geocaches of meat

[Neo_Geo]

Elevation is a HUGE weakness in GPS!!! I have taken my Legend (WAAS capable) on many commercial airline flights where elevation readings were ASTRONOMICALLY off! In one case, I was departing Quito, Ecuador (elevation about 9,000 feet), and the Legend was telling me something like 300 or 400 FEET! In several instances, it took about a full 60 seconds for the GPSr to figure out the true altitude. I flew eleven legs from NY to Ecuador and back and experienced several strange anomalies such as this (not necessarily related to altitude). Time will always tell. While in Ecuador, I had the pleasure of visiting the Galapagos Islands. While GPSing near the coast line, sometimes I was above sea level, and sometimes below! Sometimes I was walking on water (Maybe Jesus had a Garmin Legend) and boating on dry land too! I'm pretty sure that this is why some models come with barometric altimeters.

[+SpongeRob]

Not sure I agree. Was just at clingmans dome in TN and at the top of the observation tower I was reading 6700 feet. That's real close considering the elevation tower was a good 40 feet up. I think you just need to wait for a good lock.

 

I had a whole article on Clingmans Dome on my website.

 

- Rob

 

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SpongeRob

rwmech@keenpeople.com

www.keenpeople.com

WPWU826

 

Cache'n Retrievers

[+Tahoe Skier5000]

The satellite geometry has a large effect on the elevation readings. On the III+, there's actually a readout that tells you the geometry quality (called DOP) which was pretty cool. In short, if the satellites you have a lock on are very close together, or low in the sky, you can count on the elevation being a pretty far off. The ideal setup would be to have all of the satellites spread out at about 45 degrees from the horizon.

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N38 49.027 W120 01.064

Garmin V

[Citizensmith]

As a previous poster mentioned, the less accurate elevation is due to the spread of the satellites being less useful for Z axis determination. Getting even a half decent reading would require 9 or more well spread satellites. The DOP (Depth of Precision? can't remember actually) is a factor of both how many satellites you have locked and where they are positioned. The really high end GPS units (trimble and such) download almanac files from the satellites. These can then be processed on a computer to get any updates to satellite orbits due to course corrections, and provide plots of how many satellites and what spread you'll get at what time of day. It's very useful as it means you'll know when you'll only be able to lock 5 and get crappy results, or when you can hit 9 or 10 and be spot on. At its best I'll get about 1.1 to 1.5 feet of horizontal accuracy from my unit, and at the same time about 5 feet of vertical accuracy. And that's from a $5000 GPS I use at work.

[Kerry.]

Actually every GPS has to download an Almanac file otherwise the thing simply wouldn't work.

 

Again this misconception that the more satellites the better the accuarcy, not so. The DOP (and that is Dilution Of Precision) has to do with the strength of the geometry of the satellites not the total number.

 

"1.1 to 1.5 feet" not in real time you won't, post processed you could but hindsight might be ok for hiding but not for finding.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+tirediron]

Thanks all... I guess that pretty much answers my question. I will try leaving it out in a spot with a known elevation and see how it does after a half-hour or so.

[Citizensmith]

quote:

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Originally posted by Kerry:

The DOP (and that is Dilution Of Precision)

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Thanks

 

quote:

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Again this misconception that the more satellites the better the accuarcy, not so.

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I'd say both are right. 3 perfectly placed satellites still wouldn't be any better than 9 jumbled ones. Of course what we really want is both.

 

quote:

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"1.1 to 1.5 feet" not in real time you won't, post processed you could but hindsight might be ok for hiding but not for finding.

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Nope, no post processing, but I do do a lot of averaging and the GPS is often mounted on a bipod.

 

And yeah, I wouldn't ever go hunting with this thing. But I may use it occasionally to go hide stuff.

[Kerry.]

quote:

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Accuracy (RMS)3 after differential correction

GPS Pathfinder postprocessing (4) . . . . . . . 50 cm + 1 ppm

GPS Pathfinder Office carrier postprocessing

With 5 minutes tracking satellites. . . . . . . 30 cm + 5 ppm

With 10 minutes tracking satellites . . . . . . 20 cm + 5 ppm

With 20 minutes tracking satellites . . . . . . 10 cm + 5 ppm

With 45 minutes tracking satellites . . . . . . 1 cm + 5 ppm

Real-time . . . . . . . . . . . . . . . . . . . Submeter(5)

 

(3) Horizontal accuracy. Requires data to be collected with minimum of 4 satellites, maximum PDOP of 6, minimum

SNR of 6, minimum elevation of 15 degrees, and reasonable multipath conditions. Ionosperic conditions,

multipath signals or obstruction of the sky by buildings or heavy tree canopy may degrade precision by

interfering with signal reception.

(4) Postprocessing with GPS Pathfi nder Offi ce software or GPS Pathfi nder Express service.

(5) RTCM SC-104 standard format broadcast from a Trimble reference station. Accuracy varies with proximity to

base station.

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Smithy, Those are the accuracy specifications for a ProXR. You might note they are RMS accuracy figures after differential correction or post processed. Differential corrections are rather different to "averaging". One simply won't get 1 foot accuracy as a pure SPS user without some form of corrections and that excludes averaging (as such).

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[dzavetsky]

AFter reading your post, I remembered to check this on my Legend....

 

I was in teh Laurel Mountains on my motorcycle....saw a sign that said something like 2648 feet or so... (I think)....my Legend read 2646 or so right at the sign...not scientifice, by any means, but seems close to me....

 

quote:

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Originally posted by tirediron:

I have been doing some caches around San Diego during the last week and noticed that the elevation that my eTrex Legend is displaying is way out, eg -25' when I am in fact at least ten feet above, 0' when I am closer to 100' and such. I know it's not supposed to be bang on, but has anyone else noticed this much discrepancy?

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