Elevation Is Inaccurate.

[cajun66]

I have a new GPS that I got yesterday for Christmas. It's a Garmin Etrex Legend.

I am absolutely new to the GPS and am canstantly amazed at the amount of info I've gotten from it. But I've noticed that the elevation cannot be correct. I've gotten incosistent readings at similar locations and it fluctuates when I'm stationary. Is this normal?

[+Sputnik 57]

Altitude is the generally the least accurate of the position information you get from a GPS receiver. 50 to 100 foot variances seem to be fairly common in my experience. Someone with detailed knowlegde of the geometry used by GPS receivers can probably explain why.

[cajun66]

I'm not really worried about it as Everywhere I go is pretty much sea level anyway.

I was mostly curious. Thanks for the quick response.

[peter]

Considering that we're determining our position relative to satellites that are about 12000 miles away and moving at thousands of miles per hour it's amazing that any of the position measurements are as accurate as they are. And the horizontal lat/long positions fluctuate as well due to assorted measurement errors. Typically under good reception conditions the horizontal position may vary by up to about 20' and the altitude by about 40'. But obstructions that block some satellite signals can make the errors considerably larger.

 

As Sputnik57 indicated, the geometry plays a significant role in the accuracy. GPS works by determining the distance to multiple (pref. 4 or more) satellites that are in known orbits. That will give you the most accurate results if some of the satellites are to each side of you rather than all on one side. As a result the east - west (long.) position is statistically the most accurate since the satellites are equally likely to be east or west of us. The north - south (lat.) position is a little less accurate since we tend to have more satellites to the south of us than to the north when we're in the northern hemisphere. And the up - down (altitude) position is least accurate since we can only receive signals from satellites that are above us.

[Magellin]

Altitude is from "MEAN HIGH TIDE SEA LEVEL"

I would not worry to much about the ALT since you know where you are when you are there, you should use a TOPO MAP (USGS PAPER) to verify your ALT and POS.

I live @ 7' AMHT yet it shows at times 20'above, and whilst in the boat it can show from+1 to +10 yet I can place the unit into the gulf and the readings dont change.

 

Don't worry bout +50/100 discreps, if you are at the correct long/lat you are standing on it!

[+TotemLake]

I remember reading a thread a couple years back that indicated the altitude error rate is 1.5 times the provided EPE. I'd give a pointer but you know the usual excuse.

[+fratermus]

Altitude is from "MEAN HIGH TIDE SEA LEVEL"

Ahh, so maybe some big waves are coming in when the OP is trying to get altiude reaadings. :-)