# elevation: -17 feet??

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I am in new orleans...

I knew that we are supposedly 5 feet below sea level... but 17 feet below??

or does this -17 feet elevation have anything to do with sea level?? also.. with 2.40 c on my legend and WAAS enabled still only accuracy to 22 feet..

If 22 feet is tolerance, then I'd say that -17 is within specs.

That being said, tolerances are really defined for 2D coordinates. Also, I have heard that it is MUCH more difficult to get an accurate elevation reading than position reading. The explanation I was given was that from orbit, it is much easier to pinpoint a position on the surface of the planet, but it's real hard to get a good read on how high above the mean sea level you are.

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Go! And don't be afraid to get a little wet!

To get a good satellite geometry, for altitude measurement, requires the use of satellites on the other side of the earth, too. But they are blocked by the earth itself, so that doesn't work.

-17 feet is rather "normal". I've seen more than -100 feet on my unit. Besides, GPS altitude is above the geoide, which isn't exactly the same as Mean Sea Level.

Anders

quote:
Originally posted by Anders:

-17 feet is rather "normal". I've seen more than -100 feet on my unit. Besides, GPS altitude is above the geoide, which isn't exactly the same as Mean Sea Level.

Anders

I thought modern Garmins used tables and interpolation to adjust the elevation relative to the geoide to altitude relative to (approximate local) sea level?

I've also 'heard' (for what it's worth) that the accuracy for altitude is 'typically' about 1.5 the accuracy for horizontal position; so if you're getting an EPE of ~20ft, you can expect an elevation accuracy of ~30ft (±). Has anyone else heard this rule-of-thumb?

df

Most consumer GPSr provide the Orthometric Height which is the height above the geoid (the height above mean sea level. MSL).

The Geoid Undulation or Geoidal Separation, is the difference between the Earth's Geoid and the GRS80 ellipsoid.

Your GPSr actually calculates the height above the GRS80 ellipsoid (HAE) and by use of a lookup table calculates... Orthometric Height (MSL) = HAE - Geoid Undulation.

The Orthometric height displayed will always have 1.5 to 2 times the error of that of the horizontal position for the same point.

OK, so they do fix it to resemble the altitude above the sea level. But the measurement precision must still be an important factor, since I've also seen that the DOP vertically usually is 1.5 to 2 times larger than it's horizontally.

Anders

This makes me wonder what kinds of readings one would recieve in Death Valley. There negative elevation would be what to expect since it is below see level, down to near -300ft.

When I was on a recent trip to the shore I occacsionally saw negative elevations. I just wrote it off to calculation errors. I know that the elevation on my Legend is very inaccurate and don't put much stock in it.

I'm not lost!

I just don't know where I am.

measuring the elevation of the water at low-tide? Living in the desert, I have no idea what the tidal variance could be, but it would make an interesting experiment. If you get an answer, please let me know.

Mike. KD9KC.

El Paso, Texas.

Seventeen minutes after her FIRST call for help, police officers arrived to find Ronyale White dead.

Prohibiting self defense is the ultimate crime. Police carry guns to protect themselves. What protects YOU ???

I was just in Brawley, California over the weekend and I got readings from -130 to -150, which is very accurate for that area. My topo maps were within 10-15 feet almost the entire time.

From there we went to Palm Springs to the top of the San Juanito mountain, at over 8500 feet, so I got to see both extremes! I'm going to plot it with a program I have that does elevation profiles and 3D profiles. Should be interesing!

My experience when driving with my GPSV is that the elevation is extremely accurate. When I see a "summit elevation XXXX feet" sign I usually check it and it's almost always within 10 feet or even closer! Pretty amazing I think.

I went deep sea fishing not too long ago and got readings down to about -15 feet. It's bound to vary with tides and if you are in a good sea swell, whether you are at the top or the bottom of that swell when the GPS updates. Figure that on top of accuracy and other factors, it all makes sense.

On the way back it was deeper because of the weight of all the fish we caught made the boat lower in the water.

I've spent a lot of time on the coast, if I set my GPS near the high tide mark and give it a few minutes to stabilize, it usually reads close to 0 or a little higher.

Me and that dog are gonna take a walk in the woods, and only one of us is coming back...

[This message was edited by brdad on August 21, 2002 at 08:31 PM.]

I went deep sea fishing not too long ago and got readings down to about -15 feet. It's bound to vary with tides and if you are in a good sea swell, whether you are at the top or the bottom of that swell when the GPS updates. Figure that on top of accuracy and other factors, it all makes sense.

On the way back it was deeper because of the weight of all the fish we caught made the boat lower in the water.

I've spent a lot of time on the coast, if I set my GPS near the high tide mark and give it a few minutes to stabilize, it usually reads close to 0 or a little higher.

Me and that dog are gonna take a walk in the woods, and only one of us is coming back...

[This message was edited by brdad on August 21, 2002 at 08:31 PM.]

Lowest cache in Sweden is only at -2.41 m...

Anders

quote:
Originally posted by Anders:

Lowest cache in Sweden is only at -2.41 m...

Anders

Not really - I have placed one in Sweden at about -4 m. Can't tell you which one, that would be a spoiler. But I can tell you it is in a very wet environment

quote:
Originally posted by Anders:

Lowest cache in Sweden is only at -2.41 m...

Anders

Not really - I have placed one in Sweden at about -4 m. Can't tell you which one, that would be a spoiler. But I can tell you it is in a very wet environment

OK, so the lowest cache in Sweden, that brags about it, is at -2.41 meters. Is that better, Gustaf?

Anders

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