## Recommended Posts

This might be a weird question but how come my GPSr is giving me elevation readings I know are wrong. I live on the beach and its telling me i'm 25-40ft above sea level. Is this normal?

GPSr's are half as accurate in determining elevations as they for horizontal fixes so what your seeing is just the normal error and is well within the specifications for a recreational GPSr.

Edited by PDOP's

Something I've always wondered is this: Since the level of the sea at any point varies with the tide, what is actually considered "sea level"? Even though you live on the beach, maybe you are 25-40 feet above sea level at low tide? Just a thought

What is sea level? Most maps indicate elevations as "MSL" (Mean Sea Level) or "ASL" (Average Sea Level). Not being a cartographer, I assume this means the average between high and low tides taken over a period of time. Sort of like datum: NAD1927 or WGS84; it has to be updated periodically.

Plate tectonics or something....

Anyway, while at the beach just be happy that the elevations are positive!

I believe it also has something to do with how the Earth is modeled using the map datum. The Earth not being exactly round, the datum models the Earth into a round Geoid(?), thus where you are the model may be above or below the actual surface of the Earth. Something like that; I'm no expert. Also true is that elevation is less acurate that horizontal coordinates because of how your GPS triangulates the satellite signal. Something like how these parenthesis are closer horizontally than lengthwise () .

NAD27 is not an updated WSG84. They are 2 different ways to model the Earth, starting from 2 different points. NAD27( for US only) is a spot in Kansas (Humes Ranch). WSG84(worldwide) is Geocentric based on satellite orbits. Nothing to do with the changing Earth.

Edited by EraSeek

The height a GPS derives from the conversion of cartesian coordinates is generally referred to as an ellipsoidal height, which is based on a mathematical model. The difference between this height and what is generally referred to as sea level varies thoughout the world so to derive sea level heights the GPS generally uses another model (geoid model) to determine separation values to apply to ellipsoidal heights to get sea level heights. Does/can get rather complicated and more involved than most GPS users are interested in.

Generally heights are based on mean sea level, which is based over all tide ranges over approx 20 years. However using GPS principles over the past many years, it has been found in some countries that in effect "mean sea level" has been found to be not as level as was assumed.

Over large areas the geoid model has inaccuracies but apart from that the initial GPS derived ellipsoid height is about 2-2.5 times less accuarte than the horizontal accuracy.

In practice the height derived by a recreational GPS is basically for general interest only, don't place too much critical importance on height.

Cheers, Kerry.

Edited by Kerry.
The height a GPS derives from the conversion of cartesian coordinates is generally referred to as an ellipsoidal height, which is based on a mathematical model. The difference between this height and what is generally referred to as sea level varies thoughout the world so to derive sea level heights the GPS generally uses another model (geoid model) to determine separation values to apply to ellipsoidal heights to get sea level heights. Does/can get rather complicated and more involved than most GPS users are interested in.

Generally heights are based on mean sea level, which is based over all tide ranges over approx 20 years. However using GPS principles over the past many years, it has been found in some countries that in effect "mean sea level" has been found to be not as level as was assumed.

Over large areas the geoid model has inaccuracies but apart from that the initial GPS derived ellipsoid height is about 2-2.5 times less accuarte than the horizontal accuracy.

In practice the height derived by a recreational GPS is basically for general interest only, don't place too much critical importance on height.

Cheers, Kerry.

Whoa........ what he said!

I believe what is referred to as sea level is determined by atmospheric pressure, since most altimeters are barometric.

Well..i wouldnt worry tooooo much...when i was living in my house down near the water my unit often told me I was twenty plus feet under it!...and so far I havnt had to break out the water wings!

The altitude on consumer GPSr units that calc it from the GPS system is not a very accurate reading. This is why there are a number of higher end models available with barometric altimeters, which give you a direct readout of density altitude.

Of course to make the density altitude read the same as actual altitude above MSL you need to correct for barometric pressure.

Altitude measurement is not all that easy with out continuing correction. Pilots correct their altimeter before takeoff and update it with the barometric pressure for the airport they are about to land at (and in route as needed).

You are not going to find any hand held device that with give you truely accurate altitude readings without being updated in real time for barometric pressure.

The good news is you usually don't need very high accuracy for altitude readings.

Dave_W6DPS

i was caching during the winter of this year, as the war in iraq was gearing up. my magellan showed me about 150' from the cache which was actually about 3/4 mile away and showed an elevation of 10,500 feet...not likely in central nj. it did that for about 20 minutes, then cleared up and never did it again. weird.

another time i was fishing on a charter boat and playing with my gps. the guy next to me asked "what's our elevation?". without missing a beat i told him "sea level". he got a dumb look on his face and that was the end of our conversation.

well that's my gps weird elevation story. good night....

another time i was fishing on a charter boat and playing with my gps. the guy next to me asked "what's our elevation?". without missing a beat i told him "sea level". he got a dumb look on his face and that was the end of our conversation.

Coulda been a charter boat on a mountain lake.

I stopped looking at the elevation, if I'm walking are standing still.

If your driving it works better, but still not sure if its right.

Last year I driving on a small highway in central Lousianna, (Hwy 28) The magallen said -20. I could see this reading in New olreans but not in middle of the state.

I drive a truck for a living, not sure which device I like best Magallen or Nokia

No wait the magallen is, it doesn't talk back

W Ray

The Z component of your coordinate displayed is an orthometric ht derived by combining your geoid ht and your ellipsoid ht(H=h-N). The GH is typically a negative value in North America. The term MSL has simply carried on for no particular reason. The old NAVD 29 elevations were based on an adjustment of numerous tidal BMs but NGVD 88 is based actually on one Bm in the St. Lawerence Seaway. The error in your Z will typically run 1.7 times that of the error you encounter horizontally. WGS 84 is an earth centered coordinate system and its origin has nothing to do with NAVSTAR. The various iterations of WGS go back to the 60's. Can anyone guess what the DOD needed a world-wide goedetic corrdinate system for?

Now grab your snow shoes and get outside!

Slightly off topic.

I heard an interesting tidbit on the radio today. There's some kind of flatness scale (I don't remember the name) similar to the hardness scale for minerals. A pancake is a .6, but the state of Kansas is a .3. So it seems that Kansas is really flatter than a pancake.

WGS 84 is an earth centered coordinate system and its origin has nothing to do with NAVSTAR. The various iterations of WGS go back to the 60's.

Hmm. Well here's what THEY say:

"WGS 84 was defined in January 1987 using Doppler satellite surveying techniques. It was used as the reference frame for broadcast GPS Ephemerides (orbits) beginning January 23, 1987. At 0000 GMT January 2, 1994, WGS 84 was upgraded in accuracy using GPS measurements. The formal name then became WGS 84 (G730) since the upgrade date coincided with the start of GPS Week 730. It became the reference frame for broadcast orbits on June 28, 1994. At 0000 GMT September 30, 1996 (the start of GPS Week 873), WGS 84 was redefined again and was more closely aligned with International Earth Rotation Service (IERS) Terrestrial Reference Frame (ITRF) 94. It is now formally called WGS 84 (G873). WGS 84 (G873) was adopted as the reference frame for broadcast orbits on January 29, 1997. "

Also I said it was Humes Ranch in Kansas earlier, but Its Meades Ranch.

WGS84 was the fourth generation of an Earth-centred reference frame developed by the US Defense Mapping Agency. WGS84 basically superceded WGS72, WGS66 and WGS60 all of which were predecessors to WGS84, much the same as WGS84 has been redefined several times since WGS84 was first adopted using techniques including GPS, laser measurements and other techniques.

WGS84 is based on GRS80 (Geodetic Reference System 1980), which are basically identical except for one minor exception. Until Jan 27, 1987 GPS still used WGS72. WGS72 was also used for the Transit Doppler System until Jan 1989, which was operational long before GPS was launched. At the time it was the best (earth-centred reference) there was.

As more and more data is collected WGS84 will probably be redefined again but any changes these days won't be as marked as those in the past, not that many users would have even noticed changes made to the GPS reference in recent years.

Defining WGS84 (as it became to be known) was basically in progress before GPS was launched.

Cheers, Kerry.

another time i was fishing on a charter boat and playing with my gps.  the guy next to me asked "what's our elevation?".  without missing a beat i told him "sea level".  he got a dumb look on his face and that was the end of our conversation.

Coulda been a charter boat on a mountain lake.

except that he was standing next to me on the boat in the florida keys. i suppose there could've been that kind of confusion otherwise.......

I operated a MX-1502 Doppler rig for DMA in the early 80's and at the time the technology seemed incredible. You simply put the unit together (60lbs) hook it up to a truck battery, lay out a pile of ant cable, slap a data tape into it and wait for four days to acquire a position. The was high tech let me tell you!

Cheers,

Pat

VT,

I know what the DOD needed a world-wide coordinate system for

1. Submarines

2. Things that take pictures of submarines

3. Things that go boom that come from submarines

4. Space Travel

5. "environmental" high-resolution radar mapping of the earths surface

6. Starting coordinates for high-speed low-drag single-use cylindrical sub-orbital flying objects with high mass, low volume and large atomic numbers

7. End ccordinates for high-speed low-drag single-use cylindrical objects

And as we used to say in the USAF...Things that go bump in the night.

Now, a trick question...why aren't GPS satellites Geosynchronous? (hint: What is the antenna angle at the north pole?) (Hint #2: Where is the antenna located on a submarine?)

Slightly off topic.

Ever use your GPSR in a plane at 36,000 feet elevation? Having only used a GPSR for 2.5 years, and not living in mountain country, other than flying, I have not been over 4 or 5,000 feet high since then. Kind of startling to see that high of an elevation. Interestingly, it took about 10 minutes to finally lock in and start spitting out numbers.

Where is the antenna located on a submarine?

That would depend on which of the many antennas you are talking about, and what they are designed to work with...

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.