# Elevation

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My son and I have a bet on this, How does a GPS know the elevation you are at? Thanx in advance

Same way it knows your latitude and longitude. It's all derived from the calculations of timing from the various satellites.

Edited to spell "Trimble" correctly.

Edited by Neo_Geo

well the short answer is most use a barometric altimeter to track recent changes in ambient air pressure and give you the elevation readings.. The barometric altimeter will accurately report elevations ranging from -2000 to 30,000 feet.

This is why you cannot get an accurate elevation reading in a plane (because the cockpit is normalized.)

so who wins?

Edited by kgag

well the short answer is most use a barometric altimeter to track recent changes in ambient air pressure and give you the elevation readings..

This is why you cannot get an accurate elevation reading in a plane (because the cockpit is normalized.)

so who wins?

Do they? I never knew that. My Meridian Gold uses GPS satellite data and that requires at least 4 satellites sending data. I like this becuase it will work in an airliner cabin. I was under the impression that most all GPSr's even those with barometric altimeters, still had the capability to use recieved satellite data to determine altitude. Dang I really do like my Meridian Gold.

The correct answer is it depends. If your GPS Unit has a built in barometric altimeter, it uses it to determine altitude, and it uses the elevation derived from the satellites to correct or calibrate the baro reading (that is how Garmins work at least). If your unit does not have the built in altimeter then it is just as Neo Geo said. It uses the calculations based on the timeing of the satellites.

How does a GPS know the elevation you are at?

He's asking how a GPS gets it's elevation information. Sensors play absolutely no part of the equation! If your unit has sensors, then the ALTIMETER function will rely on barometric pressure.

Edited by Neo_Geo

How does a GPS know the elevation you are at?

He's asking how a GPS gets it's elevation information. Sensors play absolutely no part of the equation! If your unit has sensors, then the ALTIMETER function will rely on barometric pressure.

Good point. I just wanted him to understand that some units use barometric pressure and others do not. You are absolutely correct that the GPS receiver itself uses the only satellites do determine elevation.

Most gps's dont have barometric sensors but they still know the elevation.

***As has been mentioned, a GPS gets its elevation data the same way it gets its horizontal data. From satellite geometry.***

Newer gps's with barometric sensors can use the barametric pressure to augment the gps reading and make it more accurate (airplanes altimeters are barometric sensors but these have to be set to the locals current barometric pressure to be accurate. But a GPS doesn not need a barometric sensor to tell you elevation.

My son and I have a bet on this, How does a GPS know the elevation you are at? Thanx in advance

I was under the impression that most all GPSr's even those with barometric altimeters, still had the capability to use recieved satellite data to determine altitude.

I believe they do. My 60csx uses the altimeter primarily but I can ask it what my GPS elevation is.

So who won the bet?

I did.

Well, my gpsV doesn't have an any type of "sensor" and I still get altitude.

So, if we are sure that the csx garmins use the barometer primarily and just the gps altimiter as the calibrator then there are two answers.

A gps with no barometric sensor gets its altitude from the satelltes (its all its got).

And a garmin with sensors uses both (now how about magellan and the rest??).

I was under the impression that most all GPSr's even those with barometric altimeters, still had the capability to use recieved satellite data to determine altitude.

I believe they do. My 60csx uses the altimeter primarily but I can ask it what my GPS elevation is.

I did.

I take that you are the son of IMcachingHomer...?

I would have to say I won the bet. I am the father and my son (who knows everything) will never admit I am right but since I have some replies he'll have to admit he's wrong. Thanks for all your help.

Darn son's!! Hard to live with em once they hit 14..LOL

So was your position "satellite altitude" or "pressure altitude"

I would have to say I won the bet. I am the father and my son (who knows everything) will never admit I am right but since I have some replies he'll have to admit he's wrong. Thanks for all your help.

It's a question of semantics....If by GPS, you mean the whole unit..it has two sources in the units with a barometric sensor. If you mean just the GPS "chip", it's satellite derived.

In my 60 CS, if I turn the "GPS chip" off, the barometric sensor still records and diplays altitude readings on the unit. So the source of the data is not "satellite" derived, but rather barometric sensor derived.

In a unit without barometric sensors, the only source of elevation data is from the GPS "chip" itself...satellite derived.

Looks like you owe each other a beer

The barometric altitude will only be accurate if the weather hasn't changed appreciably since the last time you calibrated it. Also, you can measure change in altitude over a short amount of time without calibrating the barometer. The altitude number it says will be wrong, but it should still be able to tell you that you are 50 feet higher than you were last time you checked.

Edited by ejnewman

It's a question of semantics....If by GPS, you mean the whole unit..it has two sources in the units with a barometric sensor. If you mean just the GPS "chip", it's satellite derived.

It's NOT a matter of semantics! It's a matter of understanding WHAT the subject matter is!

AGAIN - READ THE OP'S QUESTION!!!

How does a GPS know the elevation you are at?

Don't read anything into this! MOST GPS RECEIVERS DO NOT HAVE SENSORS!!! THE ONES THAT DO HAVE THE SENSORS USE BAROMETRIC PRESSURE FOR THE ALTIMETER FEATURE. THIS HAS ABSOLUTELY NOTHING TO DO WITH GPS (other than the fact that the device happens to be on the GPSr). THE ALTIMETER IS OFTEN CALIBRATED BY - GUESS WHAT...? - ALTITUDE DERIVED FROM GPS DATA! SENSOR-EQUIPPED MODELS OFTEN IGNORE THE GPS ALTITUDE AND DISPLAY THE BAROMETRIC PRESSURE ALTITUDE - BUT THAT'S NOT "How a GPS knows the elevation you are at"!

GPS RECEIVERS DERIVE ALTITUDE FROM CALCULATIONS FROM SATELLITE DATA - PERIOD!

Edited by Neo_Geo

It's a question of semantics....If by GPS, you mean the whole unit..it has two sources in the units with a barometric sensor. If you mean just the GPS "chip", it's satellite derived.

It's NOT a matter of semantics! It's a matter of understanding WHAT the subject matter is!

AGAIN - READ THE OP'S QUESTION!!!

How does a GPS know the elevation you are at?

Don't read anything into this! MOST GPS RECEIVERS DO NOT HAVE SENSORS!!! THE ONES THAT DO HAVE THE SENSORS USE BAROMETRIC PRESSURE FOR THE ALTIMETER FEATURE. THIS HAS ABSOLUTELY NOTHING TO DO WITH GPS (other than the fact that the device happens to be on the GPSr). THE ALTIMETER IS OFTEN CALIBRATED BY - GUESS WHAT...? - ALTITUDE DERIVED FROM GPS DATA! SENSOR-EQUIPPED MODELS OFTEN IGNORE THE GPS ALTITUDE AND DISPLAY THE BAROMETRIC PRESSURE ALTITUDE - BUT THAT'S NOT "How a GPS knows the elevation you are at"!

GPS RECEIVERS DERIVE ALTITUDE FROM CALCULATIONS FROM SATELLITE DATA - PERIOD!

Boy you must really be some kind of a pompass a**. Try not to get your undies all bunched up, come down off your throne and listen...that's right, listen to what someone is saying.

As i said, it's semantics....I see now you have changed the OP's words to "receivers"....he never said anything about the receiver...he said GPS. There are all sorts of altimeters out there, most wrist styles rely on barometric readings( you know, the ones worn by mountain climbers). Many GPS units can use both, with an option to have the "GPS" chipset (satellite) calibrate the pressure derived readings. In truth, on the Garmin units with pressure sensors, It's this autocal feature that frequently adds error to the altimeter reading. Anyone can verify this by standing in one spot and asking the unit to give you the "Satellite" derived altitudes...they vary by a wide margin over a period of minutes, depending, as does your location, on the configuration of available sats and the orientation of the unt. On a clear day with little or no change in the pressure at sea level, using the altimeter with the autocal ("satellite" derived) function off, the altimeter is much more accurate. Of course this leaves you open to errors if there is a weather realted drop in atmospheric pressure at sea level.

To each his own on how to deal with the accuracy or method of using the elevation data.....but I can simply tell you the altimeter works fine, even better, with the GPS side of the contribution turned off.

Lastly, I feel badly if no one listens to you at home, although I can sort of see why, with the attitude you have about anyone disagreeing with you. But you should leave your frustrations at home and not bring them to this forum. And try to grow up a little...using acronyms like W.T.F. illustrates your selfish, childish attitude.

My GPS 60's altitude is not very accurate. I wanted to find the height of a large hill, so I let it average for 5 minutes at the top and the bottom, and the bottom was only 6 feet below the top, while the hill was probably a 60 feet hill. I haven't trusted it since.

Check out the Trimble GPS tutorial as mentioned in POST #2. When they get to the part about magnetic compasses and barometric altimeters, post a reply here and then we'll discuss it...

And yes, I am a pompous a§§ when people refuse to listen to common sense.

It's a question of semantics....If by GPS, you mean the whole unit..it has two sources in the units with a barometric sensor. If you mean just the GPS "chip", it's satellite derived.

It's NOT a matter of semantics! It's a matter of understanding WHAT the subject matter is!

AGAIN - READ THE OP'S QUESTION!!!

How does a GPS know the elevation you are at?

Don't read anything into this! MOST GPS RECEIVERS DO NOT HAVE SENSORS!!! THE ONES THAT DO HAVE THE SENSORS USE BAROMETRIC PRESSURE FOR THE ALTIMETER FEATURE. THIS HAS ABSOLUTELY NOTHING TO DO WITH GPS (other than the fact that the device happens to be on the GPSr). THE ALTIMETER IS OFTEN CALIBRATED BY - GUESS WHAT...? - ALTITUDE DERIVED FROM GPS DATA! SENSOR-EQUIPPED MODELS OFTEN IGNORE THE GPS ALTITUDE AND DISPLAY THE BAROMETRIC PRESSURE ALTITUDE - BUT THAT'S NOT "How a GPS knows the elevation you are at"!

GPS RECEIVERS DERIVE ALTITUDE FROM CALCULATIONS FROM SATELLITE DATA - PERIOD!

Boy you must really be some kind of a pompass a**. Try not to get your undies all bunched up, come down off your throne and listen...that's right, listen to what someone is saying.

As i said, it's semantics....I see now you have changed the OP's words to "receivers"....he never said anything about the receiver...he said GPS. There are all sorts of altimeters out there, most wrist styles rely on barometric readings( you know, the ones worn by mountain climbers). Many GPS units can use both, with an option to have the "GPS" chipset (satellite) calibrate the pressure derived readings. In truth, on the Garmin units with pressure sensors, It's this autocal feature that frequently adds error to the altimeter reading. Anyone can verify this by standing in one spot and asking the unit to give you the "Satellite" derived altitudes...they vary by a wide margin over a period of minutes, depending, as does your location, on the configuration of available sats and the orientation of the unt. On a clear day with little or no change in the pressure at sea level, using the altimeter with the autocal ("satellite" derived) function off, the altimeter is much more accurate. Of course this leaves you open to errors if there is a weather realted drop in atmospheric pressure at sea level.

To each his own on how to deal with the accuracy or method of using the elevation data.....but I can simply tell you the altimeter works fine, even better, with the GPS side of the contribution turned off.

Lastly, I feel badly if no one listens to you at home, although I can sort of see why, with the attitude you have about anyone disagreeing with you. But you should leave your frustrations at home and not bring them to this forum. And try to grow up a little...using acronyms like W.T.F. illustrates your selfish, childish attitude.

What he said

Check out the Trimble GPS tutorial as mentioned in POST #2. When they get to the part about magnetic compasses and barometric altimeters, post a reply here and then we'll discuss it...

And yes, I am a pompous a§§ when people refuse to listen to common sense.

You miss the point again, but after this I'll leave you on your own to deal with your problem. Depending on the unit and user slelection, the GPS unit can use either the barometric sensor, the GPS satellite derived altitude, or a combination of both, with the GPS altitude "correcting" or calibrating the reading. I don't think you're that ignorant, but using a Garmin 60s series unit, and turning off the GPS chipset will leave you with altitude readings on the altimeter....I'll say it slowly...turn off the GPS and you still get accurate altitude readings from the unit....the readings are very accurate, and have nothing to do with GPS chipset in the unit. So, for those who can't read others writings without inferring things that aren't there..this might help you. Based on Garmin 60CS

GPS unit without barometer = Satellite derived altitude readings

GPS unit with barometer in autocal mode = GPS altitude readings derived from barometer which is calibrated by GPS derived altitude.

GPS unit with barometer and autocal off = Altitude derived from Barometric sensor only. 0, zero, nada, zip, non-existant, contribution made by the GPS chipset.

Personally, I find the GPS contribution to be the leading cause of inaccuracy. I have repeatedly checked satellite derived altitudes over a 1/2 hr period and they were all over the place....nice day, no weather change, I shut the autocal off and the unit is much more accurate.

GPS unit with barometer and autocal off = Altitude derived from Barometric sensor only. 0, zero, nada, zip, non-existant, contribution made by the GPS chipset.

MY POINT EXACTLY!!! Any altitude information given by the barometric altimeter HAS NOTHING TO DO WITH GPS!!!

AGAIN - READ THE OP'S QUESTION! REALLY READ IT THIS TIME - ONE-WORD-AT-A-TIME to the end:

My son and I have a bet on this, How does a GPS know the elevation you are at? Thanx in advance

"How does a GPS know the elevation you are at?"

When you start throwing in things like "units with altimeters", "barometer and autocal off", you're reading stuff into the question. An eTrex Yellow is a simple GPS receiver. It has no barometric sensor. It simply gets its altitude information from calculating distances from the satellites! That is how a GPS knows the elevation you are at! A unit with a barometric altimeter DOESN'T EVEN NEED THE ALTIMETER to know its altitude!

I see now you have changed the OP's words to "receivers"....he never said anything about the receiver...he said GPS.

Well, the GPS system consists of satellites, ground stations and receivers. I made the assumption that he was talking about a receiver. My apologies to the OP if he has a satellite or ground station. Still, it would not change the answer.

Regardless of the answer, you may find this appropriate:

Mark Twain: "When I was a boy of 14, my father was so ignorant I could hardly stand to have the old man around. But when I got to be 21, I was astonished at how much the old man had learned in seven years."

wow sorry I brought up the barometric altimeter thing....

If we read the OP very carefully the answer is a "GPS" has absolutly no idea what altitude you are at, a "GPS" does not even know that you or your GPS receiver exists.

Of course this does ASSUME that you are wearing your tin-foil hat so the GPS can't read your brain waves and even if you do forget your hat a GPS still won't know what your alltitude is unless you know what it is so it can extract the information from your brain.

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