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60csx Altimeter: Fixed Elevation


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I am having a great time trying out all the features of my new 60CSx. Here's something I don't understand: I choose Barometer Mode: Fixed Elevation in the altimeter setup, but I still see the elevation changing. Here are the details of the setup:

 

- GPS is Off.

- Altimeter Auto Calibration is Off.

- Altimeter is set to Fixed Elevation.

- I calibrated the altimeter to my known elevation (104 meters).

 

With these settings I would expect the elevation reading not to change. It can't be affected by GPS data since GPS is turned off. And I turned off auto calibration, so the altimeter shouldn't be surreptitiously re-calibrating itself. I thought the whole point of the setting "Fixed Elevation" is that only the barometer reading should change, but I see that the Elevation reading changes as well. Since this morning the Elevation reading has crept up from 104 meters to 114 meters.

 

Can anybody explain why this happens?

 

Best wishes,

 

Michael

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A barometric altimeter is subject to the current air pressure, as the pressure changes the altitude reading will change.

 

Yes, certainly that is true if "Variable Elevation" is chosen in the setup. But the point of the "Fixed Elevation" setting is to keep the elevation reading constant, so that the barometer will function like a standard fixed barometer, showing the ambient pressure. Why have a "Fixed Elevation" setting if it doen't in fact fix the elevation?

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- GPS is Off.

- Altimeter Auto Calibration is Off.

- Altimeter is set to Fixed Elevation.

- I calibrated the altimeter to my known elevation (104 meters).

 

I thought that the altimeter needed the barometer to evaluate the elevation... If you choose "Fixed elevation", don't you have to turn on GPS so it can get its elevation data from the satellites rather than the barometer?

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Good question, I've never tried that configuration and I agree with you that it "fixed elevation" should do that...

 

I'll try mine out later and see what it does.

So, what happens if you turn ON the GPS and fix the elevation? Any difference?

 

I am having a great time trying out all the features of my new 60CSx. Here's something I don't understand: I choose Barometer Mode: Fixed Elevation in the altimeter setup, but I still see the elevation changing. Here are the details of the setup:

 

- GPS is Off.

- Altimeter Auto Calibration is Off.

- Altimeter is set to Fixed Elevation.

- I calibrated the altimeter to my known elevation (104 meters).

 

With these settings I would expect the elevation reading not to change. It can't be affected by GPS data since GPS is turned off. And I turned off auto calibration, so the altimeter shouldn't be surreptitiously re-calibrating itself. I thought the whole point of the setting "Fixed Elevation" is that only the barometer reading should change, but I see that the Elevation reading changes as well. Since this morning the Elevation reading has crept up from 104 meters to 114 meters.

 

Can anybody explain why this happens?

 

Best wishes,

 

Michael

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I duplicated the OP's problem with my 60csx. I didn't have a lot of time but the altitude did change. The question is this: does it work as a reliable barometer in the above configuration? I didn't have time to test that. I suspect that the ALTITUDE is still changed based on the satellites but that the barometer millibars are being adjusted based on the air pressure (given the *starting* altitude.) Garmin may have an explanation.

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Page 76 in the manual.

 

Setting the altimeter to fixed allows you to use it as a standard barometer. After setting the altimeter to fixed go to the altimeter page, select menu>Plot Barometer. If needed you will need to set the units you would like to read the info in ie.... Inches of Hg, Millibars, Hectopascals. You can change this on the Units page.

 

Selecting "Fixed elevation" tells the unit that you are not moving and staying at a fixed location. This enables it to work as a barometer. Why concern yourself with elevation if you want to measure the barometric pressure? A standard Barometer can show an elevation change of 40'/Hr as the weather changes. Barometers are also sensitive to temperature among other environmental changes that will also show as an elevation change.

Edited by Team Trail Walker
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Selecting "Fixed elevation" tells the unit that you are not moving and staying at a fixed location. This enables it to work as a barometer. Why concern yourself with elevation if you want to measure the barometric pressure

Precisely: why does the Garmin concern itself with elevation if it's only meausuring the barometric pressure? Since GPS is turned off and I've told the unit to fix the elevation at 104 meters, that is where it should stay. Of course I could just ignore the changing altitude reading, but it does make me suspicious that something is not functioning correctly.

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You missed the point I think.

 

Setting "Fixed elevation" does not stop the elevation from changing, it tells the unit that you are not moving and you are using is as a barometer. Like I said before as the barometric pressure changes the result will show as an elevation change. That is how is works.

 

I don't think that is how it should work. I am not telling the unit that I am not moving. I am telling it that it is not moving. In particular, I am telling it that it is staying at a certain height (104 meters).

 

A barometric altimeter only measures one thing: the pressure of the air ("ambient pressure"). When there is a change in the air pressure, the sensor has no way of telling if this pressure change is due to a change in elevation or a change in the weather (i.e a change in the barometric pressure, being the air pressure at sea level). You, the human user, must decide whether to interpret the change as a change in elevation or a change in the weather. For the first option you make the assumption that the barometric ("weather") pressure does not change, for the second option you make the assumption that the elevation does not change.

 

In this case you use a known value for the elevation and use the standard barometric formula to calculate the barometric pressure. When I select "fixed elevation" for the altimeter, I am telling the unit to use the elevation value I entered to calculate the pressure. This only makes sense if the value for the elevation remains constant.

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Think of it like a Thermometer that has two scales, one Fahrenheit the other Celsius. Or the speedometer on your car has Miles and Kilometers. Barometers can have two scales on it also, Altitude and Pressure. The difference is that the barometer needs to be calibrated for it to reed correctly. Setting the altitude "fixed altitude" your telling the unit how to compensate for the ambient pressure at that altitude or adjust its scale to read correctly. With the scale set at the proper setting one can read the correct Barometric pressure.

 

Likewise, if your unit is calibrated for the current Barometric Pressure ( or altitude) and a storm rolls in it will now effect your altitude reading.

 

Pressure Change = Altitude Change

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I guess the question is, then, why would you even have to tell it, its at a fixed location? I'm mean, when you put it in barometer mode all it knows is that the pressure has gone up or down. Elevation data from the gps should make no difference and there should not even be an option on there to tell it its in a fixed location. I mean, theres an elevation screen that plots vs time or distance and then theres a barometer screen. Why would the barometer care what evelation you are at and why would it need to be told you are at a fixed elevation since it shouldnt' care anyway. Only the altimeter functions of the GPS should care about that.....

 

Think of it like a Thermometer that has two scales, one Fahrenheit the other Celsius. Or the speedometer on your car has Miles and Kilometers. Barometers can have two scales on it also, Altitude and Pressure. The difference is that the barometer needs to be calibrated for it to reed correctly. Setting the altitude "fixed altitude" your telling the unit how to compensate for the ambient pressure at that altitude or adjust its scale to read correctly. With the scale set at the proper setting one can read the correct Barometric pressure.

 

Likewise, if your unit is calibrated for the current Barometric Pressure ( or altitude) and a storm rolls in it will now effect your altitude reading.

 

Pressure Change = Altitude Change

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KB9mvh: you make a very good point which I totally overlooked. The unit measures barometric pressure, period. It is *always* a barometer, why have a special setting to fix the altitude if it is not needed/used?

 

I suspect what is happening is this: You tell it you are at a fixed altitude (barometer mode), the unit stops using barometric pressure to calculate your altitude BUT it still uses the satellites to determine altitude. Just a guess.

 

The way to test this is to put it in barometer mode with a storm (big pressure change) approaching and see if the altitude changes a lot.

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We need a science guy to explain the workings of a barometer better than I. Someone?

 

Page 76 of the manual states that trip data is not recorded in "Fixed Mode". Also-

 

Without knowing the exact details I will say that a Barometer to read correctly at sea level and at high altitude an adjustment must be made. The Barometer in my office can be adjusted mechanically while I suspect the Barometer in the 60CSx is adjusted with an algorithm where altitude is one variable.

 

Hopefully a smart science type will jump in and explain better the whole process and correct any incorrect statements I have made in my limited knowledge.

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We need a science guy to explain the workings of a barometer better than I. Someone?

 

Page 76 of the manual states that trip data is not recorded in "Fixed Mode". Also-

 

Without knowing the exact details I will say that a Barometer to read correctly at sea level and at high altitude an adjustment must be made. The Barometer in my office can be adjusted mechanically while I suspect the Barometer in the 60CSx is adjusted with an algorithm where altitude is one variable.

 

Hopefully a smart science type will jump in and explain better the whole process and correct any incorrect statements I have made in my limited knowledge.

 

Just thoughts...

 

I'm with michael cook on this one...If you tell the unit you're at a fixed elevation, that means any and all pressure changes should be viewed as resulting from Weather variations and not altitude variations. As such the altitude reading should not change...why bother asking if your evevation is "fixed" if the unit is just going to ignore it and do what it normally does. My hunch is that the "fixed" option is a redundancy on the auto cal function, temporarily overriding the GPS sat derived auto cal feature until you turn it off.

 

When given a known altitude, the unit takes it's barometric sensor reading and then based on the altitude, derives the Barometric pressure at sea level, and "re-sets" that number as a base figure.

 

When you calibrate the unit by giving it a known atmospheric pressure, this number should be from a reference station and is based on the pressure at sea level...if the sation is not at sea level itself, it has been adjusted to reflect what the pressure would be if it were at sea level. That pressure is then "re-set" as the base figure.

 

There are only two variables that affect the barometer and cause a change in the pressure..one is the altitude which decreases with height...the other is the movements of air masses of greater or lower pressure..Highs and Lows.

 

If you interested in just using the unit as a weather barometer, removing any errors from the auto cal function should leave you with simple barometric changes that are weather related .. eg not subject to the autocal errors cause by bad sat configuration, or antenna orientation.

 

If Michael is seeing a fluctuating altitude reading in this mode, that just seems wrong to me, unless it's just one of those "not so smart" implementations by the Garmin programmers. For the common user, if he sees an altitude movement while in the "fixed elevation" mode, it's natural to think the unit is not working correctly. Not quite sure on this as I haven't used the feature a heck of a lot...but ambient pressure to me is the total unadjusted pressure reading at Your current location. Whereas the barometric pressure is the pressure corrected to what it would be if you were at sea level.

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Picked this up off the internet:

 

An altimeter is a device designed to estimate altitude above mean sea level (MSL). It does

 

so solely on the basis of atmospheric pressure. In fact, all altimeters have TWO dials--one

 

for altitude and one for pressure. An altimeter is basically a mechanical calculator which

 

translates a change in atmospheric pressure into a change in altitude based on the idea of

 

a 'standard' atmosphere.

 

This hypothetical atmosphere starts at 'mean sea level' with a temperature of 15 C

 

and 29.92 inches of mercury (inHg). It decreases in pressure at a rate of about 1 inHg per

 

1000 ft elevation and decreases in temperature at a rate of about 2 C per 1000 ft elevation.

 

Due to the unequal heating of the earth's surface by the sun, however, the real

 

atmosphere is far from 'standard', and if an altimeter were not adjustable, its reading would

 

be dangerously inaccurate. To account for the natural variation in barometric pressure

 

around the world, the aviation community arrived at the idea of an 'altimeter setting'.

 

All altimeters have a small knob at the bottom left which allows the altimeter to be set

 

based on the local atmospheric conditions. A pilot can adjust this knob while on the ground

 

at an airport until the altimeter reads the airport's known elevation. In addition, thousands of

 

airports around the world have automated stations which determine and broadcast over the

 

radio the exact 'altimeter setting' (in inHg) which should be used at their location to have an

 

altimeter on the runway read the correct airport elevation. Pilots flying below 18,000 ft MSL

 

listen to these broadcasts and every 100 miles or so readjust their altimeters in flight so that

 

they read appropriately for the local atmospheric conditions. At 18,000 ft MSL and above,

 

pilots simply ignore local conditions and set their altimeter to 29.92 inHg. This action

 

ensures both a high probability of being at the indicated altitude when landing and a low

 

probability of two aircrafts colliding due to non-standard atmospheric conditions when

 

cruising at high altitude.

 

The idea behind using an altimeter as a barometer is this: When adjusted to read

 

true altitude, altimeters display in their altimeter setting window the hypothetical barometric

 

pressure at mean sea level assuming a standard atmosphere. On the other hand, when

 

adjusted to read 0 ft altitude, they display in their altimeter setting window the true

 

barometric pressure. As the true barometric pressure drops, they indicate a rising altitude,

 

as though in a climbing aircraft. At the same time, the pressure read in the altimeter setting

 

window remains constant, as though based at an airport below. However, if they are

 

readjusted to read 0 ft altitude, the pressure they display will drop to reflect the true drop in

 

atmospheric pressure.

 

Thus, to use an altimeter as a barometer, simply adjust it to read 0 ft altitude and read

 

the true pressure in inches of mercury in the altimeter setting window.

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I also tried the same experiment Michael Cook outlined:

 

GPS is Off.

- Altimeter Auto Calibration is Off.

- Altimeter is set to Fixed Elevation.

I calibrated the altimeter to my known elevation (264 feet).

 

My results were similar: The elevation display initially read the calibrated value of 264 ft., then inversely tracked changes in the barometric pressure in the ratio of 10 ft. per 00.01 inches of Hg. As a further test I noted the current barometer reading (it was 29.49"), then I recalibrated the elevation up 100 ft. to 364 ft. The barometric display immediately changed to 29.59". Since the unit measured that ambient pressure had not changed but that the"fixed elevation" was one hundred feet higher, it adjusted the corrected sea level barometric display appropriately.

 

I would agree with others that in the fixed elevation mode, the elevation readout should not change, only the barometric pressure display. In fact, the screen should probably also indicate that the unit is in the fixed elevation mode, so the user doesn't wonder why the elevation display isn't changing even though he might have moved his postion up or down.

 

Don

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You're misunderstanding what the fixed/variable elevation setting for the altimeter actually does, although the manual should make it clearer.

 

Either way, you can view the altimeter pressure plot or if tracking is enabled, the elevation plot. What the setting affects is what elevation is saved in the track log. Variable records the elevation indicated by the altimeter, Fixed indicates the use of the altimeter and so the GPS elevation is recorded instead.

Edited by Frabble
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You're misunderstanding what the fixed/variable elevation setting for the altimeter actually does, although the manual should make it clearer.

 

Either way, you can view the altimeter pressure plot or if tracking is enabled, the elevation plot. What the setting affects is what elevation is saved in the track log. Variable records the elevation indicated by the altimeter, Fixed indicates the use of the altimeter and so the GPS elevation is recorded instead.

I'm not sure about the track log, but Garmin says the following with respect to the trip computer:

Because of the filtering used on the "Fixed" mode to give a more stable elevation reading from the GPS, the device will not record this information on the trip computer. This prevents giving erroneous data if the elevation is being changed in this mode.

I haven't tried it, but it stands to reason that they may not record it in the track log either.

 

And another caution:

There must be an active track log (not a saved track log or activated TracBack) to see the elevation profile. If the track log has been cleared, there will not be an elevation profile.

Experience shows that the Track Logs saved on the card are considered active logs and do contain the elevation information. Again, I don't know if they do when in "fixed" mode.

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My summary:

The altimeter data can be presented as either pressure or elevation regardless of the mode setting.

If set for Fixed Elevation you should obviously view the Altimeter page as a Pressure plot. If you're then confused by the changing Elevation on the Trip Computer page then remove or change it as a data field.

 

Why then change between Variable or Fixed Elevation? It affects the elevation recorded in the track logs. Variable uses that from the altimeter, Fixed from the GPS receiver.

 

dogwalkers2, I can't find any of those Garmin quotes in the latest 60CSx manual.

You can easily verify the mode of operation on your own unit. Obtain a 3D signal and view the GPS Elevation. Have the Altimeter mode set to Variable and calibrate to 2000 feet above the GPS elevation. Turn Tracking on and wait a few minutes for points to be plotted. Change Altimeter mode to Fixed elevation and again wait a few minutes. Download and view the track log in MapSource. The elevation recorded will first be that of the Altimeter, then GPS.

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... found them.

The Fixed mode filtering referred to the Rino series.

Agree with the second quote. Can't see this as a problem since elevation data is included in the active track log whichever mode.

Ya, in the mouse over, it mentions the Rino series. If you go to the whole article (which is just as short as what shows up on the mouse over), it lists all the units it applies to and basically includes everything that has the sensors (Vista HCx, 60CSx, the Appropriate Rinos, etc.)

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Ya, in the mouse over, it mentions the Rino series. If you go to the whole article (which is just as short as what shows up on the mouse over), it lists all the units it applies to and basically includes everything that has the sensors (Vista HCx, 60CSx, the Appropriate Rinos, etc.)

That list appears under Other Recommended links and points to other Q/As for those units and is not what is affected. I have two of those units mentioned and both display/record elevation in fixed mode, as I've suggested you do above, so that point doesn't apply here.

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That list appears under Other Recommended links and points to other Q/As for those units and is not what is affected. I have two of those units mentioned and both display/record elevation in fixed mode, as I've suggested you do above, so that point doesn't apply here.

My experience is that Garmin's published statements with regard to elevation data displayed and recorded in fixed elevation mode are just plain wrong. As you have said, the best way to find out what the unit does is to experiment with it. I think we can say that the 60CSx clearly records the barometric elevation in the active track log and in the gpx files on the SD card when in the variable elevation mode, and the gps elevation when in the fixed elevation mode; experiments you suggested clearly prove this. The 60CS by the way doesn't switch to gps elevation in the fixed mode, so I don't think there is any functional reason for them to have switched on the 60CSx - I think it's just something that slipped in while they were writing the software. I'm glad they did though; I hope they don't consider it a "bug" and change back in a future firmware update. I haven't heard of any other units having this "feature"; the 76CSx almost certainly has it, but I haven't heard of anybody checking on it or any of the newer Garmins. It would be interesting to get some feedback on that.

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That list appears under Other Recommended links and points to other Q/As for those units and is not what is affected. I have two of those units mentioned and both display/record elevation in fixed mode, as I've suggested you do above, so that point doesn't apply here.

My experience is that Garmin's published statements with regard to elevation data displayed and recorded in fixed elevation mode are just plain wrong. As you have said, the best way to find out what the unit does is to experiment with it. I think we can say that the 60CSx clearly records the barometric elevation in the active track log and in the gpx files on the SD card when in the variable elevation mode, and the gps elevation when in the fixed elevation mode; experiments you suggested clearly prove this. The 60CS by the way doesn't switch to gps elevation in the fixed mode, so I don't think there is any functional reason for them to have switched on the 60CSx - I think it's just something that slipped in while they were writing the software. I'm glad they did though; I hope they don't consider it a "bug" and change back in a future firmware update. I haven't heard of any other units having this "feature"; the 76CSx almost certainly has it, but I haven't heard of anybody checking on it or any of the newer Garmins. It would be interesting to get some feedback on that.

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Wait a minute. So if I set my altimieter to FIXED elevation then all my altimeter displays will be the GPS reading??? (instead of the pressure sensor one). Man if this is true (I fly home from Dallas thursday) then the altitude might just work in a pressurized cabin of a jet.

 

Also, if this turns out to be true, Garmin deserves a SLAP for not figuring this out and letting us in on it long ago when everyone was asking about this.

 

(Or did I just mis-read this thread and no one was implying what I just said?)

 

 

Sorry about the double post; I got a php error after a 45 second timeout, so hit reply agsin; evidently the reply had be posted before the error occurred.

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Wait a minute. So if I set my altimieter to FIXED elevation then all my altimeter displays will be the GPS reading??? (instead of the pressure sensor one). Man if this is true (I fly home from Dallas thursday) then the altitude might just work in a pressurized cabin of a jet.

No, the displays remain the barometric elevation. But the gps elevation is recorded in the active track log and in the gpx logs on the SD card, so you can have a record of your actual flight elevations when you get home. During the flight you can view the current elevation via a menu option on the satellite page, or by generating a waypoint and averaging it, but you can't get a continuous readout of the gps elevation.

 

Why Garmin won't implement a GPS elevation option is a mystery to me. It **should** be a very simple software change (but this spoken by someone who has never seen their software, so who knows?). Normally I'm very satisfied with the barometric altitude, but flying in a pressurized plane is the one place where being able to use the GPS elevation would be nice.

 

To reiterate, this works specifically for the 60CSx; the 60CS definitely does not have this "feature"; the 76CSx probably does, and maybe the other "x" models, but I don't have any direct knowledge other than for the 60CS and the 60CSx (well, for the original Vista - it doesn't have it either :laughing: )

 

Just to elaborate a little further: In playing with it some more today, if you are in fixed elevation you don't have to average the waypoint to see the GPS elevation - it's the one recorded when you mark it if you are in fixed elevation. Also, if you have the elevation plot displayed on the Altimeter page, it appears to be the GPS elevation except for the right-most few points. The elevation read out at the bottom will still be the barometric elevation, but if you switch to pan by pushing the left arrow on toggle button it will display the GPS elevation (but at a slightly earlier time).

Edited by Hertzog
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