Gps Altimeter Vs. Barometric Altimeter

[Bite_Me]

I'm looking for a new GPS and I'm trying to decide between the Garmin 60csx and 60cx handhelds. I was wondering why the 60csx has two ways of determining the altitude?? By way of locking unto 4 or more satellites or using the barometric pressure. I would think the GPS altimeter would be more accurate than the barometric altimeter, given the fact air pressure is constantly changing. So what is the advantage of including a barometric altimeter into a GPS? Those who own the 60csx or 76csx units do you find the altitude readings from the GPS altimeter are always close to the readings from the barometric altimeter?

 

Thanks,

[Michael Cook]

GPS does not determine altitude as accurately as it determines the horizontal co-ordinates. A barometric altimeter can give you better accuracy, provided you understand its limitations.

 

The important thing to consider with a barometric altimeter is that it only measures the pressure of the air around it ("ambient pressure"). When this pressure changes, the change could be a result of a change in the weather ("barometric pressure" or pressure measured at sea level), a change in altitude, or a combination of both. It's up to you to decide. If the barometer is fixed to the wall of the room, you can safely assume that any change that it measures in the air pressure is due to a change in the weather. If you have a barometer with you on a one hour balloon flight, you may reasonably assume that changes in the measured air pressure during this hour are mostly due to changes in altitude.

 

We have to count on the fact that the barometric pressure doesn't usually change very rapidly. In the case of the GPSr, if you go on a hike or a ride that lasts a few hours, it is reasonable to assume that the changes in measured air pressure are largely due to changes in altitude. Whenever you are at a known altitude, or can find out the barometric pressure at sea level, you should use the opportunity to re-calibrate the altimeter. For instance, if you start out from somewhere where there is a fixed barometer, you calibrate the altimeter to the value given by this barometer. If, when you reach point "x", you have the possibility of phoning somebody who can give you the present reading of the fixed barometer, you can re-calibrate the altimeter and calculate the difference in altitude between where you started and point "x" to an accuracy of a few feet.

 

A little personal test: I left my 60CSx near the window of my home (where the GPS reception is admittedly pretty bad) for a few hours. Within this time I noticed GPS elevation readings as different as 72 meters and 123 meters. During the same time the barometric altimeter registered values between 102 meters and 109 meters.

[+Prime Suspect]

I'm looking for a new GPS and I'm trying to decide between the Garmin 60csx and 60cx handhelds. I was wondering why the 60csx has two ways of determining the altitude?? By way of locking unto 4 or more satellites or using the barometric pressure. I would think the GPS altimeter would be more accurate than the barometric altimeter, given the fact air pressure is constantly changing. So what is the advantage of including a barometric altimeter into a GPS? Those who own the 60csx or 76csx units do you find the altitude readings from the GPS altimeter are always close to the readings from the barometric altimeter?

 

Thanks,

The rule of thumb (if I remember this right) is that the vertical error is 3 times the horizontal error. So if your GPS is showing an accuracy of 15 feet, your altitude accuracy is around 45 feet. The altimeter should usually be accurate to within 10 feet or less, if you use automatic calibration, and considerably better than that if you've recently manually calibrated it.

 

On the other hand, if you're in a commercial airliner, the pressurized cabin will cause the altimeter to think it's at a lower height than it really is. The GPS altitude will be much more accurate in this case.

[Neo_Geo]

One of the main reasons for having the barometric altimeter is that if altitude information is critical, it will continue providing altitude information if you lose your satellite reception under poor conditions. The altimeter must be calibrated before each use, and one of the easiest ways of calbrating it is by using the GPS data if you have good reception from the satellites.

[+hurley_108]

That all said, I've emailed Garmin on several occasions asking them to provide some way of getting real-time GPS altitude. As is, there are only three ways of using the GPS altitude: the GPS altitude box on the satellite page (static), the calibrate altimeter page (static), and the average waypoint function (pseudostatic - average). I really wish they would create a second altitude field in the list of data fields you can display on, say, the trip computer page, that would deliver the GPS altitude; and also an option in the tracklog page to track GPS or Barometric altitude. This would allow us to decide which inaccuracies are least important.

 

\hopes there's someone from Garmin lurking.... =]

[+phatdawg]

The CSx also has a compass. I have a 76 CSx and this is a nice feature. Spend the extra $$ and get the best thing going.

 

 

 

I'm looking for a new GPS and I'm trying to decide between the Garmin 60csx and 60cx handhelds. I was wondering why the 60csx has two ways of determining the altitude?? By way of locking unto 4 or more satellites or using the barometric pressure. I would think the GPS altimeter would be more accurate than the barometric altimeter, given the fact air pressure is constantly changing. So what is the advantage of including a barometric altimeter into a GPS? Those who own the 60csx or 76csx units do you find the altitude readings from the GPS altimeter are always close to the readings from the barometric altimeter?

 

Thanks,

[+Red90]

In my experience (5 years with a Etrex Vista and 4 months with a 60Cx)..... The 60Cx ability to hold good accuracy almost all of the time allows it to have a good stable altitude reading all of the time. I have NEVER lost lock while hiking even in very heavy bush. The altitude plots are stable and smooth. In the end it is basically as good as the barometer units.

 

This is from the 60Cx. Lots of very heavy bush at lower elevations.

Edited August 1, 2006 by Red90

[Alphawolf]

Forget the barometric altimeter...It's just one more thing to worry about calibrating (constantly), and who cares if you are really at 7506 ft. instead of 7450 ft. like the GPS says? The barometric altimeter ends up (after you play with it a while and it loses it's "cool" factor) being about as useful as pockets in your underwear...JMHO

 

And...while I'm at it...I feel the same way about the electronic compass. Give me my Foretrex 101, a 1:24,000 scale map, a grid card and my Silva compass any day over that combined stuff...

Edited August 1, 2006 by Alphawolf

[Bite_Me]

Doing more research I downloaded the 60cx and 60csx manuals. The 60csx manual says this unit will self calibrate the barometric altimeter each time you turn it on. I thought this was cool, because accurate altitude readings are important to me. Then I found the following review of the barometric altimeter and electronic compass for the 76s unit. The 76s "probably" have the same sensors as the newer 60csx unit. The review said the barometric calibration can take an hour! Yikes. Here is the link to the review and a link to the JPEG graphic showing the long calibration time.

 

http://www.gpsinformation.net/waas/g76s/g76s-map.html

 

http://www.gpsinformation.net/waas/g76s/baro-plot-r.jpg

 

Do you 60csx or 76csx owners find it takes a while for the altitude reading to stabilize?

 

Any more comments would be appreciated as I'm still on the fence between the two units.

 

Thanks,

[Michael Cook]

Automatic calibration is a bit of a strange thing, since the the GPS readings used for the calibration are usually less accurate than the barometer readings! I guess this means that in auto-calibration mode the instrument does a lot of averaging over time to minimize the effects of GPS errors, and only re-sets the barometer when there has been a significant difference between barometer calculated altitude and GPS elevation more than a certain time. If the auto-calibration sets the barometer too often, you will lose the benefits of the accuracy of the barometric altimeter.

 

The point is this: the barometer has no way of knowing your actual altitude, but it can measure very accurately a change in altitude. So the best thing is only to use auto-calibration if you have no better information. If you happen to know your correct altitude (because it's marked on the map, for instance), calibrate the altimeter to this altitude.

 

I just re-calibrate the altimeter when I get the chance: it's usually not far from the truth and the re-calibration only takes a few seconds once you've got used to it.

 

To help you in your decision, maybe we need to know more about why accurate altitude information is important for you. And don't forget the other difference: the compass. Some people love it, others couldn't care less.

 

Doing more research I downloaded the 60cx and 60csx manuals. The 60csx manual says this unit will self calibrate the barometric altimeter each time you turn it on. I thought this was cool, because accurate altitude readings are important to me. Then I found the following review of the barometric altimeter and electronic compass for the 76s unit. The 76s "probably" have the same sensors as the newer 60csx unit. The review said the barometric calibration can take an hour! Yikes. Here is the link to the review and a link to the JPEG graphic showing the long calibration time.

 

http://www.gpsinformation.net/waas/g76s/g76s-map.html

 

http://www.gpsinformation.net/waas/g76s/baro-plot-r.jpg

 

Do you 60csx or 76csx owners find it takes a while for the altitude reading to stabilize?

 

Any more comments would be appreciated as I'm still on the fence between the two units.

 

Thanks,

[Bite_Me]

The altitude reading is important to me can best be explained how I currently use my current crappy yellow etrex GPS on a hike, which is my main usage for a GPS. I leave it on the display showing my current altitude so I know how much more I have to climb to get to the destination, usually a summit. How much more I need to climb is more important to me then how much further I have to go.

 

Why do you say the barometer reading is more accurate versus the GPS reading? To get the most accurate waypoint reading, it seems you use this "averaging" function where the altitude is selected using the GPS alitmeter, not the barometer altimeter.

 

Automatic calibration is a bit of a strange thing, since the the GPS readings used for the calibration are usually less accurate than the barometer readings! I guess this means that in auto-calibration mode the instrument does a lot of averaging over time to minimize the effects of GPS errors, and only re-sets the barometer when there has been a significant difference between barometer calculated altitude and GPS elevation more than a certain time. If the auto-calibration sets the barometer too often, you will lose the benefits of the accuracy of the barometric altimeter.

 

The point is this: the barometer has no way of knowing your actual altitude, but it can measure very accurately a change in altitude. So the best thing is only to use auto-calibration if you have no better information. If you happen to know your correct altitude (because it's marked on the map, for instance), calibrate the altimeter to this altitude.

 

I just re-calibrate the altimeter when I get the chance: it's usually not far from the truth and the re-calibration only takes a few seconds once you've got used to it.

 

To help you in your decision, maybe we need to know more about why accurate altitude information is important for you. And don't forget the other difference: the compass. Some people love it, others couldn't care less.

 

[+Red90]

The problem with the compass is that the ones on Garmin GPSrs are crap. They need calibration at every battery change and MUST be held VERY level and this means you need to be stopped. The 60 series GPS reception drops in half when you hold it level............

 

If the compass was better, then it would be useful, but it it sucks so it is not worth it. The other thing is that the 60Cx updates so quickly and accuartely that only a small movement is needed (compared to older units) to get the GPS "compass" display to align accurately.

[Michael Cook]

Why do you say the barometer reading is more accurate versus the GPS reading? To get the most accurate waypoint reading, it seems you use this "averaging" function where the altitude is selected using the GPS alitmeter, not the barometer altimeter.

What the barometric altimeter measures very accurately, particularly over a short time span, is the change in altitude. But you must tell it at what altitude it starts, since it doesn't have a way of measuring the actual altitude from sea level. If someone dumps you somewhere with just a barometer, there is no way of deducing your altitude from the barometer reading alone. In this situation, if you now walk up a hill, what the barometer can tell you is how much higher you now are in relation to your point of depart.

 

GPS does measure the actual altitude, but the value it gives can change dramatically in just a few minutes, even if you are in fact standing still, depending on satellite positions and quality of reception.

 

So we have two very different ways of determining altitude: a barometer, which needs to be calibrated and will then be accurate over a short period of time, but as the hours pass will "drift" because of changes in the weather, and GPS, which is most of the time less accurate than the barometer but is not affected by the weather.

 

Let's say you calibrate your barometric altimeter to a known value (the altitude is indicated on the map), then run up a hill in a few minutes. Counting on the fact that the barometric pressure at sea level rarely changes much in just a few minutes, the altimeter should give you a very accurate estimation of your present altitude, whereas the value calculated by the GPS may be way off. If you now go on walking for several hours, things can look different: maybe the barometric pressure at sea level has changed by a significant amount, affecting the altimeter reading, and maybe the GPS has had the good fortune to have several moments where it had an accurate elevation reading.

 

Obviously both methods have their strengths and weaknesses, and if Garmin has got things right, they should be able, by combining the informations given by the two different systems, to produce more accurate and consistent results than with one system alone. I guess what the auto-calibration feature in the Garmin 60CSx does (or at least tries to do) is to look at the quality of the signals and the geometry of the satellites and then only to use the GPS-determined value to re-calibrate the barometer when it has calculated that the GPS reading is accurate enough. Maybe it looks at the moments where the GPS reading is the most accurate and makes an average.

 

For some more information on all this, download this Garmin brochure: Using a Garmin GPS with Paper Maps and look at pages 38 and 39.

[+Searching_ut]

The altitude reading is important to me can best be explained how I currently use my current crappy yellow etrex GPS on a hike, which is my main usage for a GPS. I leave it on the display showing my current altitude so I know how much more I have to climb to get to the destination, usually a summit. How much more I need to climb is more important to me then how much further I have to go.

 

 

Since you've been using an eTrex, you're quite familiar with the accuracy you can expect from a GPS only solution. Being a peak bagging sort myself, who admittedly isn't getting any younger, I find elevation changes and amount of climbing remaining to be quite important numbers to have. (I like to judge if I'll have the energy to get back before dark or not) I've found that in most cases, a GPS will give you altitude data accurate to within a couple hundred feet or so in the mountains most of the time. Altimeter readings being about the same accuracy wise as they can vary in that sort of terrain as well. What the combination GPS barometer does is smooth, or filter out the altitude readings so that your track data, and altitude gained and lost on the trip computer will be more accurate. With a GPS only solution you would expect a hike that involved most of the day and 7,000 feet or so of elevation gain/loss to read somewhere in the neighborhood of 10,000+ feet of alt change because of the accumulated position error of the GPS. With the Barometric altitude used in the Sensor models, you can expect the trip data and track to more closely represent the actual amount of climbing and decending you did. The only problem is the real data takes away from your bragging rights a great deal

 

Bottom line, the barometer altitude solution probably won't help much in letting you know how much more you have to climb. It will come in handy to figure out how much climbing and decending you really did at the end of the day.

[+Alan2]

My Vista can be switched to show ambient presssure in millibars in addition to altitude in feet or meters. I'm surprised no one mentioned the advantage of using the barometric reading for weather changes; something that might be more useful than the elevation readings to people who sail or are hiking in mountains and concered about unanticiparted storms etc.

 

(Disclaimer:- For myself, I admit I use it mainly for the "wow" factor like checking to see the elevations when my ears "pop". As far as storms, I usually don't hike if I'm expecting rain!)

 

Can the 60cs and 76cs be switched to pressure readings?

Edited August 2, 2006 by Alan2

[bgarland]

Yepper, they both can be set for either reading display.

[+Brian Stirling]

Yepper, they both can be set for either reading display.

 

I have had my 60CSx for about 2 months now and while I like it lot I do not think the baro altimeter has been implemented very well at all. If I drive with it I often see altitude errors nearly 200 feet whereas the lat/lon error is more like 15 feet. I maybe wrong, but I hink Garmin is using the code for aircraft use and does not expect the altitude to go up and down as frequently as it does when driving in the mountains. My experience with other GPSr's, all of them Garmin BTW, that do not have a baro altimeter feature are that they were MORE accurate at indicating alt when moving whereas the 60CSx may be more accurate if you stay still for any length of time.

 

What I'd like to see is a menu feature that permits you to change the way the unit calculates and displays alt. That is, when driving in the mountains with constant up/down movements it might be better to bias the calculation in favor of the GPS reading whereas when the verticle velocity is low enough the bias should swing to the baro altimeter.

 

If you stay still long enough the unit can average the GPS alt reading and adjust the baro offsets to provide a very accurate alt reading and with the baro unit so calibrated you should be able to use the baro reading for more accurate reading moment-to-moment. Also, if you know the correct altitude and the correct baro pressure it should be a simple task to calculate the sea level air pressure and monitor it over time to know something about the weather.

 

Anyway, I think Garmin could have down a better job with this feature and I do hope they improve it through firmware updates...

 

Brian

[+BigLarry]

The barometric altimeter works great for me. My eTrex Vista and my new 60CSx agree with each other to better than 5' accuracy over the length of a ride when using the barometric altitude, calibrating to a known altitude at the start of a ride.

 

When I try to use the GPS altitude to calibrate at the start of a ride, I find it's very far off. At a known altitude, I find the GPS derived altitude can easliy be off by as much as 200' on my eTrex Vista. Furthermore, the indicated altitude will change by 100' over a few minutes, perhaps as part of it's averaging to the better value? I find the newer SiRF recevier doesn't give that much better accuracy than my Vista in areas with good reception - at least not enough to make the GPS altitude useful for me.

 

In contrast, the barometric altimeter is very stable. Over the space of a 6 to 8 hour bike ride, when I come back to my start point, I see a drift typically under 50'. On a two hour ride, the drift is ususally less than 30'. I find I can predict when I'm coming to the peak of a mountain very accurately using the barometric altimeter.

 

Also, the total climb data is more accurate using the 1' resolution of the barometric altimeter. The 100' resolution of the GPS based altimeter won't detect all the little hills on the trail and won't register a lot of the climbing I've done. Although the up and down drift of the GPS altitude could add in some bonus extra climb,... even if you're not moving.

 

In general, I'm very happy with the utiliity of the barometric altimeter, and never got good accuacy from the GPS altimeter, even on my newer 60CSx that I've used on a few rides. I wouldn't purchase a GPS without the barometric altimeter, even with the best of today's GPS recevier accuracy.

[+Alan2]

Which reminds me that the Vista's program has a jumpmaster routine for parachuting that uses the barometric altimeter where continuous accuracy is critical.

 

(I don't think another disclaimer from me is required regarding my use of this feature!)