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Atimeter Worries


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I have a Vista. I want a 60cx or 60csx. My problem in choosing is that the Vista altimeter died twice (said I was at 60,000+ ft), requiring me to send it back to Garmin for repair the first time and replacement the second time (no charge and new unit the 2nd time). I'd rather sacrifice the accuracy for reliability if the new ones aren't any better than the old ones but I'd like to get both. Does anybody have an opinion or experience to relate?

 

I'd also like to relate that the Vista froze up completely a few months ago and Garmin couldn't fix it. They sent me another new or refurbished Vista for 1/2 the price of a normal repair. They couldn't transfer the data into the new one, but I had it all on the computer anyway. As good as the customer service is, I'm still disappointed with the overall reliability of the Vista. I've had to send it in 3 times in 3 years, and I only used it for the fall hunting seasons. I prefer Garmin over Magellan, but am a little gun shy. I was going for a legend C when I found out about the 60 series.

 

Since I can't find any stories similar to mine, I'm thinking I just got a couple lemons.

 

I'm planning on joining you folks in some geocaching soon. Thnaks for the pointers!

 

Kelly

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I'm going to add a question about the barometric altimeter of the Vista, MeriPlat and other units.

 

Say your barometer reads 1,000 feet above sea level while a high pressure system is overhead. What is the difference it will read when a low pressure system is overhead? Is it only a few feet or a few dozen feets difference in the reading.

 

Thanks.

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Say your barometer reads 1,000 feet above sea level while a high pressure system is overhead. What is the difference it will read when a low pressure system is overhead? Is it only a few feet or a few dozen feets difference in the reading.

I once calculated how big this difference could be for another topic:

As an extreme example, if you had calibrated your barometric altimeter using nominal pressure at sea level (29.92 in Hg) and read your altitude using it while sitting in a boat in the eye of Hurricane Katrina, it would have indicated an altitude of around 2700 feet at sea level!

So I would say that you should expect variations of up to a couple hundred feet under more normal weather conditions.

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I'm going to add a question about the barometric altimeter of the Vista, MeriPlat and other units.

 

Say your barometer reads 1,000 feet above sea level while a high pressure system is overhead. What is the difference it will read when a low pressure system is overhead? Is it only a few feet or a few dozen feets difference in the reading.

 

If the pressure goes down one tenth of an inch, your altimeter will go up 100 feet. If the pressures goes down one inch, your altimeter will go up 1000 feet. This assumes you or the GPS do not reset it.

Edited by Son of Blue
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Here’s my observations on the barometric pressure readings for the Garmin GPSMAP 60CSx.

 

I’ve had it for approximately 2 weeks and have not had any issues or problems. I have a fairly expensive weather station at home so I’m lucky enough to have an accurate barometer reading when I calibrate the GPSr. If I calibrate it using my known elevation the barometer reading is always within .01 inHg. If I calibrate using my known baro reading from my weather station, the Garmin is always within 10 feet of the true elevation. So the GPSr (as far as barometer) seems to be accurate.

 

In the “inHg” measurement world .01 = 10 feet, .10 = 100 feet and 1.00 inHg = 1000 feet. So as far as I can tell the Garmin GPSMAP 60CSx is not a true barometer (it is not a weather station). In other words when you input a known barometer reading or known elevation, and then begin an ascent or descent it is using the baro data to determine, or get a more accurate determination of the elevation. So if you are standing still for a long period of time and a low pressure system begins to move in around you, the GPSr will think that you are climbing.

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Related question for my Explorist 600 with a barometer and a pressure altimeter:

 

How is this best used?

 

Set the altimeter when you start a hike, and trust it only for short intervals like an hour?

 

I could see it useful on cache instructions, like climb down the cliff 40 feet,

where GPS really does not help.

 

It seems like variations in air pressure make it unreliable for longer periods.

 

And never bother watching it in a building pressurized by air conditioning - you will think

you are in an elevator.

 

Robert

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You are correct. The more often you calibrate the GPSr the more accurate your information is going to be. Of course if you are hiking the only known data is going to probably be your elevation (assuming you are not carrying your trusty wx station with you).

 

It all comes down the the old saying "garbage in, garbage out".

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Let me ask this:

 

Why is there so much talk about how inaccurate the GPS (none Baro) elevations are? If the GPSr is advertising, say, +/- 20' accuracy, why isn't that verical as well as horizontal?

 

And if you have the baro version, that atuocalibrates itself, why isn't that fairly accurate?

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Calibration has to be done regularly with altimeters. They are based on air pressure and that is constantly changing. One of the methods is to do pre-planning and get a list of known altitudes at various spots in your planned path. When you reach one you then re-calibrate the altimeter. Topo software will give a reasonable estimate of the altitude and you can enter these as waypoints and check your altimeter when you reach one of these.

 

JDandDD

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And never bother watching it in a building pressurized by air conditioning - you will think

you are in an elevator.

 

 

You should see what it does when you put it on your dash and drive with your window open...maybe add the odd cross-breeze in there too...

 

Seriously though, I use the Garmin Summit and find it to be fairly accurate over long periods of use. I live at sea level and on occasion drive off into the mountains and over an 8 hour period with elevation changes of over 1500 metres and add a couple weather systems, it has no problem telling me how high I am. Hasn't been off by more than 5 metres....which also happens to be the best EPE it can get from the sats anyways. It's usually within 1 metre accurate.

 

There were discussions here last summer about the barometric altimeters...their likes & dislikes and such...and also explanations of how they work et al. Run a search in the forums for it...

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Let me ask this:

 

Why is there so much talk about how inaccurate the GPS (none Baro) elevations are? If the GPSr is advertising, say, +/- 20' accuracy, why isn't that verical as well as horizontal?

 

And if you have the baro version, that atuocalibrates itself, why isn't that fairly accurate?

 

I haven't generally found my GPSr's elevation estimate to be grossly inaccurate, but the primary reason that the elevation measurement tends to be less accurate than the horizontal component of your position is that the time of arrival calculations made upon which GPS is based are generally less sensitive to elevation than horizontal position. (That's because GPS satellites are usually not close to the zenith. There is also likely a strong contribution to the insensitivity from the fact that GPS satellites, at least from what I can tell from my display, all tend to be at not too greatly different elevation look angles. If all GPS satellites that your GPSr is communicating with just happened to be at the exact same elevation angle, there would be no sensitivity to your elevation/altitude.) The less sensitivity any calculation has to any specific physical quantity, the harder it is to accurately estimate that physical quantity.

Edited by ghs
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Deneye, do you have the Auto-calibrate turned on? If so its actually doing re-calibrates from the GPS altitude calculations regularly. I find that my Magellan give a pretty accurate reading, with +-5m as well, despite many books saying that it will be off by as much as 50m.

 

Anyway, as the Summit manual says, to be truly accurate you have to do the manual calibrate using very well known altitudes and pressures. I think that's likely one of those disclaimers so that people can't sue when the altitude is incorrectly calculated as it sometimes will be.

 

JDandDD

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In my humble opinion Garmin should not label it as a "barometer". It should be label exactly what it is - an altimeter, and like an altimeter, the pilot is always adjusting the kollsman window to the current altimeter setting (i.e. in Hg) from the closest airport, Flight Service Station, etc to ensure he/she has an accurate altitude reading on their altimeter.

 

I stand corrected. It is labeled as altimeter. I guess what I'm referring to is the pressure plots. To me it's useless data.

Edited by Zeke&Josie
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In my humble opinion Garmin should not label it as a "barometer". It should be label exactly what it is - an altimeter ...

 

I stand corrected. It is labeled as altimeter. I guess what I'm referring to is the pressure plots. To me it's useless data.

 

I'm not sure what distinction you're trying to make here. Physically barometers and altimeters are the same thing, i.e. they're instruments that measure air pressure, with the only distinction being how the dial is labeled. Mark it in 'inches of Hg' and hang it on your wall at home and it's a barometer. Take the same instrument but label the dial in 'Feet of Altitude' and toss it in your hiking pack and it's an altimeter.

 

Since the Garmin units have electronic displays it's easy to use it for both functions as long as you take into account how it works. If I get up in the morning in camp where there are no convenient weather forecasts I may well want to check how the pressure has been changing. In that case I'll use the Garmin (with sensors) as a barometer and observing a rapidly dropping pressure over the last few hours might be a good data point in deciding not to try for a summit climb. OTOH, if the weather indications are ok and we do start climbing then the altimeter readings (with suitable manual and/or automatic calibrations) can be a useful indicator of our progress.

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This thread is exactly why I have no need or desire for an altimeter.

 

Something else to get anal about, fret about and worry about calibrating, when, in fact it has absolutely no impact on a day of hiking or geocaching!

 

My altitude is what it is...Like I'm going to worry about my actual altitude being 20 ft. different from what the GPS says it is...

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Son of Blue- you'll find the Vista'a on=board magnetic compass more important for caching than the barometric altimeter. At least I have and I've been using a Vista for 5 years. At this point I'd recommend a Vista C with more memory or if you can afford it a 60csx which has bothe the barometric altimeter, magnetic compass, extra memory and the SirfIII GPS chipset - the most senstive in the market to date.

Edited by Alan2
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I'm going to add a question about the barometric altimeter of the Vista, MeriPlat and other units.

 

Say your barometer reads 1,000 feet above sea level while a high pressure system is overhead. What is the difference it will read when a low pressure system is overhead? Is it only a few feet or a few dozen feets difference in the reading.

 

Thanks.

 

Are you quite certain that the meridian platinum has a barometric altimeter? :blink:

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So let me get this right.

 

A GPS altimeter will use the GPS satelittes to help auto calibrate the pressure altimeter in those units if you set it up that way. That makes sense. I think pilots have to calibrate their equipment each time before they take off.

 

I guess you could also use a benchmark with the exact known altitude to configure the barometric altimeter.

 

And one more question...

 

Can you use the magnetic compass on a GPS inside your car? I've read where they will point to your engine as being north instead of magnetic north. Can someone clear this up one way or the other? Thanks.

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... If all GPS satellites that your GPSr is communicating with just happened to be at the exact same elevation angle, there would be no sensitivity to your elevation/altitude.)

 

Unfortunately, this explanation is not correct. Four or more satellites at the same elevation (but different azimuths) are perfectly capable of giving you an elevation.

 

The elevation is less accurate than the position only partly as a result of geometry; all the satellites are on the same "side" of the GPS, so the geometry is not as good. But the main effect is error in the ionospheric delay. If the ionospheric Total Electron Content (TEC) is larger than the model in your GPS, then the signals from all the satellites will be delayed and they will all look farther from your GPS than they really are, and your GPS will think you are lower than you actually are.

 

That's why WAAS is so important for aviation; a GPS-based instrument approach needs an accurate elevation! WAAS fixes this problem by giving your GPS the actual ionospheric TEC, which dramatically improves the accuracy of its elevation measurement.

 

As a result, if you are getting WAAS corrections, your GPS elevation is likely just about as accurate with or without a barometric altimeter; on the other hand, if you are not getting WAAS corrections, a barometric altimeter, properly calibrated, can give you better elevation data.

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... If all GPS satellites that your GPSr is communicating with just happened to be at the exact same elevation angle, there would be no sensitivity to your elevation/altitude.)

 

Unfortunately, this explanation is not correct. Four or more satellites at the same elevation (but different azimuths) are perfectly capable of giving you an elevation.

 

 

You don't give up, do you? :blink: (It seems that you're confirming a not very good suspicion of mine.) I originally said something else. But you made me think. (You did the last time too, whether you believe it or not.) Looking at the full expressions for the different propagation paths, there appears to be a real slight dependence on elevation/altitude even if the satellite elevation angles are the same. But when you make the standard binomial/Taylor series expansions of those expressions, that dependence goes away. (Now, that I said that, I have this fear that I'm eventually going to realize that I was right the first time.) Even using the full expressions, that dependence is so slight that the resulting elevation calculated would be entirely inaccurate, especially given the way digital computers work. And given typical geometries, inaccuracies in the ionospheric time delay corrections would likely affect transverse position accuracy more than the vertical component.

Edited by ghs
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Hmm... I was wrong about the 4-satellite solution for altitude; because the position is calculated from pseudoranges, rather than actual ranges, the situation in which the satellites are coplanar can indeed be degenerate for any number of satellites. Interesting.

 

However, the ionospheric corrections do affect altitude more than horizontal position.

 

But I'm not gonna argue about it.

Edited by fizzymagic
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The basic idea behind having a barometric altimeter in your GPSr is to continue to give you atitude information if you lose your GPS signal. How often has that ever happened to you? When you DO have a good lock on the sats, your altitude data is generally accurate enough, so it can be used to calibrate the altimeter. So if your altitude is critical (think search & rescue for mountain skiers, skydiving or a backup navigation device for a private pilot), the altimeter feature would be a good thing. Personally, I like to know my elevation only occasionally as a curiosity and nothing more, so the altimeter isn't very important to me. As a matter of fact, I consider it a hassle because it needs calibrating all the time, and it's just something more to go wrong with the GPSr. And when used in a pressurized cabin on an airliner, it'll tell you that your altitude is only 5,000 feet when the plane is really at 41,000 feet.

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One thing Ive noticed, is that the elevation on my 60Cx, is so much more accurate than my older 60C, which is interesting.

 

The best GPS I've ever had with the barometer/altimeter, is the older map76S, and it gave a fairly good elevation profile of trails..

 

The problem with getting a good elevation reading on a GPS, is that the satellites are all above you, and none of them can transmit from below you(other side of the planet), and this would be like having all the satellites in the sky east of your longitude, this would make your east-west error a bit greater, than the north-south error.

 

-

Geoff

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Some people are most interested in absolute elevation. If you need this, then yes, a barometric altimeter needs frequent recalibration. If you don't need a high degree of precision then GPS altitude probably is good enough. For me, I'm most interested in relative elevation. When I'm at the top of a hill, how many feet have I climbed? How much climbing was in this ride relative to others? I've found that barometric altimeters, even without calibration, can give this answer very stably. I haven't had a track with GPS altitude only to compare to my VistaC, but judging by comments like Geoff's above, it probably won't be as nice and might either miss smaller hills or create them where there are none. As is true of many other things, a lot depends on what your intended use and expectations are. I can't see the altimeter being useful for geocaching per se most of the time.

 

Keith

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Deneye, do you have the Auto-calibrate turned on? If so its actually doing re-calibrates from the GPS altitude calculations regularly. I find that my Magellan give a pretty accurate reading, with +-5m as well, despite many books saying that it will be off by as much as 50m.

 

Anyway, as the Summit manual says, to be truly accurate you have to do the manual calibrate using very well known altitudes and pressures. I think that's likely one of those disclaimers so that people can't sue when the altitude is incorrectly calculated as it sometimes will be.

 

JDandDD

 

Yes. And if I really need the elevation measurements, about 10 minutes after aquiring satellites I'll calibrate the unit with the known value if possible. If I don't know the actual elevation, I'll sometimes set it to zero as a benchmark.

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Personally, I like to know my elevation only occasionally as a curiosity and nothing more, so the altimeter isn't very important to me. As a matter of fact, I consider it a hassle because it needs calibrating all the time, and it's just something more to go wrong with the GPSr. And when used in a pressurized cabin on an airliner, it'll tell you that your altitude is only 5,000 feet when the plane is really at 41,000 feet.

 

I find it ironic what many people (not you in particular Neo Geo, but people in general) have to say about altimeters in GPSrs while at the same time thinking how cool it is to use their unit during an air flight....

 

:blink:

 

One person's trash is another's treasure...

Edited by Deneye
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I can't see the altimeter being useful for geocaching per se most of the time.

 

Sorry for the multiple posts...

 

Some days it's unfortunate that geocaching uses only 2 of the 3 dimensions we live in. I recently hunted a cache that was placed on the side of a hill. Due to reflecting signals, I wasn't getting very accurate location readings which was a pain because there was nearly a 50 metre elevation change between the upper & lower limits of location on this hillside. Knowing the elevation of the cache, even with a 5 metre EPE would have helped lessen all that over-exercising I did that day :blink:

 

I had to buy a box of donuts in order to build up my private reserves again...

 

(yes, I DNF'd it)

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Personally, I like to know my elevation only occasionally as a curiosity and nothing more, so the altimeter isn't very important to me. As a matter of fact, I consider it a hassle because it needs calibrating all the time, and it's just something more to go wrong with the GPSr. And when used in a pressurized cabin on an airliner, it'll tell you that your altitude is only 5,000 feet when the plane is really at 41,000 feet.

 

I find it ironic what many people (not you in particular Neo Geo, but people in general) have to say about altimeters in GPSrs while at the same time thinking how cool it is to use their unit during an air flight....

 

:)

 

One person's trash is another's treasure...

 

Just to clarify, in the comment I made that you quoted, I was refering to the barometric altimeter function of GPSrs equipped with sensors (as opposed to the GPSr's ability to derive altitude data from GPS data). I do appreciate the GPSr's ability to show altitude information. As I mentioned, I'm only interested in elevation as a curiosity on very few occasions. Traveling on a plane IS most definitely one of those few occasions. When I DO want to know my altitude, I generally like to know it within 100 feet or so (but that information is NOT critical to my survival, and thus not "important" to me). The only irony I can see in this is that some people want to buy a GPSr for air travel thinking that a barometric altimeter would be the way to go. I've seen a few people here express their disappointment in false readings on airliners, so I throw this information out there from time to time for any lurkers who may be thinking that way.

Edited by Neo_Geo
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I think the most useful concept I have picked up from this thread is don't bother

setting the barometric altimiter to the actual elevation - just set it to zero (easier by far),

and use it as a relative offset from your starting point (trail head, parking lot, etc.)

 

That would not have occured to me from just reading the directions...

 

Robert

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