Garmin Barometric Altimeters

[+michaelnel]

I have had several Garmin GPSRs with barometric altimeters, (Colorado, GPSMap 60csx (three of them), Oregon 450 and now a GPSMap 62s). None of them have ever displayed a remotely accurate elevation longer than momentarily.

 

Before leaving the house to go geocaching (I do almost all of mine within 5 miles of my home in San Francisco), I reset the stuff in the trip meter, delete the tracks, and calibrate the altimeter. I know I am 19 feet above sea level in my room here, so I set it at that.

 

Then I will go out caching for the day, and later look at the elevation plot in basecamp. It shows me at 19 feet for just a few minutes, then often shows me going 60-70 feet below sea level when I am 2 blocks from my house and the elevation hasn't changed more than 10 feet (I live in one of the rare flat parts of San Francisco), and peaks of sometimes several hundred feet when I have done nothing more than very gradual climbs that take me maybe up to 3 or 400 feet over a mile or two. Not really extreme stuff, but the elevation plot is just NUTS.

 

I like having the 3 axis electronic compass, but at least in my experience on all these units, the barometric altimeter is on crack.

 

I have tried it in both auto-calibrating and non-auto-calibrating modes too (I assume auto is when GPS-derived altitude is used to "correct" the barometric), but it's nuts either way. I don't know why I bother to even calibrate it.

 

Yes I do know that the barometric pressure changes during the day, but I am not talking days when a big front sweeps through, I am talking days when it has been the same weather for a week and I doubt the barometric pressure is changing much.

Edited June 4, 2011 by michaelnel

[+dfx]

... and I doubt ...

And this may just be where you're wrong.

[+michaelnel]

... and I doubt ...

And this may just be where you're wrong.

 

And I might not be.

 

I just looked at a plot from today. I calibrated it at 19 feet and left the house. By the time I had walked a tenth of a mile on flat ground it had me at 60 feet below sea level, a net change of 80 feet.

 

That must have been one fast moving front.

 

I just looked at data from a nearby weather station. My walk was from around noon to 4 PM. During that time, the barometric pressure rose from 29.85 to 29.90.

Edited June 5, 2011 by michaelnel

[+mpilchfamily]

Temperature, altitude, weather and air density all contribute to pressure changes. I don't know how sensitive the altimeter on the unit is or how it even tells your height from barometric pressure. But doing a quick search online showed an average barometric change on a calm day fluctuates a little less then an inch. Only wish i knew how big of a height difference that makes.

[MtnHermit]

Some thoughts:

• Why does it matter? It's not as if you need to know the elevation.
  
• I live in Colorado and if my altimeter is within 100 feet, I count myself lucky. But most often my OR 450 is within +/- 20. I seldom go for a hike where the net gain/loss is less than 1000 feet.
  
• Have you ever done a stationary average using the GPS average mode? How close is the altitude then?
  

 

I've used analog dial altimeters and Casio digital watch altimeters and I could count on them being off 100 feet per 1000 of change. The OR 450 is easily the best altimeter I've owned. BTW I do use auto-calibrate.

 

Bottom line, I think you're expecting too much from a consumer grade device.

[+michaelnel]

Bottom line, yes, I think it is a useless feature that really doesn't provide even moderate accuracy. I think it is primarily useful to Garmin's Marketing Department, as another "feature".

 

I would gladly pay less for a unit that didn't have it. Problem is, I do value the electronic compass, and they usually come as a package deal.

 

Oh well, I think the best thing is to ignore it.

[+Team CowboyPapa]

 

I just looked at data from a nearby weather station. My walk was from around noon to 4 PM. During that time, the barometric pressure rose from 29.85 to 29.90.

That is equivalent to an elevation decrease of 45 feet.

Edited June 5, 2011 by Team CowboyPapa

[+Team CowboyPapa]

Only wish i knew how big of a height difference that makes.

Roger that. Check this:

http://www.digitaldutch.com/atmoscalc/

[+mpilchfamily]

Thanks!

[+Smitzy]

I work on commercial vessels as a navigator. I don't use the altimeter to measure my altitude. I live near the water on a mostly flat island so I can't speak to it's use. However.... The barometer function I use all the time while out geocaching hiking and fishing. If used with a reliable forecast and a basic understanding of meteorology. You can get an idea of where you are in the weather and what is happening around you generally. knowing the exact pressure is helpful but not absolutely necessary. The change over time in the most important thing. If you set you GPSMAP 62 to monitor the pressure it can do it even when the power if off. it will give you up to 12hrs of a log. Under normal use its not too rough on the bat. Zoom out and you see the trend not the exact pressure. It smooths out all the spikes. Coupling this data with a print out of the weather highs and lows and some good cloud spotting. You can give yourself a good idea of what the weather might do to you. It's better then taking a 6hr old forecast made up 100s of miles away by a guy at a desk. Weather changes this feature will help you stay on top of it.

[+Red90]

Leave it on auto calibrate. It will stay quite close to correct. Manual calibrations are only good for a short period.

[sparksd]

Welcome to the world of baro altimeters! I've used them for climbing, hiking, and trail running for over 40 years now so I'm used to their vagaries, particularly here in the Pacific NW where air pressure can change quickly and significantly. I have always recalibrated my altimeter when I reach a point of known elevation, usually taken off of a topo map. And even though I do it regularly, I almost always have to change the current setting. This is the nature of baro altimeters, even my trusty/crusty old Thommen, a model of accuracy.

 

GPS-based altitude estimations have improved tremendously in accuracy as new GPS chips have hit the market and improvements in the constellation have continued to be made (you should have seen it when Selective Availability was turned on!). When these estimations are applied to correct (autocalibrate) a unit's baro altimeter, you'll get better average baro accuracy over time. But because of how baro altimeters work, they will always "wander" and while GPS-based altitude is pretty good these days, it is nowhere near horizontal position accuracy (and won't be until they figure out a way to get GPS signals from satellites on the other side of the planet to contribute to the position solution).

 

Bottom line: altitude on a handheld GPS unit, baro or GPS-based, will always be less than perfect. And as the navigator pointed out, a baro altimeter can help you see tending weather patterns, something I really appreciated in the high mountains (if my altimeter told me I was many hundreds of feet closer to the summit without leaving my tent, I knew it was a good time to head home).

 

Dave

[+CacheFreakTim]

I have found the barometric altimeters in handheld GPS units to not be accurate over the years. I'm not sure if they are getting any more accurate because the last time I tried to use mine I had similar results from what you described. If I ever want to know the vertical distance I walked I just view my track on a topo map and find it is far more accurate.

[+rav_bunneh]

One can easily imagine the relative cool inside a house, then walking outside with the gps feet of the radiant hot heat of the sidewalk. Yes, that WAS one fast moving front as it doesn't take long to go from one set of conditions to another. :3

 

If you really want to confuse the gps walk down the street and go into each shop along the way.

[+splashy]

It won't show anything, the pressure is showing not the temperature.

[sparksd]

It won't show anything, the pressure is showing not the temperature.

 

Actually, it is showing altitude, which is derived from air pressure. The unit is using the pressure in conjunction with standard atmosphere tables to determine altitude. If the air temperature varies greatly from the standard model then the altitude estimation can be off. Most good barometric altimeters have incorporated air temperature correction but some cheaper units (e.g., cheap watches with altimeters) do not. From what I've seen with the 62st, it is temperature-corrected.

 

Side note - apparently, early models of the 62s had an issue where pressing the buttons or squeezing the case caused altitude spikes due to temporary increased air pressure within the unit due to depression of the button or compression of the case but that has been fixed in later models. I believe the air pressure sensor openings are inside the battery case.

 

Dave

[+dfx]

The unit is using the pressure in conjunction with standard atmosphere tables to determine altitude. If the air temperature varies greatly from the standard model then the altitude estimation can be off.

 

Not sure if I follow here. Temperature changes influence gas pressure only in enclosed (i.e. hermetically sealed) spaces. If a room inside the house is cooler than outside, the air there will be denser, but pressure will be the same.

[sparksd]

The unit is using the pressure in conjunction with standard atmosphere tables to determine altitude. If the air temperature varies greatly from the standard model then the altitude estimation can be off.

 

Not sure if I follow here. Temperature changes influence gas pressure only in enclosed (i.e. hermetically sealed) spaces. If a room inside the house is cooler than outside, the air there will be denser, but pressure will be the same.

 

You're right - it doesn't affect pressure but what temperature does affect are the components measuring the pressure by causing them to slightly contract and/or expand, affecting the altitude solution. Temperature-compensated altimeters are "tuned" to take this effect into account. Uncompensated altimeters can show significant error due to temperature variation.

 

Dave

[sparksd]

One of the best descriptions of barometric altimeters and how they work is the Avocet Vertech FAQ collection --

 

http://www.avocet.com/vertechpages/vertfaq.html

 

I have owned one of these (in addition to a couple of other altimeters) for many years, though it is currently inoperable due to a dead battery.

 

Dave

[yogazoo]

owning several Garmin units I've noted that while the barometric altimeters will show the pressure changes and trends, trying to derive an accurate number is impossible. All three of my units will give different pressure numbers. So, If I calibrate my elevation to a known pressure station reading my elevations will all be different. If I calibrate my pressure reading by a known elevation I will get 3 different pressure numbers. It's a relative change thing and Red90's right, leave it on auto-calibrate and forget it. You elevation is the important number after all and auto-calibrate will zero in on that (by adjusting the relation to pressure).

[+julianh]

Definitely use auto-calibrate - in my experience, with auto-calibrate on, it should keep an accurate elevation record (+/- 5 m say) all day, adjusting for changes in barometric pressure.

 

Things that can upset the barometric altimeter:

 

1) Your speed - air speed changes air pressure. If you are in a moving car for example you may find your elevation reads wrong by 10 m or more at highway speeds. The actual error depends on a whole lot of factors - whether windows are open or shut, etc. (Also - don't expect it to work in a pressurised aeroplane, where air pressure is artificially maintained to approximately 2,000 m effective elevation.)

 

2) Wind speed - if you are in very gusty conditions, you can see apparent elevation rise and fall with the wind gusts. If you are standing in strong steady winds, try turning your back to the wind, then face the wind - you will probably see your apparent elevation change by a couple of metres.

 

3) Gripping the unit too tightly, or covering the vent hole - the pressure sensor is inside the casing, and needs to be freely vented to atmosphere to register the air pressure. If you squeeze your unit tightly, the air pressure inside the unit will rise briefly, showing an apparent drop in elevation. This will correct itself in a couple of seconds when you relax your grip.

 

4) Faulty unit? Being aware of the above factors, and proper calibration / usage, I have found my Summit HC to be accurate to about +/- 5 m all day every day for normal walking / hiking use. If you are consistent;ly getting bigger errors than this, it suggests something may be faulty.

 

Hope this helps!

Edited June 6, 2011 by julianh

[+julianh]

One can easily imagine the relative cool inside a house, then walking outside with the gps feet of the radiant hot heat of the sidewalk. Yes, that WAS one fast moving front as it doesn't take long to go from one set of conditions to another. :3

 

If you really want to confuse the gps walk down the street and go into each shop along the way.

Yes, but that doesn't change the air pressure, so this won't register on your barometric altimeter.

[+ecanderson]

If a room inside the house is cooler than outside, the air there will be denser, but pressure will be the same.

You're losing track of how the air got to be 'different' indoors. Popping into and out of buildings will often drive a baro sensor crazy due to the significant positive pressure maintained by many HVAC installations. Things get even wilder when elevator systems are involved.

[+ecanderson]

If you really want to confuse the gps walk down the street and go into each shop along the way.

... and don't let 'em talk you out of this! The effect is even more pronounced in many office buildings.

[+julianh]

If you really want to confuse the gps walk down the street and go into each shop along the way.

... and don't let 'em talk you out of this! The effect is even more pronounced in many office buildings.

Geez! How high do you guys pressurise your buildings in the States?!

 

I would have thought the maximum pressure differential between inside and outside of a mechanically ventilated building is probably only 5 mm water gauge (50 Pa) or so? (Think about it - how hard do you have to push to open the door to get inside or outside? 5 mm water gauge pressure differential would create a total pressure load on a standard 2.1 m x 0.9 m door panel of about 10 kg force - do you have to push / pull with a force of more than 10 kg just to overcome the air pressure differential and open the door?)

 

Each mm of water gauge pressure differential is roughly equal to one metre of elevation difference, so I wouldn't have thought the HVAC pressure effect would lead to apparent elevation errors of more than about 5 metres or so.

[+dfx]

Each mm of water gauge pressure differential is roughly equal to one metre of elevation difference, so I wouldn't have thought the HVAC pressure effect would lead to apparent elevation errors of more than about 5 metres or so.

 

Yeah that's what I thought. I don't see how any significant pressure difference can be created without almost hermetically sealing all doors and windows. But w/e.

[+khumbu_calypso]

I am still pretty new to all of this GPS fun, but lately I have been trying to understand how these barometric altimeters work, how atmospheric pressure is measured and how it relates to altitude. How elevation is measured and reported on maps and GPSr's. Geoids, spheroids, ellipsoids, blah, blah, blah. In the end I just want to have some measure of confidence that the number on the screen of my 62s is at least close to accurate, and if nothing else, that the numbers it spits out are at least consistently inaccurate so that I can at least trust the elevation change data it gives me, or that the barometric pressure trending is trustworthy.

 

So I have been following this thread and experimenting with my 62s quite a bit over the past few days. I have been quite surprised in fact at how accurate it seems to be after reading here that the general consensus seems to be that they are pretty inaccurate.

 

Today I drove to the airport where the elevation is known and there also happens to be a national weather station that reports barometric pressure by the minute on the internet. I first calibrated the barometric altimeter using the known elevation of 1084 m. When I did so the barometer then read within 0.1 millibars. I don't know how much of an error 1 tenth of a millibar is or if that is significant at all. I then calibrated using the known barometric pressure. When I did so, the elevation reported was 1080 m, or 4 m lower than the known elevation. An error of 4 m seems pretty reasonable to me since the landscape around the airport is generally pretty flat, but definitely not perfectly flat and I don't know where exactly the 1084 m was measured. I also wonder how high above (or below) my standing position the weather station's barometer is. Either of these factors, I would think, could easily account for a 4 m or 0.1 millibar error.

 

So with my newly calibrated barometric altimeter, I set off to find a few benchmarks of known elevation. I managed to find two. At the first one, my 62s reported the elevation to be 1082 m. The benchmark's reported elevation was 1081.6 m. A mere 40 cm variance that is most likely attributable to rounding. At the second benchmark, my 62s reported 1057 m, where the benchmark's known elevation was 1056.1 m. This time 90 cm variance so can't really chalk it up entirely to rounding error.

 

Since dinner I have had my 62s sitting out on my back deck. I have been checking the barometer readings every hour or so and comparing them to the reports coming from a WeatherBug backyard weather station located 20 km away from my house and sitting about 34 m lower in elevation according to GoogleEarth. Each time I compare the two, my 62s reports the barometric pressure to be 0.6 millibars lower than the number reported by the weather station. Does 0.6 millibars equate to roughly 34 m? From what I can tell that is a very complex question that involves a lot more than a simple calculation. However, the fact that the numbers are consistently 0.6 millibars apart is good enough for me.

 

Since calibrating the 62s at the airport, I have also noticed that the 62s reports the elevation at my house within 1 m of what GoogleEarth says that it should be. I checked 4 or 5 other places as I travelled around today with my aircard connected laptop running Google Earth. None of the checks I did were out by more than 5 m from what Google Earth said they should be. I don't know what DEM Google Earth is based on, but I would guess that it is not meant to provide accuracy better than 5 m for vertical data.

 

All in all, its been a good excercise that has demonstrated to me that I can trust the 62s to give me the information that I want with precision reasonable for my purposes. Was that hike I just did an 800 m elevation gain or 700 m? Is the weather going to get really ugly, really soon? Should I hunker down now, or can we keep plugging along to the next camp site?

Edited June 7, 2011 by khumbu_calypso

[+khumbu_calypso]

Other observations...

 

When I am driving, if I roll down the window, the negative air pressure created makes the altimeter jump by as much as 30 m.

 

When I turn on the AC in my truck, the increased pressure makes the altimeter drop by up to 10 m.

 

Even when parked stationary in my truck with the AC off, rolling down the window makes the altimeter jump a meter or two.

 

With regards to the question about the effect of building HVAC systems, in my line of work I routinely commission building envelope assessments to determine how tightly constructed the homes my company builds are. I don't recall the exact numbers that the engineers quote (its all a lot of "blah, blah, blah"), but it seems to me that HVAC systems that are not properly balanced can produce positive pressures inside tightly constructed homes in the 25-35 kilopascal range. Or at least that number rings a bell when I recall the last time I had one of those conversations.

[+ecanderson]

Each mm of water gauge pressure differential is roughly equal to one metre of elevation difference, so I wouldn't have thought the HVAC pressure effect would lead to apparent elevation errors of more than about 5 metres or so.

 

Yeah that's what I thought. I don't see how any significant pressure difference can be created without almost hermetically sealing all doors and windows. But w/e.

Even wind pressure differential on two sides of a building can create a pressurized 'system' (one in Golden, CO comes to mind immediately - doors on opposite sides). Haven't you ever had trouble opening a door due to these effects???

[+michaelnel]

So, I just checked the local airport weather, which reports the barometer is 30.05 inches of hg. I went to plug this into my 62s, which is set for inches hg, and it only gives one digit to the right of the decimal, so I was able to enter 30.0, but not 30.05.

 

Is that last digit not significant?

[+Red90]

So, I just checked the local airport weather, which reports the barometer is 30.05 inches of hg. I went to plug this into my 62s, which is set for inches hg, and it only gives one digit to the right of the decimal, so I was able to enter 30.0, but not 30.05.

 

Is that last digit not significant?

 

You should be entering your current accurate elevation. The unit know barometric pressure.

 

As I've tried to convince people above the auto calibration using the GPS elevation works the best.

Edited June 7, 2011 by Red90

[+julianh]

 

With regards to the question about the effect of building HVAC systems, in my line of work I routinely commission building envelope assessments to determine how tightly constructed the homes my company builds are. I don't recall the exact numbers that the engineers quote (its all a lot of "blah, blah, blah"), but it seems to me that HVAC systems that are not properly balanced can produce positive pressures inside tightly constructed homes in the 25-35 kilopascal range. Or at least that number rings a bell when I recall the last time I had one of those conversations.

25 to 35 Pa, not kPa. There are 101.6 kPa in one standard atmosphere, so 25 to 35 kPa is 25% to 35% of an atmoshere - or 4 to 5 pounds force for every square INCH of surface area in American units. That is a LOT more than hurricane force winds, and it would sure make your ears pop!

 

As per my previous post - 25 to 35 Pa would only generate 2 or 3 metres or so elevation error.

[+julianh]

Even wind pressure differential on two sides of a building can create a pressurized 'system' (one in Golden, CO comes to mind immediately - doors on opposite sides). Haven't you ever had trouble opening a door due to these effects???

Yeah, strong wind blowing onto or across a door can certainly create quite large forces that you have to push or pull against when opening or closing a door. However, the differential pressures due to normal HVAC systems are a LOT smaller than strong wind pressures - otherwise, people would be getting their arms ripped off EVERY time they went to open a door!

[+michaelnel]

 

You should be entering your current accurate elevation. The unit know barometric pressure.

 

That's what I normally do. I wanted to see if entering a known barometric pressure would cause it to display the known altitude, but my real question is this:

 

Since barometric pressure in inches of Hg is conventionally given to two decimal places, why does Garmin only allow you to enter the value with one decimal place?

 

As I've tried to convince people above the auto calibration using the GPS elevation works the best.

 

Yes, I do have it set for auto calibration normally, the above was just an experiment.

[+julianh]

So, I just checked the local airport weather, which reports the barometer is 30.05 inches of hg. I went to plug this into my 62s, which is set for inches hg, and it only gives one digit to the right of the decimal, so I was able to enter 30.0, but not 30.05.

 

Is that last digit not significant?

As others have said, you are generally better off calibrating using known elevation rather than barometric pressure, if possible. However, if all you know is the barometric pressure, you are better off entering the pressure in metric units (hPa) because the Garmin units allow you to enter to an accuracy of 0.1 hPa (an error of 1 part in 10,000), which is a LOT more precise than 0.1 inches of Hg (i.e. an error of 1 part in 300). Don't stress about it too much, though - as long as you have auto-calibrate turned on, the unit will correct the initial error itself within 15 - 30 minutes typically, once it has a reasonable GPS fix.

 

Yes, it would be great to be able to enter the second decimal place if using in Hg - this is a long-standing problem with Garmin units - it has been reported numerous times to Garmin over the last 5 - 10 years, but they have never done anything about it.

[+michaelnel]

Thanks for actually answering my question. Some folks seem to have trouble doing that.

[+ecanderson]

Thanks for actually answering my question. Some folks seem to have trouble doing that.

As to your other question - a 0.01mm difference is about 10' of altitude around sea level. I really doubt that the sensor in a Garmin manages changes in that range very well to begin with. No surprise that they don't even allow for entering values that small.

[+Red90]

As to your other question - a 0.01mm difference is about 10' of altitude around sea level.

 

But the precision he stated was 0.1 INCHES not 0.01 mm...... If 10' = 0.01 mm 0.1 inches = 2540 feet.

Edited June 7, 2011 by Red90

[+Red90]

Thanks for actually answering my question. Some folks seem to have trouble doing that.

 

Well... I would have answered, but I'd have to go on the internet and figure out what this antiquated unit of measure equaled in real life. The way I look at it, YOU could do this as easily as me, so why would I do research to answer your question?

[+dfx]

As to your other question - a 0.01mm difference is about 10' of altitude around sea level.

 

But the precision he stated was 0.1 INCHES not 0.01 mm...... If 10' = 0.01 mm 0.1 inches = 2540 feet.

 

Wow, ok. Let's see. On my Oregon 450, when I switch the pressure units to mmHg, the altimeter calibration data field shows me 759.4. When I switch it to inches Hg, it shows me 29.9. So that's quite a bit of difference in number precision, 0.1 mm vs. 2.54 mm, or around 25 times as much/little.

 

Which means that if you want to calibrate to 30.05 inHg of pressure, you can convert it to 763.3 mm and enter that, and that should give you much more precision.

 

Regaring atmospheric pressure. Mean pressure at sea level is around 100 kPa. At 9,000 m of altitude, it's around 30 kPa. Pressure doesn't change linearly with altitude, but it's somewhat close, so let's assume that pressure change is 7.777777.... Pa per meter. Converting this number to other units, you get:

 

0.0022967769 inHg per meter

0.058338292 mmHg per meter

0.000700057577 inHg per foot

0.017781511 mmHg per foot

 

Inverting those numbers, you get:

 

435.3927 meters per 1 inHg

17.1414 meters per 1 mmHg

1428.4539 feet per 1 inHg

56.23819 feet per 1 mmHg

 

So there, just to add to the confusion with some actual numbers.

Edited June 8, 2011 by dfx

[+julianh]

As to your other question - a 0.01mm difference is about 10' of altitude around sea level. I really doubt that the sensor in a Garmin manages changes in that range very well to begin with. No surprise that they don't even allow for entering values that small.

Seems to be a lot of confusion about air pressure units here. For the record - 1 atmosphere at sea level is approximately equal to:

101,300 Pa / 101.3 kPa / 1,013 hPa

14.7 psi

29.9 inches Hg

408 inches H2O (34 feet)

 

Each metre (say 3 feet) of elevation gain (near sea level) reduces your ambient pressure by approximately:

0.0001 atmospheres

11.8 Pa / 0.012 kPa / 0.12 hPa

0.0017 psi

0.0035 inches Hg

0.047 inches H2O

 

Does anyone want any other pressure unit conversion factors? (Bushels per acre? Gallons of water per square millimetre? Elephants per square mile? )

 

Hope this helps!

 

EDIT:

@dfx - I think my conversion factors are more accurate than your factors for elevation changes near sea level - the rate of change of pressure with elevation is decidedly non-linear over the altitude range you interpolated.

Edited June 8, 2011 by julianh

[+dfx]

@dfx - I think my conversion factors are more accurate than your factors for elevation changes near sea level - the rate of change of pressure with elevation is decidedly non-linear over the altitude range you interpolated.

 

I used this graph as source - close enough for a rough approximation.

[+julianh]

@dfx - I think my conversion factors are more accurate than your factors for elevation changes near sea level - the rate of change of pressure with elevation is decidedly non-linear over the altitude range you interpolated.

 

I used this graph as source - close enough for a rough approximation.

A "rough approximation" maybe - you probably shouldn't quote your "rough approximations" to 8 or 9 digits of precision!

 

(If you look at the graph, you will see that the relative rate of change of pressure with elevation - given by the slope of the curve - is significantly greater near sea level than it is at higher elevations.)

[+janandsteve]

.....sitting about 34 m lower in elevation according to GoogleEarth.....

GE elevation data is inaccurate here in the UK - not sure how accurate it is at your location......

 

See our previous discussions here esp post #19: http://forums.Groundspeak.com/GC/index.php?showtopic=268993&st=0&p=4627137&fromsearch=1entry4627137

[+ecanderson]

As to your other question - a 0.01mm difference is about 10' of altitude around sea level.

 

But the precision he stated was 0.1 INCHES not 0.01 mm...... If 10' = 0.01 mm 0.1 inches = 2540 feet.

No, it was the hundredths that he wasn't allowed to enter. 10 of those is only 100 feet. He's allowed to enter it up to XX.X accuracy per his post. Still not very precise, but a lot closer than half a mile. Depending upon altitude, 0.1 in Hg gets you about 100'. Where did the 2540 come from? Edited June 8, 2011 by ecanderson

[+Team CowboyPapa]

....Depending upon altitude, 0.1 in Hg gets you about 1000'.

More like 92.5' at SL. Does it vary up to 1000' at 40,000' AMSL?

[+ecanderson]

....Depending upon altitude, 0.1 in Hg gets you about 1000'.

More like 92.5' at SL. Does it vary up to 1000' at 40,000' AMSL?

Oops - got a bit overzealous with my zeros while scratching my head over the 2540. No, meant 100' (as in 10 of those 0.01's I was mentioning earlier) - made the necessary edit.

 

I'm good with the 0.0035 inches Hg / meter that the other fellow quoted. That gets me to about 0.01 per 10' (and not far from 0.1 per 100').

[+Team CowboyPapa]

....Depending upon altitude, 0.1 in Hg gets you about 1000'.

More like 92.5' at SL. Does it vary up to 1000' at 40,000' AMSL?

Oops - got a bit overzealous with my zeros while scratching my head over the 2540. No, meant 100' (as in 10 of those 0.01's I was mentioning earlier) - made the necessary edit.

 

I'm good with the 0.0035 inches Hg / meter that the other fellow quoted. That gets me to about 0.01 per 10' (and not far from 0.1 per 100').

Roger that!

[+Team CowboyPapa]

Oh yeah, we had a low tide this AM about 8AM PDT. I guess I'll walk my dogs down there to check it out....back later.

Edited June 8, 2011 by Team CowboyPapa

[+Rubberhead]

I've found humidity affects mine, especially at sea level. I'll set mine at the landing (sea level) and be, just like you said, 30-40 feet below sea level once I'm on the water. Of course, I am mainly interested in using it to forecast weather. When I'm offshore and the barometer drops, watch out...

 

But, I can set mine at the bottom of the Blueridge Parkway (3,200ish feet) and have it read only a few feet off at the top (6,054 feet). Of coure 30 feet is a lot smaller percentage at 3,200 feet than at 19ft.