altimeter/barometer help

[thatcher]

So I just got an altimeter. It has a barometer on it. Does anyone know how they work? it gives me an "absolute barometric pressure" reading which cannot be adjusted. It also gives a "sea level barometric pressure" reading that i can callibrate. which says that it "represents the present barometric reading reduced to sea level". I don't know what that means.

Can someone give me a tip on how to adjust it correctly, and how i can find use in it (like knowing what pressure suggests good weather and what suggests bad).

The thing is measured in "mbars". does anyone know what those are and how i relate them to whatever is regularly recorded?

 

anyways, you can see that I'm not Steve Pool or Harry Whoppler.

let me know if you can shead some light.

 

thanks!

[MisterMo]

This may or may not help but I'll try:

 

An altimeter and a barometer are really the same thing. They both measure air pressure which tells you both your elevation above sea level and/or what the weather might do.

 

Since barometric pressure varies with the weather your altimeter will often show that you've gone up or down several hundred feet while you've in fact been fast asleep.

Unless there's been some techy invention I don't know about it's necessary to check your altimeter a couple of times a day against known elevations to correct for weather caused changes.

 

I don't know but would guess that giving you "pressure reduced to sea level" would be an effort to make those numbers read normally for sea level so you can compare to Harry Woppler saying the pressure is 29 point such and such.

 

A millibar (one thousandth of a bar) is a standard unit of pressure, in your case, atmospheric pressure.

 

Hope this helps. What make and model is your altimiter?

[griz]

Unless there's been some techy invention I don't know about....

 

yeah, it's called GPS

[dylan_taylor]

So I just got an altimeter. It has a barometer on it. Does anyone know how they work? it gives me an "absolute barometric pressure" reading which cannot be adjusted. It also gives a "sea level barometric pressure" reading that i can callibrate. which says that it "represents the present barometric reading reduced to sea level". I don't know what that means.

Can someone give me a tip on how to adjust it correctly, and how i can find use in it (like knowing what pressure suggests good weather and what suggests bad).

The thing is measured in "mbars". does anyone know what those are and how i relate them to whatever is regularly recorded?

 

anyways, you can see that I'm not Steve Pool or Harry Whoppler.

let me know if you can shead some light.

 

thanks!

 

The short answer is check out a meteorology text from the library, and read the instruction manuel for your unit.

 

My long-winded, off the cuff answer is this: The unit uses an algorithm to calculate altitude based on barometric pressure, or vise versa. The absolute pressure is the pressure that your watch measures at your location. The "sea level barometer " is quite a useful feature. It does indeed tell you what the atmospheric pressure would be if you could suddenly jump into a hole in the ground and fall all the way to sea level.

The reason this is helpful is this: Absolute pressure is directly affected by both changes in altitude and changes in relative barometric pressure (due to changes in weather), while the sea level barometer is only affected by changes due to weather (as long as you remember to recalibrate to the correct altitude). While you are traveling in the hills, your altitude is changing, but so must your barometric pressure (which is inversly proportional to altitude). Unfortunately, barometric pressure also changes over time, because of changes in weather. If you are moving for a while after calibration, and you stop for the night, your calibration might be out of whack again already. By using your map and compass (or GPS) to pinpoint your exact location (and thus your altitude), you can now compare this true altitude with the one on your watch while you are recalibrating. I won't go into the weather forcasting any more than this oversimplified concept: in short, we are looking for positive pressure changes (or negative altitude changes) to signify a continuation of favorable weather. When we see the opposite we may suspect that a less than desirable change is on the way.

 

I especially like the sea level barometer function because:

a) you could call a local airport (or look it up on line) and see what their sea level barometer is and calibrate your altitudebased on that - even in a high altitude place like denver, etc - the sea level bar. is just a normalized standard that different people at different elevations can use to calibrate or share information. Just punch in the correct SEA barom. and voilah, your watch is set for the correct altitude.

b)On expeditions, I log sea level barometric pressure (post-calibration) in my notebook. It is usually in the neighborhood of 1010mBar for neutral pressure. lower means low pressure, higher means high pressure. As my position on the mountain changes, the sea level baromter always reads somewhere in the ballpark of 1000mbar, and this is only affected by relative changes due to weather, it is NOT affected by changes in elevation, like your "absolute" barometer is.

c. you can brainstorm other methods of calculationg an unknown relating to either altitude or pressure

 

Sorry for being long winded. I hope this helps and i hope i didn't butcher the explanation too much.

DT

[ketch]

You are asking for a whole bunch of stuff but this is the quick and dirty of it.

 

Does anyone know how they work?

 

A barometere works by measuring the pressure of the air. As you go up in altitude there is less air and so less pressure.

 

it gives me an "absolute barometric pressure" reading which cannot be adjusted. It also gives a "sea level barometric pressure" reading that i can callibrate. which says that it "represents the present barometric reading reduced to sea level". I don't know what that means.

 

As the weather changes the pressure changes and so your altitude wanders. If you know where you are on a map you can adjust pressure by setting the altitude. If you are uncertain but listening on a radio, you can set the sea level pressure (which is what they report) and then your altitude will be correct.

 

Can someone give me a tip on how to adjust it correctly, and how i can find use in it

 

As a general rule rising pressure indicates improving weather. Falling pressure indicates worsening. I.E. if you wake up and find that your indicated altitude is lower thatn when you went to bed expect the weather to be getting worse.

 

 

The thing is measured in "mbars". does anyone know what those are and how i relate them to whatever is regularly recorded?

 

The best way to relate them to what is indicated is to to be very aware of your new toy for a while. With a little experience you will reckognize patterns that match reality. Then when you are paying less attention or distracted you will know what has been going on.

 

[crazyjizzy]

I thought Dylan's answer was excellent. My points are:

1) "Absolute Barometric Pressure" is what you are reading

right there, no matter what the elevation is.

 

2) If you know the elevation at your reading site, it can

be extrapolated to sea level, for comparrison to other

information.

 

3) If I remember correctly, at sea level, the "normal" pressure, averaged thru time for conditions, is one atmosphere or Bar, or 764mm of Hg. Variations of this are measured in thousandths (m).

[MisterMo]

Unless there's been some techy invention I don't know about....

 

yeah, it's called GPS

 

Which doesn't work worth a shit in the timber, among other places?

[griz]

yeah, i forgot...all the great alpine climbs are in the timber...

 

[Drederek]

rising pressure indicates improving weather. Falling pressure indicates worsening. I.E. if you wake up and find that your indicated altitude is lower thatn when you went to bed expect the weather to be getting worse.

 

 

Wouldn't lower altitude at the same place mean higher pressure and better weather?

[ketch]

rising pressure indicates improving weather. Falling pressure indicates worsening. I.E. if you wake up and find that your indicated altitude is lower thatn when you went to bed expect the weather to be getting worse.

 

 

Wouldn't lower altitude at the same place mean higher pressure and better weather?

 

Yes it would, I am not such a good editor, huh.

[knelson]

rising pressure indicates improving weather. Falling pressure indicates worsening. I.E. if you wake up and find that your indicated altitude is lower thatn when you went to bed expect the weather to be getting worse.

 

 

Wouldn't lower altitude at the same place mean higher pressure and better weather?

 

Yup.

 

Which doesn't work worth a shit in the timber, among other places?

 

Many other places. GPS is great for location (x-y coordinates) but the way the things work, they suck at elevation (z coordinate.) That's why a lot of them have specific "altimeters" (read barometers) built in to them that are not part of the GPS. But what do I know - I don't own one. Mostly because of the fact that everyone else and their mother has one that I'm out with, and it's wrong more times than it's right about elevation.

[Drederek]

GPS is very accurate for elevation if you've got a clear cone of sky above you. It has to be a big enough cone to encompass 3 or 4 of the 24(?) satellites around the world to get an accurate shot. I don't think they're evenly spaced. Accuracy seemed to take a hit when the whole Iraq thing started - maybe they moved one over there. Timber and nearby cliffs definitely can shut it down. Mine read 14,379 atop rainier. Altimeter that hadn't been adjusted for a day read 12,800.

[crazyjizzy]

If I remember correctly, at sea level, the "normal" pressure, averaged thru time for conditions, is one atmosphere or Bar, or 764mm of Hg. Variations of this are measured in thousandths (m).

 

I have thought about this, and I think that I am wrong about several things.

 

1) One atmosphere is not 764mm Mg, but it is darn close.

 

2) Instead of metric displacement of Hg, atmospheric

pressure could also be measured in Imperial units.

Mr. Mo refered to this. I think that this would be

roughly 30" Hg.

 

3) A "Bar" is defined (I think) as the displacement of

Mercury (Hg) at "atmospheric sea-level conditions".

This arbitrary length is then divided into 1/1000ths.

 

Right now, Sea-Tac has a barometric pressure (corrected to MSL I assume) of 1017 mBars, meaning that it is 1.017 times greater that normal (good weather). Someone mentioned a pressure of 22.71. That would be in inches of Hg, and high altitude, or very bad weather.

[Ed_Seedhouse]

GPS is very accurate for elevation if you've got a clear cone of sky above you.

 

Not really. For base GPS, without differential corrections, altitude measurements are much less accurate than horizonal locaions.

 

Horizontally you can get down to around 10 meters or less with 3 or more satellites, depending on the current sky geometry. Vertically the usual attainable accuracy is around +/- 100 meters.

 

With differnetial corrections from the WAAS satellites, horizontal can get down to around 3 meters and altitude is supposed to be similar, but then WAAS birds are often not visible, especially when the horizon is not clear as, for instance, in the mountains.

 

It's understandible since there are a number of other variables involved in altitude measurements, and thus more sources of error. For one thing, determining just where "sea level" is at any given point is no trivial matter. On the scale of a few hundred meters, which is what we need, the Earth is not even close to being spherical. Your GPS stores information about this, known as the "geoid", but it doesn't have nearly enough memory to store a complete model.

 

"Sea level" is no trivial matter. How far from the center of the earth is "sea level" at the particular location you are standing at? Not easy to figure out. Just that you are standing right on the seashore doesn't mean you are standing at "sea level" for that location.

 

Ed

[crazyjizzy]

Is sea level even sea level? I am not joking, this is a real question.

 

Above the Mid Oceanic Ridges, due to the proximity of the mantle, and an attendant rise in gravity, geodacy and geophysics has shown that the level of the ocean is higher than what should be expected. This is because of localized/regionized increases in gravity pulls and piles up water.

[whoohoonickieclimbs]

Wow......who needs college when I come right here!

 

Jokes aside-I just got back from the coast and was wondering that when I standing near the water at high tide........

[MisterMo]

Is sea level even sea level? I am not joking, this is a real question.

 

Above the Mid Oceanic Ridges, due to the proximity of the mantle, and an attendant rise in gravity, geodacy and geophysics has shown that the level of the ocean is higher than what should be expected. This is because of localized/regionized increases in gravity pulls and piles up water.

 

Link

 

Pretty good page with a lot of info on stuff I never thought much about until this thread. If you scroll down a fuzz there's info on sealevel fluctuations on the order of 100m for various reasons.

 

Back closer to the original topic: I almost never use my compass. I use my altimeter both to satisfy my curiosity and, in periods of poor visibility and geographic confusion, to help figure out what's what and where's where. I don't own a GPS.

 

Among GPS owners I'm curious A)How much the things get used for shits and giggles?; B) How much they get used for navigational necessity?; and C)How much they just get lugged around doing nothing?

[whoohoonickieclimbs]

GPS owner. I just used mine when I navigated my way to the OR coast last week. It made the time go faster because it gave me the minutes to the next stop, as I plotted out my course on my laptop and had each town on there. It was very accurate! I put it on my dash board and it told me the minutes left to the next dest. COOL!

A-Above would be shits and giggle, but fun- I have heard there are some geotreasure or something you can do too, haven't done that yet.

B-Above would answer that.

C-Uh, well I use it for all kinds of things. If I want to take a hike off the beaten path in OR it allows me find my car again-I'm blonde!

[whoohoonickieclimbs]

Cool link too.

[dylan_taylor]

rising pressure indicates improving weather. Falling pressure indicates worsening. I.E. if you wake up and find that your indicated altitude is lower thatn when you went to bed expect the weather to be getting worse.

 

 

Wouldn't lower altitude at the same place mean higher pressure and better weather?

 

Yes it would, I am not such a good editor, huh.

 

Yes! I wrote it backwards, i meant lower pressure, not altitude, oops.

[griz]

 

 

Among GPS owners I'm curious A)How much the things get used for shits and giggles?; B) How much they get used for navigational necessity?; and C)How much they just get lugged around doing nothing?

 

I got mine for river trips. Allot of permit rivers require you to camp in a specific place during prime season and they can be easy to blow by if you don't have any landmarks to go off. They do a great job of keeping track of mileage. Never even used the long/lat. function and I don't hike/climb with them much at all...I just bring the old school compass.

 

They are pretty fun for shits and giggles too. I checked my speedometer on my jeep to it's speed function...very accurate. I had it track a snow climb and then just followed the arrow on it it back down while comparing it to my boot prints in the snow. Any more than 3-5 ft off my boot track and it would point me towards snow prints. Scary impressive.

 

I am surprised at the "100m off" statement. This has not been my experience at all...i'd say 10-15 meters has been the worst for me and my "experiments". Far better than a altimeter any day and the new ones don't weigh much at all. Keep in mind GPS is heavily used in aviation... from the little cessna 206 bush plane on up to 747's so again I doubt that GPS is heavily flawed on elevation/altitude.

 

 

[Drederek]

DT - I thought you may have switched that.

ES - How can GPS tell where you are without knowing the elevation? As I understand it the GPS measures the lags in signals from the satellites. If the satellites were in the same plane as the GPS I could see it being unable to tell altitude but as they are not I don't see how they cannot. they are basically triangulating your location with with the ends of 3 -4 skew lines and putting you in the globe created by their closest points. Accuracy with 3 satellites is about 100 yards and the fourth gets down to 10 yards.

The GPS is tool and toy. I like to collect waypoints and load them into my TOPO program. I always shoot the car before I head into the mountains. Tried climbing Mt Ellinor from Mt Rose trail one spring. Theres a reason theres no trail (or route for that matter) We made it up but were not going back down that way. So we decided to hike down the trail and road then back along the lake to the car. 12 miles we figured. About 3 miles down the trail I flip on the GPS. GPS says the car is .33 miles away and 2000' down. We peer over the edge at a steep, sandy scree gully. Gully spits us out 100 yds from the car. Ya I'm a believer in the GPS

[Cobra_Commander]

Is sea level even sea level? I am not joking, this is a real question.

 

Above the Mid Oceanic Ridges, due to the proximity of the mantle, and an attendant rise in gravity, geodacy and geophysics has shown that the level of the ocean is higher than what should be expected. This is because of localized/regionized increases in gravity pulls and piles up water.

 

Not to mention the significant pull of the moon and the rotation of the earth.

[Cobra_Commander]

It is true that GPS elevation measurements are traditionally much less accurate than horizontal measurements, for a number of reasons.

 

My understanding is this is because GPS elevation measurements are based on a perfect ellipsoid. These measurements are known as "height above ellipsoid" or HAE. However, we want to know what the elevation is relative to "mean sea level", or the "geoid surface". A geoid model allows translation between the HAE and MSL measurements. These models continually improve, and the NGS puts out new ones every so often. As these models become more accurate over time, GPS elevation measurements become more reliable.

 

A 20m cliff is quite significant to climbers, but to account for this at a global scale requires a very accurate understanding of the earth's shape. It is much easier to be fairly accurate in the horizontal.

 

It should be noted that with the right techniques, GPS elevation measurements can be used for projects that require the most accurate of measurements. It just needs to be applied correctly.

[Dru]

Plot your 3D position from GPS on the topo map you brought and see what elevation the map says you are at.