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Altimeter, why does it need to be Calibrated over and over again?


jlw82
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I don't understand why the Altimeter needs to be calibrated all the time!

 

I know the exact pressure and how far above sea level I live. If I let my Oregon 450 (3.9) lay over night, calibrated via pressure or elevation the elevation have changed a lot over night. Eve the slightness presure change makes a huge difference.

 

So, what is the best settings to get good Altimeter readings on a Oeregn 450? Overall.

 

Im aware of this site, http://garminoregon.wikispaces.com/

 

And, is the pressure the same indoors, as outdoors?

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ambient air pressure can change from one day to the other, this phenomenon is usually called "weather". :ph34r:

 

since the altimeter can detect differences in air pressure within one meter of elevation changes, even the slightest difference in ambient pressure makes a huge difference. and yeah, unless you stay in an airtight sealed room, air pressure inside should be the same as outside.

 

personally i leave the altimeter set to "auto calibrate", but i never observed how well that setting works and how accurate the readings are with it.

Edited by dfx
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And, is the pressure the same indoors, as outdoors?
For sure, else your house would blow apart. :ph34r:

 

You probably don't have tornadoes in Sweden, but it is the rapid pressure drop of a tornado that causes buildings to literally explode.

 

As to calibration, a small change in pressure causes a very large change in altitude. If you have a 24K map or equivalent, simple use the nearest contour to set the elevation. Works for me.

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You probably don't have tornadoes in Sweden, but it is the rapid pressure drop of a tornado that causes buildings to literally explode.

 

There not enough of a difference in pressure to cause a house to 'pop'. It's the massive wind force that does the damage.

 

The engineering team at Texas Tech's Institute for Disaster Research (Minor et al., 1977) point out that the pressure drop inside a tornado with 260 mph winds is only about 10%, or just 1.4 pounds per square inch. Most buildings can vent this difference through its normal openings in about three seconds. That is sufficient time even if the tornado is moving forward at a very rapid 60 mph. In the real world, the discussion is pointless. That violent a tornado would totally blow apart a house before the central low pressure ever arrived. Venting of air to relieve pressure would not be an issue.
Edited by BlueDeuce
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And, is the pressure the same indoors, as outdoors?

Not necessarily - especially in office buildings. The HVAC may be set up to provide some positive pressure inside. Sometimes this can be caused by prevailing wind against the openings in a building, too. The differential can be significant. Surely you've opened a tight door on the exterior of an office building and heard the "whoosh"? Even a relatively "neutral" building can have the pressure wander due to the operation of elevators.
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I still dont get it. The Oregon 450 have a Barometer that keep track on the airpressure. If I place the oregon on a table, and it lay flat on the same spot for a day I see how it tracked to airpressure. But why does it not compensate the altitude?

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I still dont get it. The Oregon 450 have a Barometer that keep track on the airpressure. If I place the oregon on a table, and it lay flat on the same spot for a day I see how it tracked to airpressure. But why does it not compensate the altitude?

It assumes constant pressure so if the pressure goes down it thinks you are climbing. How would it know the weather changed?
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I don't have an Oregon 450. Does it have the auto calibration function that is supposed to correct for ambient pressure changes due to weather effects at constant elevation?

 

If so, can it be set up to display simultaneously:

GPS derived elevation,

Ambient pressure (uncorrected),

Ambient pressure derived elevation (corrected, or auto-calibrated)?

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I don't have an Oregon 450. Does it have the auto calibration function that is supposed to correct for ambient pressure changes due to weather effects at constant elevation?

 

If so, can it be set up to display simultaneously:

GPS derived elevation,

Ambient pressure (uncorrected),

Ambient pressure derived elevation (corrected, or auto-calibrated)?

as i understand it, the auto-calibration uses the elevation calculated from GPS data to compensate for weather changes. it's probably not necessary to do any manual calibration at all when using that, but i can't vouch for its accuracy.

 

in the GPS status page, the elevation from the GPS data will be shown. in all other screens (?), the elevation from the barometer will be shown, with whatever calibration method has been performed and enabled. i don't think it's possible to show the raw air pressure data.

Edited by dfx
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I don't have an Oregon 450. Does it have the auto calibration function that is supposed to correct for ambient pressure changes due to weather effects at constant elevation?

 

If so, can it be set up to display simultaneously:

GPS derived elevation,

Ambient pressure (uncorrected),

Ambient pressure derived elevation (corrected, or auto-calibrated)?

 

i don't think it's possible to show the raw air pressure data.

That's unfortunate. If it did show those three, I would have suggested an experimental regimen by which the effectiveness of the auto-calibration could have been determined. However, not to be able to collect the corrected and uncorr.ected side-by-side for later reduced comparison one doesn't really have a standard for comparison.

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Rats - don't have mine in front of me now - but I can't get over the feeling that I've seen the raw sensor data on the diag page.

good idea, just checked that out. there's temperature, voltages, some unknown "frequency", but no pressure.

Edited by dfx
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Rats - don't have mine in front of me now - but I can't get over the feeling that I've seen the raw sensor data on the diag page.

good idea, just checked that out. there's temperature, voltages, some unknown "frequency", but no pressure.

 

On the Elevation Plot on the 550 the fields at the top can display Ambient Pressure and Barometer

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I've found auto-calibrate to keep my 60CSx accurate to within 10' of altitude elevation. If you really need better than that, you will need to calibrate it every day.

 

I've found my 60CSx to be quite accurate as well. I check the reported altitude against the TOPO map elevations right around my house and other known local elevation markers. It's always very close.

 

And, the proper term is elevation...not altitude. Elevation refers to a point on the surface of the earth above mean sea level (MSL). Altitude always refers to a point in the atmosphere above MSL. Obviously, they can sometimes be the same. In an aircraft, if elevation = altitude, you'd BETTER be taking off, taxiing or landing or you've had a really bad day. :blink:

 

Edited to add: I just checked the elevation at my house with the "auto correct" setting on, as it always is. I can't remember the last time I calibrated it. GPS elevation on the satellite status page menu is 5462'. The elevation reported on the trip odometer page states 5470'. Close enough. Also note that the 60CSx uses the term ELEVATION on both of those pages...not altitude. ;)

Edited by sviking
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I don't have an Oregon 450. Does it have the auto calibration function that is supposed to correct for ambient pressure changes due to weather effects at constant elevation?

 

If so, can it be set up to display simultaneously:

GPS derived elevation,

Ambient pressure (uncorrected),

Ambient pressure derived elevation (corrected, or auto-calibrated)?

 

i don't think it's possible to show the raw air pressure data.

That's unfortunate. If it did show those three, I would have suggested an experimental regimen ....

Well, over and out for me.

 

I don't need to post information for it to be stricken through and replaced with drivel and inconsequential gibberish in red.

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I don't need to post information for it to be stricken through and replaced with drivel and inconsequential gibberish in red.

 

Well, when you're wrong...you're wrong. :blink:

 

And, that's "you're" as in "you" in general...not a specific person. Before you comment on such as you're so often wont to do... ;)

Edited by sviking
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i don't think it's possible to show the raw air pressure data.
Rats - don't have mine in front of me now - but I can't get over the feeling that I've seen the raw sensor data on the diag page.

 

The 60CSx can definitely show raw air pressure data...and display the ambient pressure plot over time. Select "view pressure plot" or you can plot the barometer. Straight air pressure data and the current pressure (inches HG, millibars or hectopascals) is displayed on the lower right.

 

I would be very surprised (and disappointed) if the Oregon couldn't do this as well.

Edited by sviking
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I have a question. Why do we need it to be dead on accurate? (I hope we aren't flying planes using an Oregon as an Altimeter)

 

If I was hunting for a cache that the elevation needed to be accurate to be able to find it I would look for another stable point to set my gps. 'Round here there's benchmark that you can use to ...take a benchmark.

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I have a question. Why do we need it to be dead on accurate? (I hope we aren't flying planes using an Oregon as an Altimeter)

 

If I was hunting for a cache that the elevation needed to be accurate to be able to find it I would look for another stable point to set my gps. 'Round here there's benchmark that you can use to ...take a benchmark.

 

It doesn't have to be "dead on accurate". A lot of us just like so see how accurate it CAN be. For most uses, the elevation read off the topographic map is good enough, if you have one loaded and in use. The GPS will interpolate the altitude between contour lines when you move the cursor around. As compared to the altimeter, which reads ELEVATION, on my 60CSx, it's pretty close, as well. I have a great topo map for the state of AZ from gpsfiledepot.com. All their maps are free. :blink:

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I've found auto-calibrate to keep my 60CSx accurate to within 10' of altitude elevation. If you really need better than that, you will need to calibrate it every day.

 

I've found my 60CSx to be quite accurate as well. I check the reported altitude against the TOPO map elevations right around my house and other known local elevation markers. It's always very close.

 

And, the proper term is elevation...not altitude. Elevation refers to a point on the surface of the earth above mean sea level (MSL). Altitude always refers to a point in the atmosphere above MSL. Obviously, they can sometimes be the same. In an aircraft, if elevation = altitude, you'd BETTER be taking off, taxiing or landing or you've had a really bad day. :blink:

 

Thanks for straightening us out. Please stop by tomorrow on your next ego trip.

Edited by Team CowboyPapa
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I've found auto-calibrate to keep my 60CSx accurate to within 10' of altitude elevation. If you really need better than that, you will need to calibrate it every day.

Time to ease up on that one and keep the metaphors straight. Apart from the odd tree and parking structure, our caching use of a GPS is 0' AGL, so from your aeronautical aspect, the use of elevation is pretty much irrelevant to us. Especially out where I live, the relevant point tends to be the delta in altitude, and "true altitude" at that. The face of the GPS isn't reporting elevation above surrounding terrain .. it's reporting (as best it can) the altitude ASL. On a typical caching day, I'm perfectly happy if the elevation (in your terms) remains at zero all day. I hate climbing pine trees.

 

You need to further understand that while elevation has a very specific meaning when you're in the cockpit, it is used in a rather different way in the world of geography. It's not the first time an English word takes on a different connotation based upon context. Your paradigm isn't the only one out there.

Edited by ecanderson
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Apart from the odd tree and parking structure, our caching use of a GPS is 0' AGL, so from your aeronautical aspect, the use of elevation is pretty much irrelevant to us.

 

"To us"? You speaking for everyone now? The OP asked the question about elevation (altitude, whatever) and said nothing about caching.

 

And, if it's so irrelevant, then why does the actual GPS hardware use the term ELEVATION? If a GPS is running a topo map, why isn't there a setting to read AGL? Why even care? There is NO way to reliably get AGL without a RAD ALT (radar altimeter), unless you personally have that feature in a handheld GPS...which you do not...

 

If someone ever floats a stabilized cache in a hot air balloon or something, then I might agree with you. :D

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If I was hunting for a cache ...

Although this site is hosted by Groundspeak, some of the folks here have no interest in caching and have never even looked for one. While the caching hobby may be prone to "numbers" envy, hiking can be as well in its own way ... "elevation envy".

 

I can understand that. You just have to wonder how much advice you need to give based on what the poster is trying to achieve. 'Hey I'm going into the Northern Rockies on a three week expedition, how do I recalibrate...". vs "Hey, I'm playing with this thing to find a nearby cache and just wondering...."

 

Serious questions deserve serious answers. I'd like to know if the OP was getting into something that might be beneficial to direct them to a better measurement system.

Edited by BlueDeuce
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elevation has a very specific meaning when you're in the cockpit, it is used in a rather different way in the world of geography.

 

EXPLAIN your statement very carefully...

I think we both agree on the use of the word in aeronautics (height AGL). However, in geographic terminology, elevation is most often used to refer to some vertical distance ASL (aka "true altitude"). Don't think it needs to be any more careful than that. Same word, two uses. The former isn't particularly relevant to caching or hiking with a GPS since we at ground level (or close to it) the majority of the time. The latter is relevant, and is how the word is most often used in that context.
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I think we both agree on the use of the word in aeronautics (height AGL). However, in geographic terminology, elevation is most often used to refer to some vertical distance ASL (aka "true altitude"). Don't think it needs to be any more careful than that. Same word, two uses. The former isn't particularly relevant to caching or hiking with a GPS since we at ground level (or close to it) the majority of the time. The latter is relevant, and is how the word is most often used in that context.

 

What does AGL in feet mean to you?

 

What is this "ASL" plane of reference when it comes to elevation/altitude? If it's MSL (mean sea level), then guess what? It's ELEVATION when referring a point on the surface of the earth. You know...where peoples' feet tend to be when they're caching or hiking.

 

So, yet again, exactly how is knowing ELEVATION irrelevant? Why do our GPS units report ELEVATION? Why are topographic contours labeled in ELEVATION? Why are road signs listed in ELEVATION? As in the height of that point on the ground above MSL? If it's so irrelevant, why do so many people ask about or even discuss it like, you know...the very origin of this thread?

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...why do so many people ask about or even discuss it ...
Ah, a useful question buried amidst wordplay and arguments. Ahead of (and during) a trek, I wanna know the up* and down** of where I'm going, for the sake of planning the easiest routes. I'm lazy that way. That's about it for me. What do you look at it for?

 

---

* up: the direction away from the earth's center of gravity

** down: the other way.

Edited by lee_rimar
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What is this "ASL" plane of reference when it comes to elevation/altitude? If it's MSL (mean sea level), then guess what? It's ELEVATION when referring a point on the surface of the earth. You know...where peoples' feet tend to be when they're caching or hiking.

his point is that "elevation" is a relative term: elevation above something. this something can be ground level, sea level, or anything else for that matter. now when you just say "elevation" without saying relative to what, then it depends on the context what exactly you mean.

 

altitude on the other hand always refers to elevation above sea level.

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Anyone I've ever hiked with always refers to our current elevation, or our total elevation gain.

Only time I hear anyone refer to altitude is the captain announcing it over the PA in a plane. :lol:

 

I'd firmly agree with the first sentence ref "elevation gain".

 

To be completely fair, in certain parts of the world, hikers do also refer separately to altitude ASL a lot, as when climbing what we call "14ers" here in Colorado (gasp, wheeze, gasp - is there any air up here?), but we're one of the exceptions due to local geography where it's physiologically relevant. Turns into somewhat of a mixed bag of terms as a result. Even so, the plots people do from their hikes all reference "elevation" here in its "geographic" usage.

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