altimeter
Results 1 to 12 of 12

Thread: altimeter

  1. #1
    Frog Whisperer
    Reputation: Touch0Gray's Avatar
    Join Date
    May 2003
    Posts
    40,575

    altimeter

    ok, I was in a jewelry store today and was playing with a Tissot with an altimeter in it and was noting to the owner that it was fluctuating...from 938 to 945...he laughed and said sure....it changes as the waves come it...We got a good laugh about it and I went on my way...
    OK...I was wondering why it was fluctuating, I was not moving, granted today was a stormy day and the barometric pressure MAY have been fluctuating too...I mean it was no more accurate than my GPS that's for sure..

    This evening I looked up measuring sea level and found this:
    " Question - from what point is sea level measured?

    Sea level, better known as Mean Sea Level is the average height of the
    sea between high and low tide. Obviously, this can vary greatly depending
    on climate and the affects of the gravitational attraction of the moon,
    sun, and other planets and their positions relative to each other. Satellites
    monitor sea level to see if it changes with time and climate change."

    Could someone expalin this to me?.....should the barometric altimeter be constant it it isn't moving??

  2. #2
    eminence grease
    Reputation: terry b's Avatar
    Join Date
    Jan 2004
    Posts
    18,538
    They vary like crazy, lots of things affect barometric pressure, especially inside. Little things like someone opening and closing a door.

    My wife, the Luddite used to laugh at my Casio altimeter watch. We'd be at the beach, and the altitude alarm would be going off. Telling us we were 10-20 feet below sea level. Consumer altimeters are good enough, they're just not perfect.
    You'd be better off with a netbook, they do everything better.

    My travel blog: http://tbaroundtheworld.blogspot.com

  3. #3
    Frog Whisperer
    Reputation: Touch0Gray's Avatar
    Join Date
    May 2003
    Posts
    40,575
    hehehe..I assumed it wasn't the waves thng......With your answer in mind I am glad I didn't buy it...not that I was going too..or pay retail if I had. My E-trex Legend varies less under open sky and is plenty accurate for my needs.....I just like gadgets!

  4. #4
    eminence grease
    Reputation: terry b's Avatar
    Join Date
    Jan 2004
    Posts
    18,538
    Quote Originally Posted by Touch0Gray
    hehehe..I assumed it wasn't the waves thng......With your answer in mind I am glad I didn't buy it...not that I was going too..or pay retail if I had. My E-trex Legend varies less under open sky and is plenty accurate for my needs.....I just like gadgets!
    Yea, I have a barometric Vista and a Legend (which is of course GPS altitude) and they are far less whacky than my watch was. My weather station is rock solid - very little spurious variation.
    You'd be better off with a netbook, they do everything better.

    My travel blog: http://tbaroundtheworld.blogspot.com

  5. #5
    Cat 6
    Reputation: chuckice's Avatar
    Join Date
    Aug 2004
    Posts
    4,693
    Quote Originally Posted by Touch0Gray
    ok, I was in a jewelry store today and was playing with a Tissot with an altimeter in it and was noting to the owner that it was fluctuating...from 938 to 945...he laughed and said sure....it changes as the waves come it...We got a good laugh about it and I went on my way...
    OK...I was wondering why it was fluctuating, I was not moving, granted today was a stormy day and the barometric pressure MAY have been fluctuating too...I mean it was no more accurate than my GPS that's for sure..

    This evening I looked up measuring sea level and found this:
    " Question - from what point is sea level measured?

    Sea level, better known as Mean Sea Level is the average height of the
    sea between high and low tide. Obviously, this can vary greatly depending
    on climate and the affects of the gravitational attraction of the moon,
    sun, and other planets and their positions relative to each other. Satellites
    monitor sea level to see if it changes with time and climate change."

    Could someone expalin this to me?.....should the barometric altimeter be constant it it isn't moving??
    BTW...if it was the tissot t-touch it's a pretty cool watch.

  6. #6
    Martini time?
    Reputation: twrecks's Avatar
    Join Date
    Feb 2005
    Posts
    206
    Going back to my Air Force instrument tech training days...

    Barometric altitude is distance above sea level. If you flew from SFO (sea level) and landed in Denver, your barometric altitude would be 6000 feet or so, useful for takeoff and landing configurations, fuel consumption, stopping distances etc. Much more helpful for landing in Denver would be radar altitude, which would give the distance from your butt to the ground, useful for not running into mountains.

    Baro pressure is always set to 29.92 once you are in the air, not sure how it works on a watch.

  7. #7
    Frog Whisperer
    Reputation: Touch0Gray's Avatar
    Join Date
    May 2003
    Posts
    40,575
    Terry just out of curiosity, how does the elevation compare from the vista to the legend, ever checked?... I mean barometric vs gps elevation?.... My legend seem pretty solid unless I am getting a lot of reflected signals to the gps, but I can usually tell by jumps in the reading.

    And yeah it was the touch.....nice watch BUT the sucker is as big as my head!
    Tissot makes a nice automatic (spring driven) which is really my choice in watches... I understand they make the touch in Ti to get the weight down and in gold to make it REALLY freaking heavy!

  8. #8
    eminence grease
    Reputation: terry b's Avatar
    Join Date
    Jan 2004
    Posts
    18,538
    Quote Originally Posted by Touch0Gray
    Terry just out of curiosity, how does the elevation compare from the vista to the legend, ever checked?... I mean barometric vs gps elevation?.... My legend seem pretty solid unless I am getting a lot of reflected signals to the gps, but I can usually tell by jumps in the reading.

    I think the Vista on average is more precise, but the Legend is far more stable. The Legend only changes when you move, vs. the Vista which fluctuates due to the pressure. I also have a Forerunner and what I have found with it (GPS altitude) is that I will be 1/2 way up a climb before it shows any altitude change due to the communication lag time with the satellites and the associated calculations.

    A decent analogy might be to consider the Vista to be analog altitude and the Legend to be digital. A waveform generated by the analog unit would be smooth while the waveform generated by the digital unit would be a step function.

    I'll bet they'd both say 14,148 if you were standing on top of Mt. Evans, but the map of how you got there would not be the same.
    You'd be better off with a netbook, they do everything better.

    My travel blog: http://tbaroundtheworld.blogspot.com

  9. #9
    Moderatus Puisne
    Reputation: Argentius's Avatar
    Join Date
    Aug 2004
    Posts
    15,882
    How much of that would smooth out when you downloaded the forerunner, do you think?

    I'm getting a Garmin soon, and have a HAC4 (barometric altimeter) right now. The HAC4's screen updates slowly on some features (like grade), but when I download it, the data smooth out. At least, as much as they can with a max 20-second sample rate.

  10. #10
    Shirtcocker
    Reputation: Bocephus Jones II's Avatar
    Join Date
    Oct 2004
    Posts
    60,638
    Quote Originally Posted by Argentius
    How much of that would smooth out when you downloaded the forerunner, do you think?

    I'm getting a Garmin soon, and have a HAC4 (barometric altimeter) right now. The HAC4's screen updates slowly on some features (like grade), but when I download it, the data smooth out. At least, as much as they can with a max 20-second sample rate.
    I stopped even bothering to calibrate my Polar S720i...it's all wacky here in the mountains. It still is pretty accurate for calulating vertical feet climbed as long as a big storm doesn't roll in mid-ride.
    "I regard the brain as a computer which will stop working when its components fail. There is no heaven or afterlife for broken down computers; that is a fairy story for people afraid of the dark." -S. Hawking

  11. #11
    It's all ball bearings
    Reputation: BenWA's Avatar
    Join Date
    Aug 2004
    Posts
    5,258
    meh, that's the nature of altimeters. Less than 10' of variability really isn't that bad at all, even for the most expensive and sophisticated aircraft altimeters that use pressure. Altimeters don't measure true altitude, they measure air pressure...pressure is used as a proxy for altitude based on atmospheric pressure lapse rates. The problem is that pressure can change easily with local disturbance, such as mentioned above (door opening or closing, air circulation in a room or wind hitting the sensor).

    The most accurate way to measure altitude is with surveying equipment like a Total Station theodolite. Obviously this isn't practical when hiking, biking, or flying in an airplane...or GPS with a fully corrected altitude signal (which is fruit that only the military can pluck).

    Even airplanes have to accept the fact that there is some amount of slop within the measurements of pressure altimeters (this is partially why IFR traffic has to maintain specified minimum altitude separation of 1000' from other aircraft). What matters is that all airplanes that are next to each other (or near terrain) are set on the "same page". Airplanes below class A airspace (18,000') set their altimeters to whatever the reported altimeter setting is in the immediate vicinity (at the nearest airport or from ATC) and they constantly re-calibrate every time a new altimeter setting is issued for their area. Above 18,000 (IFR only) all aircraft set to 29.92 so that everybody is on the same page up there. The only way for airplanes to measure their true altitude is with radar altimeters, altituded-corrected GPS, or precision approach radar (if the airport is equipped with such).
    Last edited by BenWA; 08-25-2006 at 11:31 AM.

  12. #12
    eminence grease
    Reputation: terry b's Avatar
    Join Date
    Jan 2004
    Posts
    18,538
    Quote Originally Posted by Argentius
    How much of that would smooth out when you downloaded the forerunner, do you think?

    I'm getting a Garmin soon, and have a HAC4 (barometric altimeter) right now. The HAC4's screen updates slowly on some features (like grade), but when I download it, the data smooth out. At least, as much as they can with a max 20-second sample rate.

    Total variation is pretty much lost in the noise once it's downloaded - the x-axis of the graph is divided up in 30 minute increments so the momentary lag is not visible. I only see it when I've been climbing for 30-60 seconds and while the road is steep, the altimeter shows the same value. While the graph looks "digital", the lack of data between the points doesn't offer a lot of useful information. Even though an actual climb is not digital - grades and descents are analog sine waves.

    Nothing to render the instrument unuseable, rather it's just a nit that annoys data-nutcases like me.
    Attached Images Attached Images
    You'd be better off with a netbook, they do everything better.

    My travel blog: http://tbaroundtheworld.blogspot.com

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

THE SITE

ABOUT ROADBIKEREVIEW

VISIT US AT

© Copyright 2020 VerticalScope Inc. All rights reserved.