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Barometric Altimeter vs GPS Altimeter (3D Speed)

jasonmfry
over 3 years ago

Does the watch use Barometric or GPS for altimeter readings during an activity? I have 3D speed enabled. 

Reason for ask, I had a F5X and currently have a F7XSS. I have (MTB) ridden the same course with both watches and the same settings with the exception of all satellites multiband for the F7X. 

F5X consistently read 1415'-1440' of ascent. 

F7X consistently reads 1590' - 1610' of ascent.

Which is accurate? 

Thanks!

  • 0 over 3 years ago in reply to gregjohnson1956

    Altitude is generally calculated from MSL (mean sea level), so you need to find the average sea height between high and low water to be precise. In some places that's only a few feet in others it can be 50+ feet.

    If the watch is in altimeter mode then any pressure change is assumed to be a change in altitude.

    If the watch is in barometer mode then any pressure change is assumed to be a real pressure change with no change in altitude.

    If the watch is in auto mode, it tries to guess exactly what's going on and do its best to set the altitude.

    So if you leave the watch in Altimeter mode and ascend 1000 feet quickly, the watch should show an altitude gain of 1000 feet.

    However, if the weather conditions change (ie the pressure changes) which is affected by (temperature, humidity, pressure), then the watch will get the altitude change wrong.

    If the pressure doesn't change at all, then the altitude will be correct to +/- about 10 - 15 feet.

    I hike, and I use the watch in auto mode, but I also keep an eye on the weather and manually recalibrate the altitude if the pressure changes or if I am at a known point, such as a trig point or contour line on the map. 

    To be honest I don't really care if the altitude is correct or not. I use maps to tell what my ascent/descent is or transfer the saved route to OS maps and it tells me.

  • 0 over 3 years ago in reply to gspinoza

    No I didn't. where ?.

    BUT GPS can be more accurate than barometric if certain information is available.

    This depends on the number of satellites, where they in space and their relative positions to you, AND what geoid is being used and most importantly if you have access to geoid altitude corrections for the geoid in use and your location on the planet.

    For instance surveying GPS instruments with access to additional data can be accurate to within a few cm's.

  • 0 over 3 years ago in reply to 2572376
    2572376 said:

    No I didn't. where ?.

    Sorry, I confused you with someone else.

    I replied to you from my smartphone and it wasn't clear to me who was writing.

    Sorry again.

  • 0 over 3 years ago in reply to gspinoza

    no problem we have all done that :-)

  • 0 over 3 years ago

    Understanding the Accuracy of the GPS Elevation Reading

    “It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating.“

    https://support.garmin.com/en-US/?faq=QPc5x3ZFUv1QyoxITW2vZ6

    Altimeter Auto Calibration for Outdoor Handhelds and Watches


    https://support.garmin.com/en-US/?faq=FhOYuggxmV6Atph276U4h8     

  • 0 over 3 years ago in reply to gregjohnson1956

    You have to go to the beach before every hiking to calibrate your watch first Smiley

    I think the reason of using barometric is that requires less computing to determine altitude. Less battery drain. But if weather changes, barometer might see that as altitude change.

  • 0 over 3 years ago in reply to gspinoza

    GPS is actually pretty accurate, if you compensate for the geoid curvature of the earth, which the watch doesn't do.  Google geoid offset and you'll see what the offset should be for your location on the earth, and if you adjust the reported GPS altitude by that amount it should be pretty close to your actual elevation.  Its not perfect, but with this adjustment its going to rival the baro altimeter for accuracy without all the inconvenience of weather and air density changes to deal with.

  • 0 over 3 years ago in reply to 2572376

    some interesting history behind baro altimeters in planes.  Under certain weather conditions they're horribly inaccurate, but the aviation industry had standardized on them decades ago, so they have the benefit of everybody being calibrated to the same wrong elevation information, so they don't run into each other during flight.  they use the radio altimeter only when landing because if they used it during flight it would put them at the wrong elevation (because its more accurate) and interfere with all the other planes using the baro altimeter readings.

  • 0 over 3 years ago in reply to 5693709

    But they also communicate with ATC on approach to get the local barometric pressure at the time to manually adjust altitude too. 

  • 0 over 3 years ago in reply to Dr Phil

    yes, because they want to calibrate the baro altimeter as often as practical.  And, they can use the baro altimeter for approach if desired.  From what I understand the baro altimeter is still the go-to instrument for aviation because its the one piece of equipment every plane will have on board, the other altitude measuring devices are secondary because even though they're potentially more accurate, they're not the standard by which all other planes in the sky are determining their altitude.

    And just to clarify what I'm saying, its not that baro altimeters are inaccurate as instrumentation, they're actually very accurate to the standard to which they're calibrated, and they will all read very close under the same conditions.  The inaccuracy comes about because all baro altimeters are calibrated to what's called a standard atmosphere, and most of the time the actual atmosphere doesn't match the characteristics of the standard atmosphere.  That causes them all to be inaccurate, but they're all inaccurate by the same amount, so they all read the same thing, even if its not technically the correct altitude.  So, if they're supposed to fly at 6,000ft and when their altimeter reads 6000ft its really 5500ft, there's no problem because all the planes that are supposed to be flying at 6,000ft are all flying at the same altitude (5500ft). 

    The flight patterns are established where each direction has its own layer of air, so there's no chance of collision with planes coming at you from the front, or either side, they're at a different altitude.  Planes at your own altitude are all flying in the same direction as you are, so you have time to react if you approach an aircraft flying at a lower speed, which wouldn't be as easy if the plane were coming at you from the side or straight ahead.

    In the case of Garmin watch altimeters, they're well calibrated to the standard atmosphere so they're very accurate.  The problem occurs because most people don't know how to compensate when the atmosphere is non-standard (practically all the time) and they rely on stupid gimmicks like auto-calibrate and auto mode which are just software algorithms that are wrong more than they are right.