Altimeter Watch Accuracy
Jul 15, 2021 at 9:22 am #3721962Matthew / BPLModerator
Lowell, recording tracks of hikes is one of the few areas of my life I’m not interested in quantifying 🤣 so I can’t speak to what happens to a track when you recalibrate. I assume you get a big step in the profile of your track.Jul 15, 2021 at 4:59 pm #3722037
If a decent signal reception is available, a modern GPS receiver should be able to give elevation data accurate to a range of 10 to 20 meters (35 to 70 feet) post correction.
And that’s exactly where the problem with GPS-based altimeter watches meant for the great outdoors lies. The GPS reception isn’t always terrific and hikers have reportedly found the altitude measurements to be off by as much as 400 feet!
Garmin confirms this, explaining that the main source of error has to do with the arrangement of the satellite configurations during fix determinations. “The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions.” As such, the values provided by GPS-based altimeters should be used with caution when navigating.
So, while it’s obvious that both systems have their pros and cons, if you’re looking for an accurate elevation data (and a way to ensure you don’t get caught in a white-out storm), you would be better off if your GPS watch uses a barometric altimeter as a system of checks and balances.
You can get receivers which also use broadcast corrections (WAAS), but you need a much larger unit for this. It was developed for air-nav, and they don’t care if the receiver is a cubic foot.
And as an added problem, what the GPS ‘altitude’ reports is really the height above the theoretical geode which approximates Earth. It is NOT height above Mean Sea Level. Deviations of 100 m are common.
CheersJul 15, 2021 at 5:21 pm #3722040Steve SBPL Member
The article has its limitations:
So you are navigating through a whiteout, trying to avoid a pesky unknown cliff under conditions that could generate a cornice, and you do not know the conditions under which your watch will apply a GPS-correction to the barometric altimeter reading or whether it the correction will be in slow increments or in one jump. Nor will it tell you whether the GPS is increasing or decreasing the apparent altitude. On the other hand, you do know that the nearby mountain is blocking all signals from one side, seriously degrading gps accuracy. Faith-based navigation has potential downsides.
(BTW, traveling down wind under such conditions can be more dangerous than upwind since you cannot see a cornice before you are on it.)Jul 16, 2021 at 2:05 am #3722070Rex SandersBPL Member
Smarter GPS devices should be able to correct for geode and datum differences while showing locations and elevations on a map. Thru-hikers are completing trails thousands of miles long with just a smartphone and an app like Gaia or Guthook.
WAAS transmits on the same L1 and L5 radio frequency bands as GPS. Newer Apple and Samsung smartphones, many Garmin devices (but not the inReach mini or Explorer), and a bunch of other handheld devices include WAAS.
I have no idea if smarter watches include these features.
— RexJul 16, 2021 at 4:03 pm #3722135
Thru-hikers are completing trails thousands of miles long with just a smartphone and an app like Gaia or Guthook.
Sure, on a marked trail.
The trouble with trying to correct for geode differences is that there are local fluctuations in (earth) density which have not been mapped. Just how much effect that has I do not know.
CheersJul 16, 2021 at 6:12 pm #3722153BonzoBPL Member
@bon-zoLocale: Virgo Supercluster
I don’t know for sure, but I don’t think the 9 Peak has WAAS access. I will check the user guide/specs when I get home.Jul 16, 2021 at 9:21 pm #3722167
I could not find any mention of WAAS access on several gear web sites or on the company’s own web site. I have me doubts.
CheersJul 17, 2021 at 12:42 pm #3722193BonzoBPL Member
@bon-zoLocale: Virgo Supercluster
I can’t find it either, Roger. As best I can tell it’s not in the manual and it’s nowhere in the watch menus. I would imagine that this means WAAS isn’t available on this device.
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