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Altimeter issues

Does anyone else notice that their altimeter requires constant re-calibration? So I went for a bike ride yesterday with the F935, a ride with around 10,000ft of elevation so it was a good test of the barometric altimeter. I did the same ride last year with the F630 (which doesn't have a barometric altimeter). I know the high point of the ride is 1489ft from the Ordnance Survey map, but comparing elevation data from both rides it looks like the GPS only elevation points on the F630 are more accurate than the data from the barometric altimeter on the F935.

This may be due to the fact that I forgot to calibrate the altimeter before the ride, but in my experience so far, even with auto calibration turned on, it seems as if the altimeter can drift by about 30m over the course of around a week unless it's calibrated on a regular basis, or preferably before the start of any activity, which is slightly annoying. Elevation gain may be perfectly accurate on the 935, but absolute elevation seems way off unless it's calibrated properly prior to starting the ride.

Here's the links to the two rides:

2017 (Forerunner 935): https://www.strava.com/activities/1063977019

2016 (Forerunner 630): https://www.strava.com/activities/621449726

0 FabioPascua over 7 years ago

As much as barometric pressure changes, alongside with your activity length (time wise).

0 ekutter-dnu over 7 years ago

This is always the downside of barometric pressure altimeters. With the 935, auto calibration appears to only operate when you hit the start button. It doesn't continuously auto-calibrate during a ride. So in mountainous terrain, GPS based altitude tends to be more accurate for detecting the high points. BP altitude will give much better numbers for total ascent though.

0 hermanhk over 7 years ago

Hi,

slightly off topic here. Can anyone tell me which is more accurate, the altimeter on the 935? or using the elevation correction on the Garmin site? I noticed when I use the elevation correction, there is a big difference

0 timgrose over 7 years ago

Accurate in what respect? Absolute values or the relative elevation profile of an activity?

The former is always going to be a function of the initial offset between the correct value and what the 935 shows.

0 thigger over 7 years ago

If it's relative change, and the activity is reasonably short (so that changes in air pressure due to weather aren't massive) then the altimeter is very good - I run along the canal and I can look at the trace and see exactly where the locks were. 'Elevation correction' often picks a nearby point just off the canal which could be quite a bit lower down.

0 ekutter-dnu over 7 years ago

Hi,

slightly off topic here. Can anyone tell me which is more accurate, the altimeter on the 935? or using the elevation correction on the Garmin site? I noticed when I use the elevation correction, there is a big difference

They all have their advantages / disadvantages

Elevation correction is just using GPS coordinates to look up known elevation value from area surveys (probably through satellite or aerial photos). As long as you have a good GPS track, things should be fairly good. Further, depending on where it is, this elevation data only has a point every 30 meters or more and interpolates for points in between. So corrected elevation won't get smaller bumps. Also if you are going along a ridge line, GPS error of 20' could have an elevation error of a 1000' when corrected. Also, corrected elevation will often mess up when you go across a bridge because it will have you following the ground below the bridge.

Barometric pressure - great at detecting small changes in elevation, bad when the weather changes during the outing, good for computing ascent
GPS elevation - great for telling you your current elevation within 100' or so, bad for small changes in elevation, terrible for computing ascent
corrected elevation - good in relatively flat terrain with a good GPS signal, bad at picking up small bumps along your route

Bottom line:
if you care about total ascent during your activity - BP is by far the best
if you care about your current absolute elevation, especially in mountainous terrain with changing weather, GPS elevation is your friend
if you just care about approximate elevation and ascent after the activity in non-mountainous terrain, correction works pretty well

0 hermanhk over 7 years ago

Thanks all,

I am more interested in the relative elevation change in my trail run, part of the reason why I got the 935 over the 735xt is the built in altimeter.

0 florin83 over 7 years ago

This is always the downside of barometric pressure altimeters. With the 935, auto calibration appears to only operate when you hit the start button. It doesn't continuously auto-calibrate during a ride. So in mountainous terrain, GPS based altitude tends to be more accurate for detecting the high points. BP altitude will give much better numbers for total ascent though.

It is very annoying that it doesn't auto-calibrate continuously like Suunto Ambit 3 Peak. After 2-3 hours of hike the altitude is 30-50m off the correct value.

0 AV8OR51 over 7 years ago

The altimeter on the Forerunner can be as accurate as this, if there no atmospheric pressure changes with in the time period. The watch was calibrated to a known field elevation about 1.5 hours ago. The altimeter in the airplane was calibrated by entering the latest barometric pressure obtained from airport weather data (METAR).ciq.forums.garmin.com/.../1229546.jpg

0 florin83 over 7 years ago

It's very annoying that after 30 minutes of running in the mountains the barometric altimeter si 30-40 meters lower than real elevation. At the start the elevation is calibrated and it's correct.
Is there a way to force the barometric altimeter to calibrate with gps at a specified time?