Total ascent figures

Discussion related to the Garmin GPSMAP 66 series GPSr
Spiney
Posts: 24
Joined: Thu Apr 04, 2019 9:35 am

Re: Total ascent figures

Post by Spiney » Fri Jun 14, 2019 11:59 pm

I have just read BLTAAKE's comments about the difference between the Trip Computer on Garmin devices and the GPX Track Log at https://forums.garmin.com/apps-software ... n-basecamp
In the past, there has been a confusion about why the Trip Computer data differed from the downloaded track log. Garmin has made two changes to their devices to address this. One is they made the Trip Computer a little smarter. It no longer logs every position solution, but only those where it decides the device has changed locations. This improved the total distance travelled, particularly for those just meandering about. The second change was to include the Trip Computer summary data with the downloaded Trip Log data.

This second change means that you no longer see summary data based on the data in the track log, but that of the trip computer. So, the device and BaseCamp agree exactly; but in truth, this is not a good thing to do.

If you are seeing Trip Computer summary data in BaseCamp, you can force recalculation based on the logged track data by deleting a single point in the track or dividing the track and then rejoining the track fragments. The former removes one data point, the latter adds one data point. The added point is a duplicate of one of the original track points.
If you follow the advice given and simply delete a single track point in BaseCamp, the improvement in accuracy of the cumulative ascent is staggering. I tried this for the track from a recent bike/hike in Scotland:

Original GPX loaded into Basecamp - ascent 2622m (WAY TOO HIGH!)
sp1.jpeg
sp1.jpeg (90.77 KiB) Viewed 891 times

Same file with trackpoint 3 deleted to force a recalculation - ascent 1890m
sp2.jpeg
sp2.jpeg (84.91 KiB) Viewed 891 times

This new figure is much closer to what I was expecting based on the map and guidebook.

Presumably Garmin's second change noted above also applies to FIT files too? I took a look at the FIT file for the walk:

I could not import the original FIT file for this hike directly into BaseCamp, so I had to convert it to a GPX first using GPSVisualizer. The initial converted GPX file had no elevation data, so I converted the FIT file again but adding elevation data from the GPSV's best digital elevation model. I then imported this new GPX file into BaseCamp. The 1880m total ascent using GPSV elevation data agreed very closely with the recalculated total having deleted a single track point and gives some validation to the number.
sp3.jpeg
sp3.jpeg (82.8 KiB) Viewed 891 times

I'd be interested to see if you get the same results and if your recalculated data is close to what you expect. If the results are consistent, simply deleting a single track point could be a relatively painless way to get a more accurate estimate of total ascent.
Last edited by Spiney on Sat Jun 15, 2019 12:01 am, edited 1 time in total.

gpsrchive
Site Admin
Posts: 631
Joined: Fri Jan 20, 2017 11:29 pm

Re: Total ascent figures

Post by gpsrchive » Sat Jun 15, 2019 12:00 am

pedigree1 wrote:
Fri Jun 14, 2019 6:04 pm
Many thanks.
oldkingog wrote:
Fri Jun 14, 2019 7:11 pm
Grateful for your work! I look forward to hearing what you learn...
I may need a week or two to get those tests started. You'll understand why soon.

pedigree1
Posts: 19
Joined: Wed Jun 12, 2019 9:48 am

Re: Total ascent figures

Post by pedigree1 » Sat Jun 15, 2019 9:42 am

Spiney wrote:
Fri Jun 14, 2019 11:59 pm
I have just read BLTAAKE's comments about the difference between the Trip Computer on Garmin devices and the GPX Track Log at https://forums.garmin.com/apps-software ... n-basecamp
In the past, there has been a confusion about why the Trip Computer data differed from the downloaded track log. Garmin has made two changes to their devices to address this. One is they made the Trip Computer a little smarter. It no longer logs every position solution, but only those where it decides the device has changed locations. This improved the total distance travelled, particularly for those just meandering about. The second change was to include the Trip Computer summary data with the downloaded Trip Log data.

This second change means that you no longer see summary data based on the data in the track log, but that of the trip computer. So, the device and BaseCamp agree exactly; but in truth, this is not a good thing to do.

If you are seeing Trip Computer summary data in BaseCamp, you can force recalculation based on the logged track data by deleting a single point in the track or dividing the track and then rejoining the track fragments. The former removes one data point, the latter adds one data point. The added point is a duplicate of one of the original track points.
If you follow the advice given and simply delete a single track point in BaseCamp, the improvement in accuracy of the cumulative ascent is staggering. I tried this for the track from a recent bike/hike in Scotland:

Original GPX loaded into Basecamp - ascent 2622m (WAY TOO HIGH!)
Image

Same file with trackpoint 3 deleted to force a recalculation - ascent 1890m
Image

This new figure is much closer to what I was expecting based on the map and guidebook.

Presumably Garmin's second change noted above also applies to FIT files too? I took a look at the FIT file for the walk:

I could not import the original FIT file for this hike directly into BaseCamp, so I had to convert it to a GPX first using GPSVisualizer. The initial converted GPX file had no elevation data, so I converted the FIT file again but adding elevation data from the GPSV's best digital elevation model. I then imported this new GPX file into BaseCamp. The 1880m total ascent using GPSV elevation data agreed very closely with the recalculated total having deleted a single track point and gives some validation to the number.

Image

I'd be interested to see if you get the same results and if your recalculated data is close to what you expect. If the results are consistent, simply deleting a single track point could be a relatively painless way to get a more accurate estimate of total ascent.
Thanks for that, very interesting! I tried your suggestion with a .fit file (recorded with v3.0 firmware) i.e. deleting a single track point in Basecamp. This took out 4m of ascent but reduced the total ascent by 100m and to much closer to the expected.
However, my issue has always been that the ascent data displayed on the unit during a hike is incorrect; it is this live data that I am more concerned about. Correcting the figures post hike is relatively easy, I can do this in other ways, it is the live feedback error that rather negates the reason for carrying the GPS unit in the first place.

oldkingog
Posts: 38
Joined: Tue Jan 08, 2019 6:01 pm

Re: Total ascent figures

Post by oldkingog » Sat Jun 15, 2019 11:34 am

[/quote]

Thanks for that, very interesting! I tried your suggestion with a .fit file (recorded with v3.0 firmware) i.e. deleting a single track point in Basecamp. This took out 4m of ascent but reduced the total ascent by 100m and to much closer to the expected.
However, my issue has always been that the ascent data displayed on the unit during a hike is incorrect; it is this live data that I am more concerned about. Correcting the figures post hike is relatively easy, I can do this in other ways, it is the live feedback error that rather negates the reason for carrying the GPS unit in the first place.
[/quote]

I couldn't have said it any better than pedigree. Definitely appreciate the tip, though!

Spiney
Posts: 24
Joined: Thu Apr 04, 2019 9:35 am

Re: Total ascent figures

Post by Spiney » Sun Jun 23, 2019 10:52 pm

And does carrying the unit in a pocket affect the barometer significantly?
I did not know anything about this issue, so I reviewed posts from various forums and Garmin Support Centre articles which collectively indicate that there may be several sources of error affecting the barometric altimeter relative to carrying the device in a pocket. It is not clear how much of an effect each of these issues may have on recorded elevation or total ascent data, but they are all said to contribute to the error of elevation measurement through pressure changes.

The barometric altimeter is affected by local variations in pressure. Mostly this is due to changes in weather or elevation change, but the pressure can be influenced by local changes in wind speeds caused by local topography in the mountains. Wind speeds are naturally increased in natural funnels like passes/cols, at the top of cliffs/edge of a mesa etc. These fluctuations can add to the elevation gain recorded. The device in a pocket may be protected against such winds until it is removed. Cyclists have been advised by Garmin that the factors that cause elevation readings to be wrong include riding solo compared to in a group, road traffic and where the device is mounted on the bike, which can all affect pressure readings.

The pressure sensor is almost certainly not temperature compensated, so taking the device out of a warm pocket into much colder air will affect the pressure sensor and can add to the elevation gain recorded. Being in a warm tent could have a similar effect.

One also needs to check that the small holes that lead to the altimeter sensor are not clogged. Cyclists have reported their elevation data flat-lining when these holes have been filled by water droplets, preventing pressure changes being recorded.

In conditions where satellite geometry is not optimum, having parts of your body between the device and the satellite may cause errors, as the water in your body attenuates the signal. Most cloth that would be covering a GPS receiver’s antenna in a pocket will pass GPS signals (e.g. backpack, jacket, etc.). This might change if the cloth that covers the GPS receiver’s antenna gets wet (e.g. from rain) as this water may also attenuate the GPS signals.

In theory, all of these factors may introduce additional errors into the estimations of Elevation and Total Ascent as recorded on a GPS device, but there is little information on their magnitude.

The best advice seems to be to carry a GPS device by mounting it at shoulder level on a rucsac strap (with a lanyard) to maximise signal reception and thus accuracy.

oldkingog
Posts: 38
Joined: Tue Jan 08, 2019 6:01 pm

Re: Total ascent figures

Post by oldkingog » Mon Jun 24, 2019 3:24 am

Interesting post, Spiney. All I need to do is carry my 66 in my hand for a five minute walk on flat ground and I'll have tallied at least 100 feet of elevation gain. Doesn't seem to matter with my unit whether or not it is in my pocket.

truckinguy
Posts: 16
Joined: Mon May 27, 2019 11:37 am

Re: Total ascent figures

Post by truckinguy » Tue Jun 25, 2019 8:46 am

When I carry my gpsr's either he montana or 66st in my shirt pocket they are way off. The chart in the track properties show lot's of spikes. But when I carry them in a pouch out on my large pack shoulder/chest strap the accuracy of the ascent is much better. I've compared this to older tracks of the same hikes back and including the early 2000 when I used an external antenna on my pack top.

Spiney
Posts: 24
Joined: Thu Apr 04, 2019 9:35 am

Re: Total ascent figures

Post by Spiney » Tue Jun 25, 2019 9:19 pm

That’s very interesting truckinguy. Does your Montana also have a barometric altimeter?

Also, please would you share how you are calibrating your 66st assuming you are using the altimeter-derived Elevation and not GPS Elevation. If you use autocalibration, how long do you leave the device to finish its calibration before setting off on your hike at the start?

truckinguy
Posts: 16
Joined: Mon May 27, 2019 11:37 am

Re: Total ascent figures

Post by truckinguy » Fri Jun 28, 2019 5:52 pm

Spiney wrote:
Tue Jun 25, 2019 9:19 pm
That’s very interesting truckinguy. Does your Montana also have a barometric altimeter?

Also, please would you share how you are calibrating your 66st assuming you are using the altimeter-derived Elevation and not GPS Elevation. If you use autocalibration, how long do you leave the device to finish its calibration before setting off on your hike at the start?
They are in default mode. Auto which uses both gps and barometer. I don't know why having the gps out front vs against my chest causes a different reading. Probably the hole for the barometer gets plugged with moisture from sweat? or just can't breath enough?
Remember a cold front or warm front etc coming in will change the barometer and change the altimeter as well fast. One can never get perfect ascent on these units. I've given up. And loading the tracks into 3 different programs give different readouts as well. Over the ground Distance is fairly accurate it's the ascent that will always be off. Just the nature of the beast. Google Altimeters,barometers,etc. You'll see why.

oldkingog
Posts: 38
Joined: Tue Jan 08, 2019 6:01 pm

Re: Total ascent figures

Post by oldkingog » Sat Jun 29, 2019 4:08 pm

truckinguy wrote:
Fri Jun 28, 2019 5:52 pm
Spiney wrote:
Tue Jun 25, 2019 9:19 pm
That’s very interesting truckinguy. Does your Montana also have a barometric altimeter?

Also, please would you share how you are calibrating your 66st assuming you are using the altimeter-derived Elevation and not GPS Elevation. If you use autocalibration, how long do you leave the device to finish its calibration before setting off on your hike at the start?
They are in default mode. Auto which uses both gps and barometer. I don't know why having the gps out front vs against my chest causes a different reading. Probably the hole for the barometer gets plugged with moisture from sweat? or just can't breath enough?
Remember a cold front or warm front etc coming in will change the barometer and change the altimeter as well fast. One can never get perfect ascent on these units. I've given up. And loading the tracks into 3 different programs give different readouts as well. Over the ground Distance is fairly accurate it's the ascent that will always be off. Just the nature of the beast. Google Altimeters,barometers,etc. You'll see why.
Just to clarify, do your elevation readings at a moment in time differ depending on location of the GPS or do the total elevation gain/loss readings differ? For me, it's not an issue of whether or not the altimeter is accurate, but rather that the ascent/descent totals are off, regardless of where I am carrying the unit.

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