//Trail Status mouse over Java // North tower loop mileage

Author Topic: North tower loop mileage  (Read 6599 times)

NateC

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North tower loop mileage
« on: October 24, 2014, 07:46:15 AM »
I know BCSP map says  N. Tower Loop trail is 3.5 miles. Is that the true mileage or have  any of you gps'd anything different? I ask because I have a Bontrager Trip 1 computer which read 4.2 last time. I know that is wrong and would like to know  what your gps based systems are saying so I know what I am shooting for as I make adjustments.
I ride a Rumblefish with 29x2.3 and have tried several wheel size settings.

MikeVK

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Re: North tower loop mileage
« Reply #1 on: October 24, 2014, 10:03:39 AM »
GPS will almost always under-report mileage on trail.  The combination of tree cover and tight turns tends to fool it.


The best solution I've found for accurate readings is truing my bike computer to a gps on straight roads with no tree cover and then riding a trail.

mtbikernate

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Re: North tower loop mileage
« Reply #2 on: October 24, 2014, 02:13:50 PM »
It is over 4 miles in length, just a hair short of Green Valley.  They are very close, but GV is longer.


Tree cover has very little to do with under-reporting of GPS distance measurements.  It has to do with the curviness of the trail and recording interval of the GPS.  It is not being fooled.  It simply is that the recording interval is insufficient to capture the complete corner.  The shorter the recording interval, the worse it is.  Also, GPS does not capture the tangent of a slope when ascending or descending.  It is simply measuring the horizontal distance.  You can do calculations later to generate the tangent of a slope, but the GPS does not do them on its own.


To capture a trail's distance the most accurately with a gps, you need a VERY high frequency and you need to be walking so your tracks have a very high point density.


1Hz is usually the best you can get on most devices.  If you're riding, that can still give you a pretty good distance between points when going downhill.  At slower speeds, it will be more accurate.  Until we see recording intervals approaching 10Hz or 100Hz on consumer gear, GPS will be significantly short.

nelliedawg82

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Re: North tower loop mileage
« Reply #3 on: October 24, 2014, 02:40:34 PM »
Interesting info Nate.


So you walk the trails you map for the MTB Project?

mtbikernate

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Re: North tower loop mileage
« Reply #4 on: October 24, 2014, 09:32:52 PM »
If I was focusing on a single trail system I might. But i am working on lots and don't have the time to walk everything. I usually ride the trail a few times and then hand draw the files I actually upload to mtbproject.  Right now I have some very high quality trail data from the dnr that WAS walked (and I think with survey grade gps hardware to boot-the point density is insane) so I mostly focus on photos and written descriptions now. And the trails on my plate now are further from home and take more work to list.

zzSQL

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Re: North tower loop mileage
« Reply #5 on: October 24, 2014, 10:48:22 PM »
There's a nice strava segment for North Tower Loop that reports 4.2 miles.

http://www.strava.com/segments/2054549

That is a really fun circuit.

NateC

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Re: North tower loop mileage
« Reply #6 on: October 24, 2014, 11:30:23 PM »
Good. Maybe I have it set right finally! I showed 4.2, but the map from park had me second guessing it. I will leave it for now. Thanks all

Webby

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Re: North tower loop mileage
« Reply #7 on: October 25, 2014, 05:27:54 PM »
Very interesting Nate,,  You done went and EDUCATED me. I too thought it was tree coverage and such. Very interesting !!  Thanks,,,

It is over 4 miles in length, just a hair short of Green Valley.  They are very close, but GV is longer.


Tree cover has very little to do with under-reporting of GPS distance measurements.  It has to do with the curviness of the trail and recording interval of the GPS.  It is not being fooled.  It simply is that the recording interval is insufficient to capture the complete corner.  The shorter the recording interval, the worse it is.  Also, GPS does not capture the tangent of a slope when ascending or descending.  It is simply measuring the horizontal distance.  You can do calculations later to generate the tangent of a slope, but the GPS does not do them on its own.


To capture a trail's distance the most accurately with a gps, you need a VERY high frequency and you need to be walking so your tracks have a very high point density.


1Hz is usually the best you can get on most devices.  If you're riding, that can still give you a pretty good distance between points when going downhill.  At slower speeds, it will be more accurate.  Until we see recording intervals approaching 10Hz or 100Hz on consumer gear, GPS will be significantly short.
Happy Happy Happy ------- Trails that is !!!

Dan71

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Re: North tower loop mileage
« Reply #8 on: October 25, 2014, 08:46:44 PM »
It is over 4 miles in length, just a hair short of Green Valley.  They are very close, but GV is longer.


Tree cover has very little to do with under-reporting of GPS distance measurements.  It has to do with the curviness of the trail and recording interval of the GPS.  It is not being fooled.  It simply is that the recording interval is insufficient to capture the complete corner.  The shorter the recording interval, the worse it is.  Also, GPS does not capture the tangent of a slope when ascending or descending.  It is simply measuring the horizontal distance.  You can do calculations later to generate the tangent of a slope, but the GPS does not do them on its own.


To capture a trail's distance the most accurately with a gps, you need a VERY high frequency and you need to be walking so your tracks have a very high point density.


1Hz is usually the best you can get on most devices.  If you're riding, that can still give you a pretty good distance between points when going downhill.  At slower speeds, it will be more accurate.  Until we see recording intervals approaching 10Hz or 100Hz on consumer gear, GPS will be significantly short.


I understand everything except you mention the shorter the recording interval, the worse it gets.  shouldn't it be the longer the recording interval worse it gets?  (Accuracy of distance measured).   Maybe I do not understand recording interval but I think that is amount of time between measurements.  If it was shorter time it measure a turn more accurate that longer time between measurements.






mtbikernate

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Re: North tower loop mileage
« Reply #9 on: October 26, 2014, 10:13:12 PM »
You are correct. I misspoke. Greater time between measurements results in even more error at speed. All I can figure is that I had that word in my head related to something else I was thinking about and I wrote it instead of the correct one




"Smart" recording is even worse at speed, IMO.  I don't know what makes it so bad but it is worse than it should be.

jnogan

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Re: North tower loop mileage
« Reply #10 on: October 30, 2014, 01:02:30 AM »
I download (or is upload) my Garmin 500 data to Garmin and Strava.  Strava consistently reports higher mileage than Garmin.  But from what you are saying Nate, it may be more accurate?

djkouza

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Re: North tower loop mileage
« Reply #11 on: October 30, 2014, 07:44:07 AM »
I download (or is upload) my Garmin 500 data to Garmin and Strava.  Strava consistently reports higher mileage than Garmin.  But from what you are saying Nate, it may be more accurate?
If your using the same data in both places the only way to account for a difference in reported distance would be if Strava was doing calculations based on elevation change to give the more accurate distance (see Nate's above comment on horizontal vs 3D distances)


FWIW a quick google got me a link to this http://www.pushthelim.it/   posted about two weeks ago on another board.  It has 10Hz, and a slew of other features.  Wonder how well made these will be.   I have a 10hz bluetooth GPS moduel that I used with an App for tracking lap times, but haven't figured out how to make strava or anything else use it.  (I'm too lazy to use another app, then d/l data and re-upload :)  )
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jnogan

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Re: North tower loop mileage
« Reply #12 on: October 30, 2014, 11:53:23 AM »
If your using the same data in both places the only way to account for a difference in reported distance would be if Strava was doing calculations based on elevation change to give the more accurate distance (see Nate's above comment on horizontal vs 3D distances)

My Strava mileage is always higher.  That they calculate "terrain following" distances makes sense.  If 4.2 miles is correct for the North Tower Loop, Garmin Connect comes in around 3.6-3.7 usually.  Strava also usually reports a higher top speed than Garmin.  That makes sense too because my top speed comes on a downhill.

I'm a casual and not frequent enough rider and I ride at BCSP only.  I use my data as a gauge to make sure I'm not regressing.  I use Strava segments as "encouragement".  My usual ride is about 7.5 miles on Garmin.  The few times I've gone a little bit longer, closer to 9 miles, Strava is still about 0.6 miles longer.  This is, by no means, a statistically analysis. 

I'm a gear head so I'm just curious what others are finding, if anything?

mtbikernate

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Re: North tower loop mileage
« Reply #13 on: October 30, 2014, 04:23:30 PM »
I HIGHLY doubt that ANY website calculates "terrain following" distances.  Most likely, IMO, is that each site has a different cutoff for what constitutes "stopped time" or "paused".  For my last ride, FYI, Garmin Connect and Strava are identical.  Garmin reports 14.69 and Strava reports 14.7.


The time differences, however, are significant.


Garmin:

2:42:42  Elapsed Time
2:08:51  Moving Time

Strava:
2:42:33  Elapsed Time
2:14:03  Moving Time
Topofusion does more calculations.  It appears to calculate distance following the terrain.  For example, it reports my distance for the SAME ride at 15.40mi.  But I'm not 100% sure, because it also is VERY different WRT timing.  TF says Elapsed Time was 2:42:33 and Moving Time was 2:38:09 so that may account for the nearly extra mile.  I wouldn't know for sure unless I also had a regular cyclocomputer doing a rollout.
« Last Edit: October 31, 2014, 01:19:32 PM by Nate Hawkins »

mtbikernate

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Re: North tower loop mileage
« Reply #14 on: October 30, 2014, 04:27:00 PM »
FWIW a quick google got me a link to this http://www.pushthelim.it/   posted about two weeks ago on another board.  It has 10Hz, and a slew of other features.  Wonder how well made these will be.   I have a 10hz bluetooth GPS moduel that I used with an App for tracking lap times, but haven't figured out how to make strava or anything else use it.  (I'm too lazy to use another app, then d/l data and re-upload :)  )


I just became aware of this device.  Looks like all aluminum housing, so the possibility is there for a really solid device.  I asked the developer a bunch of questions on mtbr today, so we'll see.  Color me intrigued, though.  Increasing sampling rate holds back GPS accuracy when measuring linear features like trails.  Slow speed straight line measuring is easy, but add speed and sinuosity and accuracy drops unless you increase sampling rate.

 

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