Top speed did not register?

[+user13371]

Had a bike ride today that included some long, straight, smooth downhill stretches. A one point my eTrex 20 said i was going 41 MPH. But when I got home and saved the track, I checked the maximum speed for the trip and it said 34.. Way didn't it retain the higher speed?

[Grasscatcher]

"said 41 mph" ?

Was that on trip odometer page? If so, then the problem probably was / is the difference in the trackpoint logging interval.

 

Look at the speed data for individual trackpoints and see if there are any that show the 41 mph.

 

Maybe the "Max MPH" is an average over a short period of points and you possibly didn't maintain the 41 for long enough.

 

or....maybe the speed "pucker factor" kicked in and shut the blood supply to your eyes off, causing them to lie to you......(only kidding)

[+user13371]

Grasscatcher: On both the "max speed" field on the GPS trip computer page as well as what showed up in recorded track. At least based on what Motion-X GPS shows on the imported track. I've seen this before - current speed in one field and max speed on another on the trip computer page: I'll see my current speed higher than the "maximum" - but this was the largest discrepancy I've noticed. I think you're probably right that there's some issue with how long I was at a given speed for it to "count" as the maximum -- probably some filtering buit in to ignore what might be erroneous data.

 

But still -- I was only at 41 for a second or two, but I know I was in the high 30's for a good stretch.

 

Re "pucker factor" - um, yeah. This bike is the most stable (and fastest) I've ever had, but there are times when riding the brakes seems prudent. A different model from the same company has disk brakes and that might be my NEXT bike.

 

Edited July 14, 2013 by user13371

[Grasscatcher]

@13371...it would be interesting to see the track profile data as plotted by different softwares.

 

Do you have any "fitness sensors" like heart rate,cadence,etc? If so then you probably have Garmin Connect.See how it plots with that.

 

Then there's also Garmin Training Center, BaseCamp, and other free programs.

 

I have found that each will display the exact same data differently.......probably due to different filtering.

 

...or even export the track data in csv format into Excel and plot it there with no filtering.

 

By the way, Neat bike!

 

Remember the old timey chain guards on fat tire bikes? It would tale ONE HUNK of sheet metal to build one for that one! HA !

[+user13371]

No fitness sensors. But when I get to the office on Monday I'll probably import the track to Google Earth for a more detailed analysis. Don't have anything comparable on iPad at home.

[+Mineral2]

careful how you save and view tracks. Some programs will remove data from the track to make it a smaller data size. However, if the max speed on your GPS doesn't match what you were seeing live while riding, well, that's another issue. As far as I'm aware, the trip computer is independent of the tracks, sampling data at a much higher rate. Speed at any given point on a track is estimated by the distance between points and the time sampled between points, and not necessarily the speed recorded by the trip computer. On the other hand, the max speed should be directly sampled from the trip computer.

[+Walts Hunting]

I have also noticed a difference in total distance and ascent between the trip computer on my Montana and what Garmin Connection shows when i upload the track. Usually not enough to worry about but did think it strange. The bad part is the Website always seems to decrease both of them.

Edited July 15, 2013 by Walts Hunting

[Grasscatcher]

I have also noticed a difference in total distance and ascent between the trip computer on my Montana and what Garmin Connection shows when i upload the track. Usually not enough to worry about but did think it strange. The bad part is the Website always seems to decrease both of them.

 

That is to be expected. The trip Odometer samples at 1/sec. Even if your track log interval is also 1/sec, when track distance is displayed by Basecamp,Garmin Connect,etc, filtering tries to take out errors and anything removed shortens the track.

 

Many people think that logging tracks at an interval of 1/sec is the "only" and most accurate way.

WRONG ! Sometimes it may be the most accurate interval to "describe" the actual physical shape of the path traveled on the ground, but at the same time it nearly ALWAYS overstates the length and elevation (changes). (ascent/descent)

[39_Steps]

But still -- I was only at 41 for a second or two, but I know I was in the high 30's for a good stretch.

 

Re "pucker factor" - um, yeah. This bike is the most stable (and fastest) I've ever had, but there are times when riding the brakes seems prudent. A different model from the same company has disk brakes and that might be my NEXT bike.

 

On topic: At a cruise speed of 10 mph, on wide, level boulevards with no excessive tree cover to interfere with the gps signal, a Delorme PN-60 may produce a one point per second track that agrees very closely, with city road vector lengths as shown in Profile View and/or Routing Views in Delorme Topo9 computer app. That's the best case scenario in post processing.

 

Off Topic: That bicycle is intriguing, and by scaling the wheels and extrapolating the measurements it appears the OP is taller than I had imagined all these years.

[+user13371]

39_Steps:

 

Off topic, The intriguing bike is a Lightning Phantom II, small frame. Front wheel is 16". You guessed correctly that I am not tall - but rather long-legged for my height and can get both feet flat on the ground at a stop. I really want a P-38-XT - someday - which I would probably order in the medium frame size.

 

On topic, I don't know if failing to capture max speed happened immediately or in post processing. Garmin eTrex keeps a default "current track" which I always assumed was raw data, and when you save that track I think it does some data reduction; track smoothing, throwing out spurious points, etc. I fouled up because by habit I clear the current track whenever I save to a named file, so I lost the raw data. I'll NOT clear the current track on my next ride.

 

-Lee

Edited July 16, 2013 by user13371

[Grasscatcher]

@ 13371-Only on older models prior to Oregon (like 60,76,older e-trex) was there any data lost when you saved. On Montanas, Oregons, Dakotas,ETrex now complete data is saved, nothing is lost. It used to be that saved tracks were different than "active log" or current track. Not true now.

 

@ Yeah, but you still have to "put up with" DeLorme.........HA !

 

Under those ideal conditions even a "chalk and string" method will produce acceptable results. However when you get into real world conditions like at hiking speeds on a 1.88 mile (3.76 M RT) uphill hike with 20 hairpin turns and nearby rock faces for just a little added multipath effect, things get somewhat different. The more points logged, the more chances for error and it always gets added in to make track excessively long.

 

Prove it to yourself. On an intricate trail, carry multiple units at the same time and have one set to log at 1/sec and vary the others. Do the same hike again except , change which unit logs at 1/sec, do it again ........and you'll find that the 1/sec unit (whichever) will always log the longest distance and quite often will be longer than "actual."

[+Mineral2]

I'd expect that on a bike, you're going fast enough that the track log should be fairly accurate. In fact, I often have the opposite problem where the track log is up to half a mile shorter than the trip odometer which makes me wonder if the track distance is calculated in two dimensions while the trip odometer is calculated in 3 (Oregon 450). My biggest pet peeve is that the track still logs when I'm not moving, and that adds a bunch of extra distance as my estimated location jumps around within the error interval. I like that the new Oregon 600 series lets you easily turn tracking on and off, but I still wish that there was an option to stop tracking when the GPS isn't actually moving. The trip computer can detect this (stopped time vs. moving time), why can't the tracking do it also?

[Grasscatcher]

I'd expect that on a bike, you're going fast enough that the track log should be fairly accurate. In fact, I often have the opposite problem where the track log is up to half a mile shorter than the trip odometer which makes me wonder if the track distance is calculated in two dimensions while the trip odometer is calculated in 3 (Oregon 450). My biggest pet peeve is that the track still logs when I'm not moving, and that adds a bunch of extra distance as my estimated location jumps around within the error interval. I like that the new Oregon 600 series lets you easily turn tracking on and off, but I still wish that there was an option to stop tracking when the GPS isn't actually moving. The trip computer can detect this (stopped time vs. moving time), why can't the tracking do it also?

Explanations(IMO) in general order....

Track shorter...yes because Odometer is sampled at 1/sec and anything other for track logging will be shorter. I'm not sure if odometer sample data is reported in 3D or not. I have other software that definitely reports in 3D for track distance and, of course, it is always longer. Also, any roughness in track logging or elevation spikes result in added length.

Logging while stopped.....yes, because you have instructed it to log a trackpoint every X time or X dist....period. It does exactly that, "wherever" it "thinks" it is at that instant.

 

Change tracklogging to "distance" to reduce random points while stopped. ....but that will also cause different distance variations. Also there are 3rd party softwares that will easily "filter" track files to (mostly) eliminate those "birdnests" of points.

 

The new Montana also easily turns track logging off and on with a one button shortcut.

 

I agree, if the trip computer can detect "stopped", then the track logging interval could be internally adjusted accordingly. It would sure save a lot of track editing.