Road Cycling It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle. -- Ernest Hemingway

Garmin GPS feedback

Old 04-08-05, 10:21 AM
  #26  
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decending at 25MPH focused on the GPS? Guess nature will take it's course and rid the gene pool of bike gadget freaks. I'm bad!
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Old 04-08-05, 12:49 PM
  #27  
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Originally Posted by lmoe
I just bought a Garmin Fortrex 201
I'm curious, how did you mount that to your bike? Or do you wear it on your wrist while riding? If so, is it difficult to keep it facing up?


Originally Posted by lmoe
1. Trees seem to block the satellites....so if I am descending a hill at 25 mph (my bike computer)....the Garmin may show my speed at 15 mph and the altimeter may not change at all. It does catch up on the elevation at the bottom of the hill, but it may take a minute or two.
Trees definitely impair the satellite signal, as does your body; the signals are quite weak. One thing you may want to try, is to set the display to show the signal strength and orientation of the sats and use that to adjust things. On many Garmin models, the "Satellite" page shows this, but IIRC you need to select something else on the Geko/Foretrex models... "Advanced skyview" or some such. Then, adjust the unit orientation and location to find the spot that gives you the best reception as you ride around. The flat part on the front of the unit, under the Garmin logo, should be kept as close to level and exposed to as much of the sky as you can manage.

You may also want to double-check that the GPS isn't in "battery saver" mode, which cuts back on the update rate. Regarding altitude, I believe your unit lacks a barometric altimeter; if that's the case, take its altitude readings with large grains of salt, particularly over short times. I've read that the vertical error on satellite measurements is something like 2.5X the horizontal error, and I've seen the satellite-reported altitude on my units vary over tens of feet with the unit just sitting still. Also, I believe the altitudes reported aren't relative to sea level, but to some other fancy things (ellipsoid? geoid?) which land-survey folks worry about more than I do.


Originally Posted by lmoe
2. What is bugging me, but without a clear understanding, is total ride average.
I compared the Garmin to two other cycle computers, each on a different bike. These cycle computers have been compared to other bicycle as well. All the cycle computers fall within the same overall ride averages, adjusting for riders who ride faster and wait, etc.

The Garmin seems to register .3 mph slower ride average than the slowest computer. I understand the the unit may need a few moments to re calculate, if it lost the satellites, but I would have expected it to be closer to the cycle computers than it is.
I've noticed the same thing. I suspect it comes down to two things:

1) The bike computers aren't really calibrated to the loaded rolling circumference of the measured wheel, which for a given wheel size varies based on the tire geometry, wear, inflation pressure, and load on that wheel. When I started using a GPS, I found that the speeds and distances reported by my bike computer were several percent high. Given that Garmin specs their GPS unit's speed accuracy to within 0.05m/s (~0.11mi/hr), in a steady state with a good sat fix, I figured I'd trust that over what my bike computer said for generic tire sizes.

I started correcting my bike computer's wheel circumference values based on feedback from the GPS, both speed and distance figures, over relatively straight trips with strong satellite signals:

circumference(new) = circumference(old) * (GPS_measured / comp_measured)

Things converged for me pretty quickly, measuring over steady, straight rides with good sat fixes. Once in a while I re-check the calibration, but now I trust the values reported by the bike computer over the GPS, especially with trees, curvy roads, and hills.

Using the same methodology, I've also found that car speedometers often vary quite a bit as well; I wish mine let me adjust the (assumed) wheel circumference.

2) The GPS counts more "moving time". GPS can be very sensitive to motion (I think it uses doppler shift on the satellite signals?); I've noticed my GPS counting up moving time as I fiddle around with my bike at stoplights, sensing the small side-to-side movements. Also, the GPS sometimes takes an extra second or two to notice I've stopped; it takes time to update its data based on the satellite fix (about once per second?), and it may be doing some smoothing. For instance, on a recent ride my Astrale reported 54m52s moving, while my Vista reported 55m14s, and it resulted in a reported moving average speed of 18.8mi/hr versus 19.0mi/hr.

Have fun with your new toy!

-JAB
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Old 04-08-05, 02:03 PM
  #28  
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You guys are incredible. I don't use not even half the features on my GPS and just the fact of not being lost is all I need to know. I like to add the following.

The first time you turn the unit on, it will take about 3 -4 minutes to find your location. After that, you can turn it off to save battery life and it will find the satalites much faster. My Garmin takes a beating on my handlebar but it works like a charm every time. It's indestructable. Unfortunately, I won't buy a new one until my existing Garmin brakes down which may be years and years from now!
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Old 05-06-05, 02:36 PM
  #29  
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I've got a question about the etrex:

While riding, the unit records the track and the elevation profile. At the end of the ride one can store the track. If you save the track, you can recall the track and profile at a later time or date.

So one could save all the tracks from a multi day ride with the elevation profiles.
Howver, does anyone know how to find what the total acsent and descent for a track is once it is stored? When you display the track, that information does not seem to be available.

many thanks,
 
Old 05-06-05, 03:33 PM
  #30  
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I use the eTrex Legend (grayscale). I turn it on and then pop it in my jersey pocket. When I get home I upload it to motionbased and see where I went.

The GPS did help me out once when we were sorta lost. We were going to go on one road (after we found it) but I looked at the gps and that route would have taken us an extra hour, which would have ment riding in the dark with no lights for an hour.
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Old 05-08-05, 01:12 AM
  #31  
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Originally Posted by rsw
While riding, the unit records the track and the elevation profile. At the end of the ride one can store the track. If you save the track, you can recall the track and profile at a later time or date.

So one could save all the tracks from a multi day ride with the elevation profiles.
Howver, does anyone know how to find what the total acsent and descent for a track is once it is stored? When you display the track, that information does not seem to be available.
Ah, track logging. I've done a bit of experimenting with this lately.

First off, a "track" is a sequence of points and optional timestamps. Each point has a latitude and longitude, as well as an optional altitude. It's like a long string of unnamed waypoints.

Second, on active vs. saved tracks: I've noticed that when you transfer the active track log to the computer ("ACTIVE LOG"), it has much more detail than track logs which have been "saved" using the GPSr's track-save function. When saving a track, the unit strips off timestamps, and then it drastically reduces the number of points through some sort of averaging. Because of this, if you want to do any detailed analysis, I recommend preserving an interesting track as the active log until you get a chance to transfer it to a computer; from there you can rename it. You can switch the track log off to keep later GPS activity from polluting the active track, if you need to use the unit before getting to a computer. Once you have the location and time information stored, you can do all kinds of analysis on it, and hard drive space is super cheap.

Third, on ascent/descent statistic collection: I believe that the GPS unit tracks ascent and descent statistics, or at least the extrema, somewhat independently of the track log. I base this on the observation that my will Vista continue to update these values after I've switched the satellite receiver off. Switching the receiver off stops the other trip statistics from accumulating, which keeps the timers from counting up while I'm trying to read them, changing my shirt, etc. Because of this, I make a mental note of the ascent/descent values as soon as I stop, and write those down ASAP before considering other numbers.

Now, to your question: how to view the total ascent and descent of a track. As far as I know, the unit provides no way to do this, nor does MapSouce. However, the necessary data is clearly stored in the track: if you double-click on a track in MapSource, you'll see the altitude numbers listed right there. You could run down the list with a calculator if you wanted, and calculate some ascent/descent numbers yourself. That'd get boring quickly, though, so we want to make the computer do it for us.

You may find that some third-party software will do this stuff for you; I know there are many GPS utilities for Windows available online (gpsbabel? GSAK? OziExplorer?), but I haven't tried any. Hopefully someone else can chime in on that. If there is a ready-made application which will calculate it for you, that's probably the best solution for most folks.

If you're feeling adventurous, you can get the computer to do the analysis yourself. First, export the track to a text file: in MapSource, double-click on your track log, which brings up the detailed view. Click on a single row in the table of track points, and then hit Control-A to select all points, then Control-C to copy them to the Windows clipboard. MapSource actually copies them as tab-delimited text.

Next, open a copy of WordPad, and push Control-V to paste in the waypoint data. Select File->Save, choose the plain text format, and save the waypoints in a file. Now your track has escaped the MapSource prison.

Now, we'll want something to run over those altitude numbers. If you've got a perl interpreter, this hopefully-not-too-obtuse script will do the job:

Code:
#!/usr/bin/perl -w
use strict;
my ($tot_asc, $tot_des, $prev_alt) = (0, 0, undef); 
while (<>) {
  my $alt = (split(/\t/))[2];
  $alt = ($alt =~ /^(\S+) (ft|m)/) ? $1 : undef;
  if (defined($alt) && defined($prev_alt)) { 
    my $delta = $alt - $prev_alt;
    if ($delta >= 0) { $tot_asc += $delta; } else { $tot_des += -$delta; }
  }
  if (defined($alt)) { $prev_alt = $alt; }
}
print "total ascent: $tot_asc, total descent: $tot_des\n";
Similar hackery is easily possible with awk, python, etc. Note that the ascent and descent values reported by this may differ from those reported on the GPSr's altimeter screen, since the unit may measure them more finely than what it reports in the track log. The values from the track log are still interesting, though.

If you're stuck without a convenient programming environment, you can also use a spreadsheet to analyze the text file. These steps worked for me, using OpenOffice Calc:

Open the text file we saved with WordPad, as type "Text CSV". Next, copy column C into a new sheet, pasting it in as column A. Then, do a find-and-replace; replacing all occurrences of " ft" with "". (Replace " m" if you're metric.) Now, column A has the raw altitude numbers. Next, select cell B2 and enter "=A2-A1" as the content. Next, select B2 and then with the mouse use the magic "auto fill" (by dragging the selection rectangle down) to expand column B from B2 down until it's as tall as column A. Now, column B holds the altitude deltas for the entire trip.

Next, for the ascent/descent accumulation. We'll use column C for ascent, column D for descent. Enter 0 in cells C1 and D1. Then, in cell C2 enter the formula "=C1+MAX(B2;0)", and in cell D2, enter the formula "=D1-MIN(B2;0)". Next, use the auto-fill magic thingy to expand from C2 down until the last row of data, and then from cell D2 down until the last row of data. At this point, the bottom cell of column C has the total ascent, and the bottom of column D is the total descent.

Simple, huh? I find the Perl script is simpler, myself. You could probably automate this with some spreadsheet template magic, but I'm no spreadsheet wiz.

Having to go use a windows machine to get the data out of MapSource in the first place with the copy-and-paste, instead of just using my laptop, got real old after about the first five times I did it. Personally, I use the freeware command-line application "gardump" from the "garmin-utils" package to download my tracks as text directly from my GPS via the serial port, and then I use scripts to do the analysis. Scripting is fun!

Hope this helped,

-JAB

Last edited by jab; 05-08-05 at 01:18 AM.
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Old 05-08-05, 02:23 AM
  #32  
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My Garmin 60CS tells me the total ascent on the altitude page.
As for the track, I export it to Excel for further analyzising and graph-making, I find this better and more versatile than the various programs I have tried. This is from a skiing trip this Easter:
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Old 05-08-05, 08:10 AM
  #33  
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Originally Posted by 240GL
My Garmin 60CS tells me the total ascent on the altitude page.
As for the track, I export it to Excel for further analyzising and graph-making, I find this better and more versatile than the various programs I have tried. This is from a skiing trip this Easter:
Excel!

You're a Norwegian stud!! I was just pasting them into MS Paint and editing from there, but that gave me no calculation ability. I'm good with Excel, too. Dunno' why I didn't think of that.

Takk, Dude!
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Old 05-08-05, 11:40 AM
  #34  
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Originally Posted by lmoe
1. Trees seem to block the satellites. ...
2. The Garmin seems to register .3 mph slower ride average than the slowest computer.
1. Hm. I put my 60CS in a saddle bag (read: under my, um, behind), and it never misses a fix. Does your unit allow connecting an external antenna? If so, get the cheap and excellent Gilsson and put it on your helmet. It is an amplifying antenna and will read a lot weaker signals than the internal antenna will.
2. The GPS will only register horizontal travel, so when going up- and downhill, GPS distances will be shorter and GPS speeds slower than the speedo readings. It all adds up for the total trip.


Originally Posted by neil0502
Takk, Dude!
Hey, that's pretty good!
Here's the how-to, sorry for my poor English: Cut and paste the tracklog. Use search and replace if you need to change the decimal point. Make a graph where height is one data set and distance from start as the other. Excel will now distribute the points equally along the X-axis: If your points are 1, 2, 4 and 8 meters from start, Excel will interpolate heights for 3, 5, 6 and 7 meters.

Erling.
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Old 05-08-05, 06:27 PM
  #35  
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Golly, there's lots of new stuff in the GPS world. I carry an old GPS-12, in a pouch on the belt of my fanny pack. I keep it with me just to see how far I"ve gone and to kinda sorta look at the altitude. How fast, couldn't care less.

If you have one of these older units, and want to spend about 40 bucks for the proprietary cable, you can down load any waypoints you save to any of the common mapping software like Street Atlas or Maptech. And while it won't give you the fancy color maps that the newer ones will, you can use the same software to plot a route and then upload it to the GPS, and then use the goto feature to see course and distance to your next turn or whatever.
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Old 05-08-05, 06:56 PM
  #36  
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I have an eTrex Venture, it's a cheap non-mapping model from a few years ago. Works great, long battery life, but I wish it would get better signal under forest tree canopy (for hiking/MTB use).
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Old 05-08-05, 06:58 PM
  #37  
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Originally Posted by 240GL
2. The GPS will only register horizontal travel, so when going up- and downhill, GPS distances will be shorter and GPS speeds slower than the speedo readings. It all adds up for the total trip.
Erling.
I have noticed there are some new models of GPS that measure cumulative altitude gain, they're marketed to mountain climbers and hikers... There's a Garmin, I forget the model, which also has a calibrated barometric altimeter in addition to its normal GPS altitude measuring abilities. eTrex Summit?
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Old 05-09-05, 09:04 AM
  #38  
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I have an eTrex legend (no color) that I use for hiking, Jeeping, and cycling... no problems with it yet for everything... I agree though that there are some tricks to getting onto the computer... On a side note, I've never had a problem with losing signals under trees or under the hardtop of the Heep...
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Old 05-09-05, 10:10 AM
  #39  
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Since I started this thread... I've bought (and love!) a Garmin GPSMAP 60C. Been very happy with it, and also with MotionBased.com
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Old 05-09-05, 01:48 PM
  #40  
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Originally Posted by 240GL
Here's the how-to, sorry for my poor English: Cut and paste the tracklog. Use search and replace if you need to change the decimal point. Make a graph where height is one data set and distance from start as the other. Excel will now distribute the points equally along the X-axis: If your points are 1, 2, 4 and 8 meters from start, Excel will interpolate heights for 3, 5, 6 and 7 meters.
Erling,

That worked beautifully! Here's the output from yesterday's ride, in Excel:

http://nbeener.com/050805_ALPINE.xls

Hyggelig treffe deg (isn't Google amazing?)
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Old 05-09-05, 02:07 PM
  #41  
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Great to hear - and see! - that it worked! If you import the data to PowerPoint as the next step, you can also add labels like names on peaks, crossroads, villages etc.; that's what I did with the graph above.

I should perhaps elaborate what I said about how the GPS measures distance and speed. The way I understand it, after reading postings on this topic on GPS boards, is that when you travel from A to B, the unit will measure the distance without taking the terrain in consideration. This will result in a difference between the data for distance (and average speed) when compared to the bike computer, which measured the actual distance including up/down. This does not contradict the fact that all GPS units will measure elevation. Units that rely on satelite readings only will be quite a bit less accurate for this purpose than those with a barometric pressure sensor.

Ha en god tur!
Erling.
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Old 05-09-05, 10:14 PM
  #42  
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Originally Posted by 240GL
I should perhaps elaborate what I said about how the GPS measures distance and speed. The way I understand it, after reading postings on this topic on GPS boards, is that when you travel from A to B, the unit will measure the distance without taking the terrain in consideration.
This is my understanding as well. Using my Garmin eTrex Vista, for example, if I set a waypoint at my current location, walk a few feet away, and edit the waypoint altitude up 5000', the "distance to waypoint" figure doesn't reflect the ~1mi vertical difference.

What I think is happening is that the GPSr is considering the shortest arc along the surface of the reference ellipsoid, or "geodesic distance". I think it also calculates altitude values relative to the ellipsoid, which only approximates the shape of the earth; the unit doesn't know enough to consider the height of the terrain, or how sea level deviates from the ellipsoid in different places. (I don't know that for a fact, I'm just guessing.)

So, in order to get distances between points on a track, I first convert the (lat,lon,alt) coordinates to (x,y,z) Earth-Centered, Earth-Fixed (ECEF) coordinates, and then take the euclidean distance between each pair. I then use that distance and the altitude change to calculate speed, slope, etc.

There are several web pages that go over this stuff in more detail; one page with some diagrams that I found useful is here, which is part of the publication, A guide to coordinate systems in Great Britain. While it sounds UK-centric, the first several chapters give an intro to the basic concepts; it helped me along.

Details on converting to ECEF are also online; Ask Dr. Math has an easy-to-follow example.

For the mathophiles in the house, Navigation on the Spheroidal Earth by Ed Williams goes into detail on equation derivations.

-JAB
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Old 05-12-05, 07:32 AM
  #43  
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" . . . . . Because of this, I make a mental note of the ascent/descent values as soon as I stop, and write those down ASAP before considering other numbers."

jab,

Thanks for the information. I am going on several tours this summer and thinking of taking my vista gps rather than my HAC4. The gps does all the travel info the HAC4 does, except it also keeps a daily record of the attitude climbed in a saved profile (which would be nice to review on a daily or entrie trip basis). I think the method I now use with the gps, the one you describe above, (writing down the daily totals on paper) will have to suffice since I will not have my laptop along.

Thanks for the help and I'll check out the your other recommendations for use when I have the l;aptop along.
 
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