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GPS onboard

Old 09-14-05, 01:24 AM
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gudel
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GPS onboard

I bought a Garmin Legend, and picked up a case that happens to have a hook and handlebar velcro attachment. It has everything the computer does and then some. Seems pretty nifty, as the GPS can be put on and detached quickly with the outside case. Will try it out in the morning!
Some pics


This gives an idea how the GPS backlit display lights up in the dark
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Old 09-14-05, 03:14 AM
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Is velcro used to atach the gps unit to the handlebar??? I thought Garmin made a proper clamp bike holder system for the etrex series.
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Old 09-14-05, 04:01 AM
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I have a Garmin etrex vista and have the mounts (not velcro, clamp to handelbars) for my city bike, my MTB and my road bike. I also use it when driving. Its moving map is great for finding your way in unfamiliar terratory on or off the road. The unit is also useful for workouts as it gives speed and distance broken down in a varity of ways. I find the size to be perfect, any bigger and you wouldn't want it on a bike and any smaller and you wouldn't be able to read it (I can without my reading glasses). I would however have prefered the new color version as it makes it easier to tell the difference between a path you traveled over and a road (or trail) as they are different colors where as the B&W unit shows both as black lines.
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Old 09-14-05, 07:43 AM
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I use this for cycling and running:



It's a Timex Bodylink System (Chrono/Receiver, GPS Transciever, HRM Chest Strap Transmitter)--geared more for training than map functionality.
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Old 09-14-05, 08:36 AM
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Originally Posted by gudel
I bought a Garmin Legend, and picked up a case that happens to have a hook and handlebar velcro attachment. It has everything the computer does and then some. Seems pretty nifty, as the GPS can be put on and detached quickly with the outside case. Will try it out in the morning!
Some pics

Gudel,

How acurate is the speed when using the GPS?
I've been thinking about flipping mine on since it's in my pack anyway.
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Old 09-14-05, 09:26 AM
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I have a Legend and the bike mount for it. The current incarnation of the bike mount is OK. Word is that the first version was too loose, and the batteries would vibrate loose if you hit a bump, and the GPS would reset.
The speed and distance are spot on. I mean, how could they not be?
I only used mine a couple of times; I fine-tuned my bike computer with them (it was about 0.2 mi off in 10 miles, so I added the right percentage of mm to the tire circumference).
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Old 09-14-05, 11:00 AM
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I love my legend. Since I ride a bent it is a great solution to the wiring problem and I don't need to worry about sensors or anything else. Love the navigational aids too. Great to go out and do a century and not have to slow down and worry about looking at signs when you get close to turns.
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Old 09-14-05, 12:29 PM
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Windup, I think the speed is pretty accurate. I'd trust the GPS more than the bike computer. Two different computers I have, contain their own mm reference for the same tire size in the manuals.
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Old 09-14-05, 01:41 PM
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On a flat trajectory, the speed is as accurate as any device you can find. However, since a GPSr only measures latitude and longitude, when you go up or down hills, the speed and distance will be measured at less than what is actually traveled.
Many GPS units now have what is called "3D" mapping, which means that if you are locked onto four or more satellites, it will display your current altitude. Unfortunately, the unit's computer is not programmed to factor that into the speed and distance traveled.
That's what accounts for the difference between a device that measures the distance on the ground, (cyclometer) and one that measures the distance between two points, (GPSr).

(edit) - Added visual aid. The GPS sees all three of these as 1 mile. Clearly, each bike is not going to travel the same distance. Thus the speed will also be incorrect.
Full size graphic

Last edited by MMACH 5; 09-14-05 at 04:25 PM. Reason: typo
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Old 09-22-05, 08:38 PM
  #10  
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There have been several threads on Bike Forums regarding GPS; a "search titles only" search for "GPS" may prove informative.


Originally Posted by MMACH 5
On a flat trajectory, the speed is as accurate as any device you can find. However, since a GPSr only measures latitude and longitude
I'm no expert, but I recall reading that modern GPS receivers derive speed measurements from more than just successive lat/lon fixes. Perhaps they also consider the doppler shift of the signals? I'm not sure.

Regarding altitude in distance measurements, I have noted that if I create two waypoints with the same lat/lon coordinates but with altitudes differing by several thousand feet, my GPS units (Garmin handhelds) don't consider them to be very far apart. I guess that these units calculate great-circle or rhumb line distances between points based solely on lat/lon, neglecting altitude. I'm not sure offhand how their units accumulate trip odometer distances. When I really care, I post-process track logs myself and take altitude into account.

Happy trails,

JAB
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Old 09-22-05, 09:02 PM
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altitude is not a distance, it's elevation people
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Old 09-22-05, 09:11 PM
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There are many threads on GPS units as alternatives to cyclometers.

Some brief things to bear in mind, though: GPS units lose contact with satellites occasionally, whereas cylcometers keep constant measurements. GPS units tend not to tally the wiggle of your wheels as you ride-- the unit straightens out your path in calculating your distance so there is about a 1% error compared to a well-calibrated cyclometer. GPS units eat batteries. Cyclometers do not.
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Old 09-23-05, 05:29 PM
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Originally Posted by Merriwether
There are many threads on GPS units as alternatives to cyclometers.

Some brief things to bear in mind, though: GPS units lose contact with satellites occasionally, whereas cylcometers keep constant measurements. GPS units tend not to tally the wiggle of your wheels as you ride-- the unit straightens out your path in calculating your distance so there is about a 1% error compared to a well-calibrated cyclometer. GPS units eat batteries. Cyclometers do not.
Very true.

IMO there is not a big point of having a GPSr on the handlebars in case you're not interested strictly in navigational features. For speed a wireless cyclometer will do fine.

I also use a Legend, btw.
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Old 09-23-05, 06:19 PM
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If you're using a Garmin GPS, I'd recommend checking out Motion Based. It does appear to take elevation into account when calculating distance, and it also can use elevation data from topographic sources, instead of the GPS (Since GPS elevation data tends to be inaccurate).

I posted some more information about them on my bloghere. Almost all of their features seem to be free, so it's worth checking out I think if you're carting the GPS around anyway.
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Old 09-23-05, 07:54 PM
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I use a garmin ForeTrex 201 and it can display vertical speed. I find this one of the most useful measures while riding in that you can (at low speeds, ignoring wind and rolling resistance) equate vertical speed directly with your power output without a Power Tap or other torque measuring system. If you know your fully laden mass then you can infer your power to lift that mass against gravity. I know that, regardless of the slope of the road (within the gearing range of my bike) if I'm doing, say, 10 meters/minute vertical I puting out X-watts. (I forgot what 10m/min equals, but around 175 Watts I think). I know, also, that if I do 15m/min for very long I will get really tired!

Last edited by McHargue; 09-23-05 at 08:01 PM.
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Old 09-23-05, 07:55 PM
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I just spent 30 mins. charting a course for tomorrow that takes me to two bike shops. This includes marking up a map, picking the best bike friendly streets and making up cue sheets. I wonder if a GPS would've made the work faster and if I could tell it to avoid highways.

Then again, the course takes me through some rough neighborhoods so a GPS might attract more attention than I want.
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Old 09-26-05, 08:09 AM
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Originally Posted by gudel
altitude is not a distance, it's elevation people
However, when your line of travel takes you up hills and down into valleys, your distance is increased.
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Old 09-26-05, 09:10 AM
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Apparently, Garmin realizes the issues that come with measuring speed and actual distance traveled when using a GPSr.

Their new GPSr designed specifically for cycling uses a magnet on the spokes to measure speed rather than relying on satellite reception.

Garmin Edge Series
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Old 09-26-05, 11:46 AM
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For me the .1 of a mile difference over 100 miles (yes that is all the difference I found between my computer and the gps on a 252 mile ride) is not worth worrying about. Also the fact that not too many computers fit recumbents makes the not enough wire or wireless distance limitations a big issue. I like not having to worry about magnets and wires too.
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Old 09-26-05, 12:33 PM
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how often does it loose the signal??? can you still geta good signal with pretty dense tree cover?

i work with GIS for a living and need to get one of these for "testing" purposes! it would be awesome to map out all my routes but i know this is sometimes easier said than done, especially when you GPS looses it's signal easily... in college i mapped some mtb trails with a real highend backpack unit. it was a big pain in the @ss because everytime we went under any tree cover you had to wait an eternity for it to get a signal. this thing would have been uselss at bike speed. luckily we were hiking and could just stop and wait for it.
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Old 09-26-05, 12:43 PM
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It takes some pretty dense tree cover to hinder most modern GPS recievers. Now, going into an area with a bunch of high-rises will give you fits. Not only will you lose signal, but when you do have it, the accuracy is several hundred feet off.
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