Edge 500 speed sensor

the mayor said:

Ok...so the 705...if I'm reading the other posts correct...will be inacurate off road because of hills and tight turns? I thought the sensor would fix that? ( or is that just with the 500?)
I did see another thread where I believe you said you used software to correct the mileage/altitude variances.

D'ooh! I've got to strap it on my bike and just do a ride and figure it out....I'm sure I'm over thinking the whole thing.

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Not inaccurate, in my experience.

Elevation is tough, since you are moving nearer and farther from the satellites 22,000 miles up there. A 100 foot up and down is pretty small from that far away. Barometric altimeters (705 has it) will provide the best accuracy, but there will still be drift. That is why uploading to a good desktop map program is essential, and make sure it uses DEM (Digital Elevation Modeling) to adjust your track against known elevation points.

The other essential is calibration to a known elevation that the mapping handhelds can preform. The 705 had this function added two software updates back:

* Added ability to use waypoint elevation as the starting elevation for an activity when the user starts within approximately 30m of this waypoint.

I have a 'home' waypoint for my house, since I ride from home most often. I also have waypoints for each trail head, and all these have the elevation set using Mapsource (back to those GPS File Depot tutoials ).

The other thing is enable one second recording in the 705. To give you a good idea, here is a track from TopoFusion, through a switchbacked area. Note the trail you can see, and note that it is 24-36 INCH wide singletrack (it is zoomed to close it is blurry). I think this is very good!



Now to use the external speed sensor with the 705, you have to turn off GPS, so you will get speed and distance data, but no elevation, and no track, or location or anything to upload. External speed sensors are great on a stationary trainer, or maybe on an indoor velodrome, but I see no use outside in an mountain bike environment. If you are in exceedingly dense deep forest cover, like a rain forest, then the GPS will use the external sensor when signal is lost, but not until.

Let me show this quote from the manual again.

Cadence data from the GSC 10 is always recorded.

The speed data from the GSC 10 is only recorded and used for distance calculation when the GPS signal is weak or the GPS is turned off.

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Parking garages and freeway underpasses are the only places my 705 looses signal.

And just to be clear, there is error inherent in EVERY method used for a cyclocomputer because wheel circumference varies according to air pressure in your tire. No tire pump is going to give you accurately repeatable inflation pressure EVERY time. It'll get you within 1-2PSI of where you want to be, but that will introduce error. Furthermore, my experience is that most people just use a preset wheel/tire size setting instead of the rollout method, and that will increase the error, also, because any experienced rider will know that not all tires "labeled" as the same size are actually going to be the same size, and that is going to be different depending on the width of your rim.

IMO, the errors inherent in GPS for use as a cyclocomputer make it no worse than a traditional cyclocomputer, firmware bugs aside. One thing that gives the GPS a leg up is that it doesn't rely on that extra transmitter. Lots of those little transmitters for traditional cyclocomputers do not transmit powerfully enough for mountain bikes and a lot of them don't work well on mtb's.