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Posted

I had used a normal cycling comp, with a magnet on the wheel and the GPS side by side, and I could not see a difference.

 

Maybe like 300 to 500 M over 100KM and the speed was pretty much the same (within point something of each other) while in the climb.

Posted

The priciple is fairly simple: the individual satelite transmits a timing signal. When this is picked up by the receiver, the time it took to get there is used to calculate distance from that satellite. Do this same calculation off another 3 satellites and the intersecting arcs of distance give you a reasonably precise position in 3 dimentions.

Since the satellites are pretty much co-incident in a plane as far as the GPS is concerned, the 3rd dimension (Z or height) is not very accurate. This is why the Garmins (for example) have the barometric altimeter. The height from the GPS is usually pretty cr@ppy...

Posted (edited)

Don't agree. Pythagoras statement l^2=x^2+y^2. to get l (actual distance) = SQRT(100,000m^2+1400m^2 ) = 100009.8m ... 100.0098km

 

Flame away

You forgot about the 1400m descent if you end where you started... ;)

 

That will DOUBLE the error! OMG!!!!

Edited by jmaccelari
Posted

If GPS is so accurate why do Garmin choose to use the wheel sensor above the GPS data on their new products?

The GPS does a lot of averaging and smoothing. Depending on the constellation and satellite visibility, errors can be up to 30m easily. The wheel sensor is calibrated by cross referencing with the measured GPS distance over a period of time so that these errors (hopefully) even out (if we assume the errors to be normally distributed, for example).

 

The "calibrated" wheel sensor is then used when satellite coverage is poor (going through a tunnel, under trees, ...).

Posted

The GPS does a lot of averaging and smoothing. Depending on the constellation and satellite visibility, errors can be up to 30m easily. The wheel sensor is calibrated by cross referencing with the measured GPS distance over a period of time so that these errors (hopefully) even out (if we assume the errors to be normally distributed, for example).

 

The "calibrated" wheel sensor is then used when satellite coverage is poor (going through a tunnel, under trees, ...).

 

The 310XT and as far as I know the Edge 500 only use the wheel sensor if it is present. Not only when sattelite signal is lost

Posted

The 310XT and as far as I know the Edge 500 only use the wheel sensor if it is present. Not only when sattelite signal is lost

I don't know how they work. If you set the wheel size manually, then it's quite logical. If the wheel size is set automatically by the unit (as with the 705, for example), then the GPS is required for the calibration on a on-going basis.

 

Once calibrated, the wheel sensor will be more accurate and less susceptible to GPS errors.

Posted (edited)

I don't know how they work. If you set the wheel size manually, then it's quite logical. If the wheel size is set automatically by the unit (as with the 705, for example), then the GPS is required for the calibration on a on-going basis.

 

Once calibrated, the wheel sensor will be more accurate and less susceptible to GPS errors.

 

 

Yes, as soon as the GPS has signal and you are moving it checks/calibrates the wheel circumference automatically but it only uses the wheel data for it's speed and distance calculations. (you can enter this size manually if you want)

 

I am having some trouble with the auto pause (shows I paused about 100 times in the 94.7) so I asked Garmin what is used for speed and distance measurement and this was their answer (I was trying to establish if it is a GPS fault or a speed/cadence sensor fault)

 

Thank you for contacting Garmin Europe.

If you have the cadence sensor paired up to the forerunner, then it will always try to take that information from the cadence sensor over GPS information.

Edited by chris_w_65
Posted (edited)

Yes, as soon as the GPS has signal and you are moving it checks/calibrates the wheel circumference automatically but it only uses the wheel data for it's speed and distance calculations.

 

I am having some trouble with the auto pause (shows I paused about 100 times in the 94.7) so I asked Garmin what is used for speed and distance measurement and this was their answer (I was trying to establish if it is a GPS fault or a speed/cadence sensor fault)

 

Thank you for contacting Garmin Europe.

If you have the cadence sensor paired up to the forerunner, then it will always try to take that information from the cadence sensor over GPS information.

 

 

Here is the speed info that my Garmin collected from the speed sensor for the 94.7. I only made 2x 40 second stops. The Garmin managed to "lose" 10 minutes of "moving time" with all the stops it recorded

 

I have changed the sensor but have been unable to test due to injury

post-18048-0-61796300-1293186830.jpg

Edited by chris_w_65
Posted

Here is the speed info that my Garmin collected from the speed sensor for the 94.7. I only made 2x 40 second stops. The Garmin managed to "lose" 10 minutes of "moving time" with all the stops it recorded

 

I have changed the sensor but have been unable to test due to injury

 

Get yourself a Tom Tom ...... people on the Hub try to convince others they are better than Garmin.

post-24-0-89791700-1293205629.jpg

Posted

Here is the speed info that my Garmin collected from the speed sensor for the 94.7. I only made 2x 40 second stops. The Garmin managed to "lose" 10 minutes of "moving time" with all the stops it recorded

 

I have changed the sensor but have been unable to test due to injury

Are you sure your sensor battery wasn't going flat? I have noted that compared to the Polar sensor, the Garmin one chows batteries big time.

Posted (edited)

That's just a snare...and a pretty poor one at that!

 

Yes you are right it is a poor one and you catch rabbits in a snare

Edited by Big H

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