GPS Query

[Klaus]

Just looking at the possibility of a GPS unit. I noticed that the more advanced one (Garmin 705) offer an extra sensor for speed and cadence. I can understand the cadence bit but the unit itself calculates the speed already, current, average, maximum. Is there a difference between measuring the speed from the wheel sensor and direct by the GPS? Just wondering. Also is there software in the public domain for downloadiung data other then myGarmin for example? Thanks for your advice!

 

[DrSquirrel]

GPS speed is not accurate, bugger what anyone says.

Wheel speed is much more accurate if the wheel diameter is measured properly.

Sadly the unit doesn't RECORD the wheel speed, it only puts in the GPS co-ords so speed is calculated at a later date via GPS.


There is a browser plugin, garmin communicator which many sites use.
You can export from the unit directly using GPSBabel and a few others.
Various applications can work directly with the Garmin units, Memory Map and My Sports Tracks are 2 that come to mind etc.

I export mine using the old Garmin Training Centre PC app just to save the raw files, and also use a website (Endomondo) to log (semi) publicly my workouts.

 

[CopperBrompton]

Garmin units calculate speed by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Garmin's specifications quote 0.1mph accuracy which is good enough for me.

 

[HLaB]

I think the speed unit is only used by a garmin to fill in the blank when gps signal was poor. Ive got the speed/cadence sensor on one bike but use the gps unit on another bike too without it. I used to run the stand alone gps and a bike computer on another bike and pretty consistently found the computer 0.1mph faster but that doesn't concern me, so on the current bike I just run the standalone gps. Ive never had a problem with gps readings but most of my rides are rural or in Dunfermline or Edinburgh where there are no tall buildings.

 

[Klaus]

I think the speed unit is only used by a garmin to fill in the blank when gps signal was poor. Ive got the speed/cadence sensor on one bike but use the gps unit on another bike too without it. I used to run the stand alone gps and a bike computer on another bike and pretty consistently found the computer 0.1mph faster but that doesn't concern me, so on the current bike I just run the standalone gps. Ive never had a problem with gps readings but most of my rides are rural or in Dunfermline or Edinburgh where there are no tall buildings.

Thanks for this and the other replies - I am not really fussed about the kind of differences that have been mentioned. Certainlyif I get a GPS I'd like to have as few bits and pieces to install/maintain as possible. At the moment I use a £1.75 bike computer - it gives me good indication of speed and distance which is quite accurate compared to what the car odo says on the same stretches. I do fancy the map capabilities of GPS though.

 

[fossyant]

I think the speed unit is only used by a garmin to fill in the blank when gps signal was poor.

That's certainly one of the uses - just don't upgrade to 3.2 - it auto pauses as you go under trees etc. Ended up going back to 3.1 after the machine system reset it's self a few weeks back........