Join Date: Jul 2008
Location: Austin, TX
Bikes: Giant OCR2, Bridgestone RB-T, Bike-E, Vision R-40, Novara Safari
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Originally Posted by mtalinm
I'm always suspicious that the GPS speed is a bit low
There's really no way for this [the GPS speed to be low] to be true unless somebody is dancing their bike around rather than riding in a sort of straight line.
The GPS measures speed by figuring out where you are, comparing that to where you were a bit ago, and dividing that by the time difference. Either the algorithm used is wrong (which would be odd, as it's very simple) or your location would get more and more and more off over time.
Ultimately, GPS speeds are extremely accurate. The only problems are if your path changes significantly between data points (every second?) or you go into places with no GPS signal (like inside.) They're not so accurate at calculating altitudes, however, which is why the 305 and 705 (but not 205 and 605) have barometric altimeters.
I think the speed sensor is there for tunnels and for use on a trainer. It's calibrated automatically by the GPS when outside, so if there is any error in the GPS-calculated speed, it would also happen with the sensor unless you jumped through some hoops.
The barometric altimeter in the 705 helps it keep more accurate climbing results. I'd suggest the 705 over the 605 if just for that. And besides, if you decide to sell it, the 705 will be easier to sell. (Does the 705 package you're looking at include the HRM and cadence sensors? If so, you can sell them to get rid of some of the price difference.)
Personally, I've not found the maps in my 705 to be that useful. Yes, it has them, and I've got them up to date, but the interface is horrid, and the cpu inside is extremely slow and so it takes forever to tell it to give you directions somewhere. My $100 or so handheld GPS is far far faster and easier to use to get directions from. If I think I'm going to need the map, I bring the handheld GPS just for that.