Southern Kettles Mileage Check
So yesterday I did the big loop - john muir blue to connector to emma carlin orange loop and back to blue and got ~30 miles on my Garmin 500. Last year I was getting ~ 28 miles for that loop and I know there have been some subtle trail changes but nothing that equates to 2 miles in my head.
Anyways, I'm just curious if anyone else has a recent mileage for that big loop. I have the speed sensor option for the Garmin and I'm just trying to get a sense for accuracy.
Checking my Forerunner I came up with 28.87 on the same loop @ 2:36:16.
My experience has been that this Forerunner comes up a little short on milage when compared to advertised race lengths and other GPS's.
I think all GPSs will understate the mileage. They have to be able to compensate for variability in satellite readings. My assumption is they have algorithms to deal with location changes (especially brief direction changes) that are within the accuracy tolerances of the signal at the time. So when you do a lot of brief direction changes (like all those zigs and zags out there) they "smooth" them out into more a trendline. I don't know for certain they do this, but my experience with readings from GPSs and my knowledge of how programmers tend to deal with these types of thing makes me think this is likely.
OP mentioned the Garmin speed sensor, which I believe acts like a cyclocomputer (tracks wheel revolutions and calculates against wheel/tire circumference). If this is the case, you need to make sure you know that it is using this rather than the GPS signals for the distance measurement. AND you need to make sure you have it calibrated correctly. I don't use cyclocomputers on my trail bikes anymore, but when I used to, I found you needed to take tire pressure and tire deflection into account if you wanted to be really accurate. I run low pressure on the trails and if I calibrate at the tire circumference without taking deflection into account, it overstates the mileage. The best way to test your cyclocomputer is to find a long stretch of flat straight rails-trail, and ride a good 5 or 10 miles at the same tire pressure you run on the trails, and compare to mileage markers on the trail and/or a gps. GPSs are pretty accurate when you don't have frequent direction changes.
All that said, 30 miles sounds reasonable to me.
Warning: may contain sarcasm and/or crap made up in an attempt to feel important.