Upcoming rides I'm leading:
Saturday, November 19: Three Sisters and Wetlands Park, 36 miles

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Geek corner: Why don't posted mileages agree?

Dear Geek Corner: I looked up the Bikely description of a recent ride you posted, and it said the ride was 46 miles long. But when I printed your route sheet, it said the ride was 48 miles. What gives? Signed, Lance.

Dear Lance: Good question! One of the really neat things about Bikely is that you can draw a route by saying "follow the road," which tells Bikely to automatically follow the path of the road as described in the Google Maps data that the program uses.

But there's a problem with this. The map data doesn't exactly correspond to the actual roads. This happens because the map data is, by its nature, sampled at certain intervals, so minor fluctuations in a path are missed. This becomes noticeable when a road is especially curvy; for example, the east side of Old La Honda Road shows up as 3.0 miles in Bikely, but it's really about 3.3 to 3.4 miles.

When I make my route sheets, I try to get the actual on-the-ground distance as accurate as possible, usually by pre-riding the route myself. The difference is usually somewhere in the range of 1% to 2% over an entire route, but that's still enough to mean a difference of 2 miles in a 100-mile route. And I'm sure you'll agree that an error of 2 miles on a route sheet can make your day really frustrating, so I try to keep that from happening.

Of course, this all depends on your bike computer being properly configured with the circumference for the tires you're using, and even pesky little things like your tire pressure affect the circumference and, therefore, how far your computer thinks you've traveled. It's a very inexact science.

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