Originally Posted by TheRCF
Yeah, but I just like to be able to say confidently that I did a century, or a metric century, or a half-century. I'm constantly pushing to reach at least 63 miles just to be absolutely sure I did a metric century. I've only done 3 centuries in four years, but I have done 13 metric centuries or further just since mid-december.
I develop airspace and we deal with map accuracy quite often. Here's what I know.
We consider a standard USGS 1:24000 map to have an accuracy of +/- 40' horizontally from any plotted point. The digital map data you get is generally based off something that's based off the same data a printed map is. But for airspace planning where a shift of 5-10 feet can mean a huge difference in the way an airplane arrives to an airport, you don't get any more accurate than the 1:24,000 USGS maps.
USGS digital elevation models, which I imagine support most of the digital data being used, is accurate at the +/- 13m level for airspace planning. All this to say, you're certainly more likely to be more accurate on your own cyclometer or GPS than what you can get from a map.
I use programs called ExpertGPS and GPS Trackmaker to do a lot of my route planning, with fantastic zoom capabilities. They provide me accuracy down to around 1-2km difference from what I bike when I "point and click" a line from start to finish along a properly calibrated digital image of a digital map. I'm doing a new century route tomorrow based on one such "point and clicked" route. I'll post back how close the GPS/cyclometer (which I've gotten within .03km on a measured course 40km) are to the point and click method.
VW