Anyway, I took the opportunity to run an experiment. I have two GPSs, an Edge 500 and an Edge 705. After owning the 705 for several years now, I finally got around to mounting this cadence/speed sensor doohickey that came with it. It works wirelessly via Ant+ Sport. In theory, this should greatly improve distance measuring when riding off-road, plus keep better track of elapsed moving time. Also on the bike with the 705, I still had a wired Cateye cyclocomputer. Thus if Aaron and I stayed together, we should measure the same distance on three different devices, two GPSs and one wired computer.
We pretty much rode all of the singletrack GB has to offer, including Stone Rowe towards the end. Aaron bombed right down the rock chute no problem, although the numerous rock step-ups on the way there gave him grief.
The experiment came out as expected. The 705 with wheel pick-up sensor and wired computer (also with magnet sensor) were within 0.6% of each other for time and distance. The 500, on the bike I rode, fell nearly 12% behind in distance. The GB trails aren't super dense either. The 500 was relying purely on satellite signal to measure traveled distance.
GPSs are really good at only one thing: placing your latitude and longitude position on planet Earth. A few meters out of 40,000km is 0.00001% accuracy. They are much less accurate in determining elevation. You'd think distance measuring would be pretty accurate, but GPS vendors use algorithms to reduce data storage and smooth out noise that may be present in the data. This can result in severely undermeasuring travelled distance in certain cases.
When traveling on mostly straight roads, GPSs can reach 1% accuracy, on par with car odometers and wired cycle computers. In tight, twisty terrain, I've seen errors as high as 30%, where the GPS always comes up short.
That is why GPS companies offer auxiliary wheel sensors, as well as optional barometric altimeters to improve elevation measurement. The wheel sensor replaces rounded off, noisy motion data from satellite signals with counted tire revolutions on dirt.
You'd think with all our technological marvels, that things like measuring distance would monotonically improve with time. But no. Many things in the realm of humans don't behave that way. Take, for example, music reproduction. We started with wax cylinders, then vinyl records. 8-track and cassette tapes were a step backwards, but they did not displace vinyl. Then came along audio CDs. They replaced everything up until that point. Some argued sound reproduced from a compact disk was inferior to a quality pressed vinyl record. I would tend to agree, although vinyl records quickly degraded with playings. In most play-back scenarios, CDs were superior. Then came the real digital revolution: MP3 players. Music reproduction quality took a huge setback. You see, people's attitudes changed. They wanted quantity over quality. 1000's of songs on a tiny, cheap device? You betcha. It didn't matter if the music was compressed beyond recognizability. That is the primary reason I still do not own a MP3 player. Memory is getting cheap enough now, and there are decent lossless compression algorithms out there, so I may jump in at some point, some 10-15 years after these things became hot.
And so it goes with measuring sporting endeavors. The chronograph was invented in the 1800's. It had accuracy to about 5 seconds per day, or 0.006%, and resolution to 1/100th of a second. Say you were running a marked mile on a track. This chronograph from nearly a century and a half ago could time your mile to within a tiny fraction of a percent. It was a purely mechanical device.
Growing up in the early 70's, I recall having a mechanical odometer on my bike. It mounted on the fork, and a spoke stud struck a little cog on the odometer. It was pre-calibrated for a 27" wheel. I bet it was accurate to within 1-2%. Very simple, very reliable. These device actually came into existence about the same time the bicycle did in the late 1800's, like this example below. Each tick is a hundredth of a mile!
In my generation, miniature electronic timing devices arrived on the scene. Timing accuracy improved by a couple orders of magnitude over mechanical movement chronographs. In addition, you could record distance and time in a single device and have it compute average speed. Carefully calibrating these devices to your tire size could net distance measurement accuracies to about 0.5%.
Now we're in the world of GPS receivers. Wired and wireless cyclocomputers are going bye-bye. Most riders who use metrics now use GPS units. Distance and moving time are accepted as accurate, which is sometimes true on the road. Those who ride or run off-road, have learned just how inaccurate these devices can be.
We've entered an era where metric gathering accuracy on average has degraded. It seems the other features of these new techno-marvels outweigh the loss in accuracy. We can record our tracks and upload them to Strava, where you are ranked against other riders' equally suspect tracks. Music fans wanted selection of tunes over quality of reproduction in MP3 players, and now cyclists prefer suspect GPS data for Strava entertainment over benchmark quality data. Maybe it is the case that music fans never really cared about "hi-fi" in the first place, and cleaver marketing tricked them into thinking it was important for a while. Perhaps cyclists never really cared about accurate average speeds or distances they rode. They were the only metrics we could gather and talk about at the time. There was no easy way to automatically capture other metrics like VAM unil GPSs and the web came along. That created a bigger sparkle in our eyes even though accuracy became suspect.
Not sure where things will go from here. Maybe memory will become so ubiquitous that music will no longer have to be compressed. Or maybe compression algorithms will become so good that even snooty music fans like myself will not be able to discern imperfections. Will music fans start pushing for quality sounding music over quantity? This will need to happen to drive the market.
Likewise with GPS technology. What will it take to improve accuracy for applications the system was never intended (original and still primary use is military)? A new, commercial generation of satellites with much tighter timing accuracy, and more of them so there are always a few satellites almost directly overhead? This would require a hefty subscription fee from millions of users to fund it. GPS receiver manufacturers would have to improve their algorithms too. You can bet the US DoD will not improve their Global Positioning Satellite system for spandex clad whining cyclists, or even let the commercial world tap into the higher levels of accuracy the current system offers. We are lucky enough the military makes an unclassified signal available at all. For all we know, the next generation system may already be up there.