We’ve just returned from a walk in the company of our U3A friends. These walks are fortnightly gentle saunters, ~5m/8kms, through some very pleasant countryside and, this spring, in some very pleasant weather. We don’t usually know exactly how long a walk is ‘cos measuring them is a little tricky. In very distant scouting days, I’d have used a 1 inchOrdnance Survey map and a mechanical wheel device with which to trace the route and approximate its length. Given all the twists and turns on a 1″/mile map, though, this was inevitably rather inaccurate.
Enter technology in the form of a Garmin eTrex h handheld GPS device. Apart from the archaic serial cable (see “GPS: Garmin Plain Silly”) required to download the eTrex’s information to a PC, a handheld device powered by two simple AA batteries that can track satellites sounds pretty high-tech, and I suppose in some ways it is. Mine even manages not to get lost under a fairly dense tree canopy – the “h” in eTrex h is something to do with high sensitivity. One companion, today’s leader, has an older eTrex (no “h”) which is constantly losing its signal under trees, so beware.
Looking for constructive uses for my toy, I thought it’d be interesting to measure some of our walks using Mr. Garmin. It’s also fun to download the route (through that archaic serial cable) – I use a piece of freeware called easyGPS – and subsequently display the route in Google Earth for added interest.
Given all the technology packed into satellite navigation systems and software development, you’d think that the resulting measurement of a walk would be spot on, would you not? Well, I would. After all, my Garmin nüvi satnav for the car tries to tell me precisely how fast I am travelling and “bongs” annoying warnings at me when I hit the speed limit, or, at least, the speed limit that it thinks exists which is frequently incorrect.
So, as we returned to our parked cars after a very enjoyable walk through some particularly fine countryside, several of our group chanted as one, “how far was that, John?” I pressed a few buttons to get the required answer from Mr. Garmin: 4.77 miles. Several walkers thought it felt longer but there you have it.
I got home (after a brief visit to a local hostelry) and downloaded the route using easyGPS: 5.23 miles.
I fired up Google Earth and sucked the saved .gpx file in to display the route on a simulated earth: 4.98 mile.
¿Que? That’s a variation of damn nearly ½ mile. Assuming that this walk must be approximately 5 miles, 0.5 mile represents a staggering 10% variation in all this wonderful technology. Since I still don’t know how long the walk actually was, I don’t know if I’ve got one accurate reading with two others having a 5% error or what. I do know that 10% over the speed limit can get you a ticket, though, so beware.
Maybe that old mechanical toothed measuring wheel on a 1” OS map wasn’t so bad after all. It certainly never broke down.
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