Join Date: Nov 2008
Location: Gagetown, New Brunswick
Bikes: Cervelo S1, Norco Faze 1 SL, Surly Big Dummy
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I've been using an Edge 705 on my bikes now for a couple of years now.
There's a couple of things you have to understand about GPS and mountain biking:
1. The accuracy of a GPS depends on the number of satellites the unit can "see" and a few other things. The bottom line is that the precision of your fix is usually in the 3-5m range. That means that when the GPS says you are at a specific point, you are really somewhere within a radius of whatever the fix accuracy is. You could be right on it, but maybe not.
2. Your average singletrack is well less than 1 m wide - it's more like 0.3m wide.
3. The update frequency is on the order of 60 Hz - meaning that you get a new position update once per second.
If you are really trucking along, it is entirely possible for you to pass through some squiggly bits of the track faster than the GPS can update, and/or the amount of "squiggle" falls within the fix accuracy error radius. When you look at the recorded track, this shows as a "smoothing" of the track.
The same problem occurs with roads, but the software in the GPS has a few things going for it:
1. Roads (and road maps) are very accuractely surveyed. Accordingly, the position of the roads on the electronic maps are very accurate;
2. Roads tend to be very wide - at least two cars wide in most cases; and
3. Cars are almost always driven on roads; road bikes are mostly driven on roads.
Accordingly, there is an option on most GPS units called "Follow Roads" or some sort, which tells the software in the unit to assume that it is driven on a road, and so if the position is off-road, correct the position over to the nearest road.
When used in a mountain biking context, the GPS has none of these crutches availible to it, and so you get the errors.
I have dozens of track maps of rides that I have done, all on the same trail, and none of them are identical. They are *similar* (after all, I was riding over the same bit of ground) but not *identical*. And the one that is the most accurate is the one in which I rode very slowly specifically to minimize the errors.
Accordingly, it is not really feasible to produce mountain biking trail maps that are "routable" like the road networks are. Yes, it can be done and kinda works, but never as seamlessly as a road map.
That being said, it can be very useful to having mapping on a mountain bike, especially if you are in an unfamilliar place. Although you won't be able to "follow a trail" with turn-by-turn directions and all that, you CAN see your location (with high precision) and where you have been (with reasonable precision) which, coupled to terrain features and a (paper) trail map, can do a good job of orienting yourself.
To make that work, you need Topo maps. The Topo maps have the ground contours on them (unlike the road maps) which are absolutely essential for orienting yourself to the ground. They also record water features to a higher level of detail and may have "blacktracks" (aka "double track") on them as well. They also tend to have other key features like water towers, power lines, and so forth.
Sadly, they don't have treelines like the paper maps do, but otherwise, they carry the same level of detail that a military map does, plus street names.
What they don't have are Points of Interest (so you can't tell it to route you to the nearest Tim Hortons)
The roads ARE routeable, so you can use the Topo maps to auto-route you to a known address.
Get the Topo maps and you'll be happy.