This note is a brief explanation of how GPSs work, how they interact with digital maps and how they create and use tracks. It's education rather than training - you'll need to translate this into the detail of your specific GPS, as they're all different. But the basics are always the same - if you get these right you'll always know where you are, even if the GPS can't tell you where you're going!
There are three parts of the GPS system, which are combined in the blob of plastic on your handlebars. They are:
(i) The GPS receiver;
(ii) The digital map; and,
(iii) The Routing Calculations.
The GPS Receiver
When you first switch your GPS on the receiver begins to listen to timing signals transmitted by satellites. There are two satellite systems, the US GPS system and the Russian GLONASS system. Many modern GPS will listen to both, which is a good thing as signal strength can vary.
The GPS starts with at least four satellites, and can use twelve or more to get a good position fix. To do this it first synchronises its internal clock with the (very accurate) satellite time signals, then jiggles around until everything lines up. The time to do this varies from a few seconds to a minute or so, but it is essential to let it finish. Once it has, it knows where you are, and it does this accurately and reliably. It knows where you are, and this knowledge is expressed as latitude and longitude. Some GPS - particularly military ones - can display this. Most cycling ones don't, as it makes little sense to the target customer.
But it does know where you are, to within a few feet.
The Digital Map
For this to make sense to the person using the GPS you need a map. One way to do it is to read the latitude and longitude off the GPS and translate them to a paper map - it's a very reliable way to do it. But it's tricky while you're riding, so the cycle GPS systems incorporate a digital map. These are pretty sophisticated, but the quality of them varies. If you have the skill and enthusiasm you can change the map in your GPS for something that suits you better, but almost everyone takes what they're given.
The digital map consists of a conventional picture map, with every point linked to its latitude and longitude. So once your GPS has worked out where it is, it can call up the digital map and show you that point. It shows you where you are in terms that make sense to you - it's a map, but, on your bike, a very little map.
As well as the digital picture and plethora of latitude and longitude points, the digital map also contains loads of information about roads, one-way streets and so on. These can be used to provide directions from where you are to somewhere you want to be. This is nothing to do with the GPS part of the device, and it's the part that gives most trouble. Why? Well, a self-contained GPS is fairly little and feeble, in computing terms, and this is the worst bit of the device on your handlebars.
As an aside, smartphones can often do a better job at this part, because they can download a lot of information from the internet and use it effectively. But only when they can get an internet connection ...
The Routing Calculations
This is where it usually goes wrong. Routes are either supplied to the GPS by loading in a file that you have made, or have copied from Strava or some-such, or they are calculated by the GPS using its own maps and stored information. So if you ask your GPS for a route to, say, Scunthorpe, it will calculate one. Depending on its digital map and sophistication this route will be more or less good. One that you have prepared separately will almost certainly be better.
There are a number of different ways to represent these routes, driven by the more sophisticated and powerful device that people are used to in cars. The more sophisticated the route description the prettier the pictures and directions the worse it works on a bicycle GPS. The simpler you keep things, the happier it, and you, will be.
Here's an example of a bit of a GPS route:
If you look closely you'll see it consists mostly of a string of latitude and longitude points. There are thousands of them. The route consists of going from one to the next, in order, and the GPS draws you a picture of this, adding in extra information like 'Turn Right Soon' and even 'You've Missed Your Turn'.
In this case, it will usually try to get you back on route. This is a bad thing, as it is feeble at it and is trying to do it while you are moving, which makes a bad job worse. Your car can do it, your phone may be able to do it, your bicycle GPS may well mess it up. This is where the trouble usually starts.
Giving It The Best Chance
Accepting that the bicycle GPS is a device with limited capability, the best results come from asking it to do as little as possible - here's what I do:
(i) Switch it on and give it time to find its position. Mostly they beep or give a message to say that they've done it. Then you know where you are. Don't do anything else until you do, or you'll be wrong from the start.
(ii) Prepare your route before your ride and load it in to the GPS. Use the simplest description to discourage the GPS routing software from sophisticated displays and calculations - it's not up to it. The example above is in '.GPX' and 'Track' format - see the 'trkpt' underlined in red? It's the simplest - always choose this.
(iii) Turn off 'recalculation' in the GPS. It's bad at it. If you miss a turn, sort it out yourself. It's usually easy enough to do. In extreme cases stop, zoom out the map until you can see your route, then ride to it.
I've done ten of thousands of miles using this simple system, and it rarely goes wrong. But, if it does ...
What To Do If It All Goes Wrong
Stop.
(i) Stop the GPS from following its route. This is either a cross on the screen or a menu option - best to look for it before you need it.
(ii) Switch the GPS off and leave it a few seconds.
(iii) Switch it back on, wait for it to work out its position, and you will know where you are.
(iv) Re-load your route, if it's a Garmin decline the kind offer to navigate back to the start.
(v) If necessary, zoom out the map so that you can see the route - it may already be obvious, or you may know where it is because you know where you are now. Ride to the route.
(vi) The GPS will continue as if nothing had happened ...
It would be nice if things were better than this, and who knows, perhaps one day they will be. Working with the kit we've got, this works.
Other Paths to Success
This is not the only way to get satisfactory results from your bicycle GPS. They are sophisticated and complex devices, and you may get a different view from someone else that works perfectly well - perhaps better. I would only caution that you listen to the whole thing - a bit of my minimalism and a bit of sophistication from another (younger) person may not combine well.
Happy trails ...
Mark
