Given a map, and characteristics of the roads connecting locations the problem is to find the shortest path between two locations. For those who are interested, the calculation being done is a “shortest path” calculation. Mostly, the two agree, but they differ frequently enough so that one should verify a downloaded route before using it. These are after all two different calculations, using similar but different implementations of the algorithms, which are running on different computers. The difficulty with calculated routes is that they do not always go where one wants them to go, and to add insult to injury, the calculated routes in the MapSource window on a PC, may differ from the calculated routes on the Garmin device. In fact, neither conclusion is correct, and both kinds of route are useful.

A calculated route is shown below: At first blush, it would seem that a calculated route would always be the best choice, and that a direct route, with its legs cutting across country, like a crow flies, would be of no use. A calculated route is a path that follows roads which are chosen by a computer program to meet certain criteria: thus a calculated route will go from “home” to “store” and should pass through “stop light,” but the path it takes, involving other roads, is the result of the criteria chosen, the algorithm involved, and the computer on which the program runs. A direct route is a set of straight lines on a map connecting “home” and “store,” while passing through “stop light.” A direct route is shown below.

If “home,” “stop light,” and “store” are waypoints with specific map locations, then a route is. To begin with, a “route” is an ordered sequence of “waypoints, “ and “waypoints” are specific locations on a map given by latitude and longitude coordinates.

I’m not sure what my initial mindset was anymore, but a key factor in understanding this stuff is the distinction between “direct routes,” and “calculated routes.” The Garmin manuals use words like “Follow Road,” “Off Road,” “Routable,” and “Non-routable,” to refer to this distinction, and to confuse the novice I suppose.

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