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Tips for hand-building routes to use as arc files.

The arcs files used by GPSBabel's arc filter can be easily created with most popular mapping programs. The results, however, can be surprising until you realize that arcs are created from straight line segments. The drivers definition of "a straight line" and the mathematical one differ. If you're selecting a relatively narrow distance of the filter, it's worth a few extra moments to build a route (and thus, an arc) that accurately models your planned travel.

To highlight the problem and the solution, we'll build an arc from Indianapolis, IN to the north side of Fort Wayne, IN. This was chosen because "straight" on I-69 isn't really very straight at all.

The first and completely obvious approach is to build a straight line route. This approach is fine if you are traveling in a helicopter, but not so fine if your intent was to find waypoints near I-69. If you're filtering 100 miles from this route, you'd get the interstate, but you can see that if you're filtering 2 miles, you're going to miss that "bend".
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(Click image to enlarge.)
Let's add one point to include the bend. Now we hit the bend, but we missed out on the majority of the northbound part of I-69.
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(Click image to enlarge.)
Adding just a few points along the way lets us capture the basic shape of the interstate for use by the arc filter. If we're filtering, say, 2 miles from the arc, this will let us capture the waypoints without detailing every minute bend of the road. The narrower your catpure, the more precise the arc must be.
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(Click image to enlarge.)