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isochrone Isochrone (equal time) is a line on a map marking all points that are reachable in a certain time from a common starting point. In sailboat routing, we have a polar diagram that tells us how fast the boat goes in given true wind speed as a function of the true wind angle (TWA), being the angle between the heading of the boat and the true wind direction. The isochrone is compiled by choosing a first heading (maybe toward the destination) and then figuring that TWA, finding that speed, and then plotting a point that distance from the starting point. Next do the same thing for a heading say 5º to the right of that, and get a new point, and then go on around the compass getting a new point every 5º, or in smaller or larger steps. Then at each point along that isochrone, do it again to get another isochrone, an hour out from that one, and so on. Then with these all computed, the program needs to use some logic to find the fastest route from start to finish. In principle, the first part is straight forward, whereas there are various mathematical and logical arguments about how to do the second step best. See also reverse isochrone.

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