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Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
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A line on a weather map joining places of equal pressure
Drawn on synoptic charts at fixed intervals of atmospheric pressure reduced to mean sea level, most commonly every 4 hectopascals, though 2 or 5 hPa spacings are also used. One hectopascal equals one millibar. Isobar spacing is the primary visual cue to wind strength, while their curvature indicates cyclonic or anticyclonic flow, with winds blowing nearly parallel to them rather than across them.
Read definition →Drawing lines showing how far the boat can get each period
The classic routeing algorithm advances the vessel from its position along a fan of headings for a fixed time step, commonly six or twelve hours, using forecast wind and the performance polar. The outer envelope of reachable points forms an isochrone, and the process repeats from points on that curve. The optimum track is recovered by tracing back from the first isochrone that reaches the destination, giving the least-time route.
Read definition →Lines on a map joining places with the same magnetic variation
Isogonic lines connect points of equal variation and are printed on magnetic variation charts and in GIS models; agonic lines are the special case where variation is zero, so magnetic and true north coincide. An agonic line currently runs through parts of the eastern United States and another through Europe and Africa, both moving steadily. Following isogonics on a routeing chart shows how much the variation correction will change over an ocean passage.
Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.