Off-course navigation
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Off course navigation is used to account for unknown variables in course to allow you to find your destination. Principally used prior to the introduction of electronic navigation it allows one to find one's destination despite the unknown factors - generally - the accuracy of the course steered or the drift to one side introduced by current or wind.
Simply put - instead of aiming directly for your destination you deliberately aim off to one side of it so that once you have travelled the correct distance towards the destination - your target must lie to a known side. For example - aim right and your destination must lie to the left. The choice of side to aim at depends on a number of factors. In a sailing vessel one might aim to be upwind or uptide side of your destination so that you can sail the easier downwind or downtide course to your destination. In an aircraft where the strength of wind might be hard to forecast but the direction can be determined by dropping drift markers you would aim upwind so that you could fly the easier downwind leg to find your target.
Hazards lying one side of your destination might also be a determining factor in your choice of side to aim for as you would aim for the safe side and then navigate towards the danger - while keeping a good lookout for the known danger. You need to have a reasonable way of estimating the distance run along your course so you know when to turn towards the destination.
Prior to the accurate determination of longitude it was common to sail north or south till one found the latitude of your destination (a relatively simple affair using a sextant or octant and with no need for accurate time) and to then sail directly east or west till one encountered one's destination at the known latitude. This method meant you did not even need to know how far you had sailed - as long as you could see a heavenly body in clear skies and so determine how far north or south you were. For example - in sailing vessels trying to find the entrance to the English Channel - you sailed North to the correct latitude mid-Atlantic and then "ran your easting down" (towards your destination) in the prevailing Westerly winds that pushed you East.
The method is somewhat subjective - relying on a mixture of experience and knowledge of one's vessel/aircraft in order to determine the amount to "aim off" and so be sure that the destination is definitely to the one (known) side. It is also conservative and means that in order to ensure safety you are likely to spend longer on your passage with a greater need for food/water stores or fuel to make your final destination.
The method was often used by helicopter pilots during the Vietnam War to find ground targets. With standard dead reckoning techniques, the idea is to precalculate the course correction due to the expected wind. However, because this is based on some estimation of the actual wind, when the aircraft is supposed to be over the target, any discrepancy could put the aircraft anywhere within a circle of error around the target, causing the pilots to have to seek out the target. Instead, by using off-course navigation, the pilot flies the track heading without allowing for the wind. Then, after the appropriate timed distance has been flown, the target will always be directly upwind of the aircraft's actual position, making it much faster and easier to find.
With the introduction of cheap navigation systems based on satellites that can pinpoint your position to within centimeters the method has largely fallen into disuse.
