When two or more LOPs are taken at different times, their intersection does not constitute a fix because the boat moved some distance between the times of the observations. To compensate for this movement, the LOPs must be adjusted to a common time by using the speed of the vessel multiplied by the time between the LOP and the fix time. For two LOPs, the usual method is to adjust the first LOP to the time of the last LOP by moving it down course for the distance traveled. This new advanced LOP is plotted parallel to the old LOP and labeled with both the time of the initial observation and the time to which it was advanced. These two times are separated by a dash. For example you take a bearing on a buoy that is slightly ahead of the vessel at 1103 and then take a beam bearing on a tower at 1115. The time between LOPs is 12 minutes, and for a boat speed of six knots, you would advance the first LOP for a distance of 1.2 nm (D= S x T / 60, or 6 x 12 / 60 = 1.2) down the track and plot it parallel to itself or just plot it at the true bearing initially observed.
The advanced LOP would be labeled 1103-1115 on top of the LOP and the magnetic bearing observed is annotated underneath the LOP (see illustration).
There are as many different ways of taking LOPs and combining them as there are ways of tying a knot. A bearing LOP can be advanced to a distance-off LOP just as easily as to a depth sounding or celestial LOP. The important point to remember is that you need to advance the older LOP to the time of the newest LOP no matter the source of the LOPs.
All the bearings you take are usually magnetic and you must convert them to true bearings to plot them. Although you can use the magnetic compass rose for plotting, it is standard practice to convert to and plot true bearings. Do you remember the memory aid "Can Dead Men Vote Twice?" The first letter of each word is used to convert from Compass to Magnetic by applying Deviation and then Variation to arrive at True. Did you also recall the "East is Least and West is Best" mnemonic for applying the correct plus or minus value to west or east variation and deviation? These plus or minus values are also applied when going from true to compass, but are reversed when converting from compass to true.
It is possible and easy to take a series of bearings on the same object and thus obtain what is called a running fix (R Fix). For the best accuracy, these relative bearings are taken when the object is approximately 45, 90, and 135 degrees from the boat. You then either advance the first two LOPs to the time of the last LOP, or you retard the last two LOPs to the time of the first LOP. An alternate method is to use the time of the beam bearing as the fix time and then advance the first LOP and retard the last LOP to that time. Usually the first method of advancing the first two LOPs to the time of the last one is the preferred method.
Sometimes it is necessary to change course between LOPs, and the advancement of the first LOP will thus be a bit different, but this is relatively easy to do. Simply draw a dashed line between the two DRs (you will need to plot a DR position for the time of each bearing) and advance the first LOP along this dashed line for the time of advance. Again, this procedure is explained in detail in the sources mentioned above.
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