It is not always possible for the navigator to observe LOPs simultaneously. Sometimes only one landmark is available. The navigator may make frequent observations of the one landmark, or he may, after one observation, lose sight of the landmark only to sight a new navigational aid. If the navigator is able to compute distances during these observations, you can get your fix. If not, or if for any reason your LOPs are obtained at different times, then you can establish a position that only partially takes into account the current. This position is the running fix, identified by the same symbol as the fix except that the time label is followed by the abbreviation "R. FIX." It is better than a DR position, but less desirable than a fix. A running fix is established by advancing the first LOP in the direction of travel of the ship (the course), a distance equal to the nautical miles the ship should have traveled during the interval between the time of the first LOP and the time of the second LOP. The point of intersection of the first LOP as advanced and the second LOP is the running fix. The advanced LOP is labeled with the times of the two LOPs separated by a dash and the direction, above and below the line.
RUNNING FIX CONSIDERATIONS
The running fix may be a well-determined position and is usually considered so. For this reason the DR track is normally replotted using the running fix as a new point of origin.
However, a running fix does not fully account for current, and the displacement of the running fix from the DR is not a true indication of current. If a head current is expected, extra allowance should be made for clearance of dangers to be passed abeam, because the plot of running fixes based upon any single landmark near the beam will indicate the ship to be farther from that danger than it actually is. If a following current is experienced, then the opposite condition will exist. This occurs because the actual distance made good is less with a head current and greater with a following current than the distance the LOP is advanced based on dead reckoning. A limit of 30 minutes should be made on the elapsed time between LOPs in a running fix.
The running fix may be a well-determined position and is usually considered so. For this reason the DR track is normally replotted using the running fix as a new point of origin.
However, a running fix does not fully account for current, and the displacement of the running fix from the DR is not a true indication of current. If a head current is expected, extra allowance should be made for clearance of dangers to be passed abeam, because the plot of running fixes based upon any single landmark near the beam will indicate the ship to be farther from that danger than it actually is. If a following current is experienced, then the opposite condition will exist. This occurs because the actual distance made good is less with a head current and greater with a following current than the distance the LOP is advanced based on dead reckoning. A limit of 30 minutes should be made on the elapsed time between LOPs in a running fix.
