JP 3-50 National Search and Rescue Manual Vol I - US Navy

established.
1. The proposed track is first plotted, and a series of DR
positions are computed for estimated progress along the track.
The DR positions at each end of the track and turning points
along the track are used. If the track legs are long,
intermediate positions should be computed.
2. A DR position is recommended for every 5 degrees of latitude
or longitude for aircraft, at least each 24 hours on the track
of a marine craft, and at least every 4 hours on the track of
lost persons in inland areas.
3. Each DR position is considered as a known position and drift
is computed for each position up to a common single time.
Thus, a series of datum points is developed. All datum points
are sequentially connected by straight lines to form a datum
line.
C. General Area Known. Neither the position nor the intended
track is known, but the general area the target was probably in, such as
a lake, a military exercise area, or an offshore fishing ground, is
known. In this case, a datum area is developed. Datum area
computations depend on many factors, such as fuel endurance, natural
boundaries, and known or suspected areas of occupancy. Datum area
computations may be reasonably exact, or only a best guess.
Figure 5-1. Aircraft Glide Area