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

known, or a yacht is on an extended coastal or ocean cruise.
The datum area may be readily apparent. In other cases, the
area can be narrowed by communications checks and deduction;
if not, large general areas may have to be searched.
512 Computation of Datum
A. Drift is movement of a search object caused by external forces.
Datum is calculated by determining which drift forces will affect the
search object, selecting the most appropriate ones, and calculating a
vector for each. The vectors are then added to the initial position to
determine datum. Drift forces should be calculated using the time
between the last known position and a time selected by the SMC. This
time of datum is normally chosen to coincide with the daily maximum
search effort, using midsearch time or the first SRU arrival time.
B. The SMC should determine which environmental forces affected
the search object during and after the incident:
1. For marine incidents, currents and winds.
2. For aircraft, primarily wind.
3. For lost persons, terrain and meteorological conditions.
C. The SMC should attempt to quantify each force affecting drift,
which is best done by vector, with bearing and length of the vector
representing target direction and speed respectively. Since objects
that float or fly are more affected by environmental forces, it is
easier to quantify their possible movement. Lost persons, while
affected by the environment, may choose or not choose to move, or may
move unpredictably.
D. Surface drift forces that act on the target are plotted as
shown in Figure 5-3. Since all drift elements are acting simultaneously
on the object, the path is along the resultant vector. Lengths of
vectors are measured in units of distance. Drift speed information is
converted to distance covered in a given time period. To determine
force vectors:
1. Determine speed of the environmental force.
2. Convert force speed to object speed.
3. Multiply object speed by duration of drift to determine vector
length.
4. Determine force direction.
5. Convert force direction to object direction.
6. Add all object vectors to derive the resultant motion vector
of the object.