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

D. Other SAR planning models, such as the Navy's NAVSAR package,
are available for determining search area and resource allocation.
Before use is made of such tools, their limitations and proper
application should be determined.
510 DATUM
The most probable location of the search object, corrected for
movement over time, is known as datum. Determining datum begins with
the reported position of the incident. Unless a distressed craft or
individual is immobilized, as in a boat grounding or debilitating
physical injury, the actual position of target during the search may be
substantially different from the initial position. Therefore, possible
movement of the search object should be counted for when calculating
datum. Datum should be recomputed periodically as drift forces continue
to affect the position of the target. Recomputed datums are usually
labeled sequentially (e.g., Datum1, Datum2, Datum3), with time of
calculation noted.
511 Initial Position
The location where the distress occurred is called the initial
position. To compute datum, the time and location of the search
object's last reliable position are first considered. This will
determine the type of datum to be computed. One of three situations
usually exists, based on the initial information obtained:
A. Position Known. The incident is witnessed or reported by radar
net, DF net, another craft, or the distressed craft itself, or position
is computed from a previously reliable position. If the position of the
incident is known, drift is determined and datum computed.
B. Track Known. The intended track is known but the position
along the track is unknown, or a single line of position, such as a DF
bearing, is obtained. If only the proposed track is known, a datum
line, a known proposed track corrected for drift, can be
5-2