Datum is the geographical reference for the area to be searched.
The following factors should be considered:
- Reported position and time of the SAR incident
- Any supplementary information such as DF bearings or sightings
- Time interval between the incident and the arrival of SAR facilities
- Estimated surface movements of the distressed craft or survival craft, depending on drift
- Drift distance is drift speed multiplied by the time interval between the incident time, or time of the last computed datum, and the commence search time
- Datum position is found by moving from the incident position, or last computed datum position, the drift distance in the drift direction and plotting the resulting position on a suitable chart.