In any maritime emergency where time elapses from the start of the incident to the arrival of rescue units it is necessary to establish the most probable survivor area. The procedure for establishing a most probable area is known as search area determination and requires the calculation of one position from another, known respectively as the datum point (DP) – the most probable position of survivors – and the last known position (LKP) – the incident position with recourse to the effects of tide and wind. Under the present manual methodology of the search and rescue planning system (USCG 1973), the DP is given an error treatment comprising drift initial position and search and rescue unit navigational (SRU) errors. Error calculations produce a search area radius (R) which is ‘boxed’ and referenced by four corner coordinates given in latitude and longitude.