Datum calculation method

There are two options for calculating the Datum position in this section of SearchWell - 

•Using condition forecasts and the Leeway table.  We use the wind speed and bearing and the current speed and bearing forecasts, and then we look at what type of casualty we are looking for and apply a formula using the wind and the current forecast so establish the direction and the speed of drift.  It is important to note that forecasts are not always accurate, and therefore, this method gives an educated guess at where the casualty might be at Datum time, but it is not necessarily accurate

•Using a Test Drift.  This is the more accurate method of calculating the Datum position, but it is best used if we are looking for the casualty soon after the incident happened.  Ideally, the rescue crew can deploy and observe the actual drift of an object with the following guidelines

oit must be as close to where LKP as is possible

oit must be as soon after the casualty was at the LKP as is possible

oit must be as similar to the casualty as is possible.  If we have access to an object that is similar, we can get a very good feel for the drift speed, but, if we cannot drift a similar object, any object will give a good idea of the drift direction, and we can estimate the drift speed.

When doing this test drift, it is important to record the start position and the end position accurately, and also to make sure that the time it was left to drift is accurate.  Typically, the object is observed for a short period of time eg 10 minutes, and the casualty might have been drifting for an hour or more.  This means that any inaccuracy in the recording of the test drift would be multiplied by 6 when using the information to calculate the total drift (there are 6 lots of 10 minutes in an hour).

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