Datum Calculation

The Datum Position answers the question "Where is the casualty likely to be when we are able to look for it?"

This section is the "heart" of SearchWell and probably of Maritime Search and Rescue, and it is a relatively complicated section for a user as SearchWell offers a lot of options.

Basically, to calculate a Datum position we need to know

•LKP: Where the casualty was last known to be and the time it was there

•Time at Datum: The time we will be in a position to look for the casualty.  This is described under the heading "Time at Datum"

•Drift: The information we will use to predict the drift speed and direction.  This is described under the heading "Method of Calculation"

In SearchWell there are two methods of calculating the Datum position using - 

•forecast conditions - in this case we calculate the position of the casualty at the end of every hour of drift until the Datum time using the forecast conditions for the area and the Leeway table to describe the casualty type

•test drift - in this case we calculate the one position at Datum time using the results of a short drift in the area of the LKP to give us a guide

Then, there is a lot of information that is displayed once the calculation has been done

•Forecasts:  If the Conditions and Leeway table option is selected we display a graphic of what the wind and current forecasts were for each hour of the drift, and what their influence on the casualty drift is (see "Forecast Output")

•Test Drift information:  This information will summarise how the casualty has drifted (see "Test Drift Information")

•Probability: There are a number of files that are created by SearchWell that can be imported into Google Earth that give a visual understanding of where the casualty might be (see "Probability Output")

•Search Box: The coordinates of the corners of various search box areas (see "Search Boxes")

Reverse Datum

There are times that we need to know where a casualty has come from eg we find an abandoned vessel floating at sea and we want to get an idea of where it came form.

In this case your LKP date and time will be later than the Datum date and time.  SearchWell then calculates the "reverse" drift

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