Plotting position

The tags capture a remarkable amount of information about where the fish has been while it was tagged. Once the clock on the tag is proved accurate, the latitude and longitude of the fish's position and movement can be established relatively accurately. Latitude is estimated by calculating the time of mid-day or mid-night, while variations in the length of the day are used to give the approximate latitude. This technique of fixing position using sunlight data is known as light-level based geolocation.   


Principal of Light-Level Based Geolocation

Where's the fish

Where would you be if you were a marlin and your tag was telling you that today was July 30, and sunrise was at 19:22 GMT and sunset 5:33 GMT? 

Midday at 00:31 GMT puts you on a longitude of 174.45E, the day's length of 10 hours and 11 minutes puts you at 36.51S.  That is to say, you are under the Auckland harbour bridge heading up the Waitemata Harbour and should probably turn around and head back out to sea. 

(Data for this exercise is taken from the time of sunrise and sunset on July 30th at Auckland - see www.rasnz.org.nz/SRSStimes.htm#July.)

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