Position Explanation


The position attached to the in situ sea surface temperature measurement is calculated by linear interpolation from the data supplied by the vessel towing the CPR. For each CPR tow the officers of the towing vessel supply a log sheet recording as a minimum the time, date and position of the deployment and recovery of the CPR. We also request that they record changes of course during the tow. Some vessels are extremely scrupulous and record their position every 3 or 6 hours whether they change direction or not. Other vessels only record the deployment and recovery positions perhaps 450 miles and 36 hours apart and we assume the course and speed has not changed in that period.

We have calculated the position of the in situ sea surface temperature by linear interpolation of the data supplied on the log sheets. These data are presumably noted by the deck officers, from the ship's GPS. We can test the accuracy of our calculated positions by comparison with certain vessels for which we have a continuous record of the GPS positions.

For example, the vessel that towed CPR route 470ZC, also has a thermo-salinograph and the data is logged at ICES. We have compared the positions at hourly intervals between 0700h GMT 22/11/2001 and 1800h 23/11/2001 for the calculated position attached to the in situ data and the recorded GPS position. We calculated the difference between these two simultaneous estimates of position. The average difference in position is 4.5 nautical miles (N=36) with the smallest error occurring at the beginning and end of the tow and the greatest error (12.1nm) about midway along the tow.