Standard Depth for Marine Measurements:
Oceanographic measurements have traditionally been taken at standard ("nominal") depths, based originally on the length of wire cable that was let out from the ship to suspend the instrument or the sampling device. The word "nominal" means that it was always well known that the label placed on the sample was probably inaccurate due to the angle of the wire in the presents of currents. Thus, a "100-m" sample could have been taken at only 99 m or 98 m, for example. Only during processing of the reversing thermometer data (or from physical measurement of the wire angle) was a final, correct depth assigned to the data. Nevertheless, oceanographers have always attempted to make their measurements at a set of standard depths, selected to capture the salient features of the stratified sea.
Other factors, including the move toward extensive use of electronic sensors (especially the CTD and its ancillary sensors), as well as the move toward use of pressure instead of depth as the Z-variable in ocean datasets, make the story of standard depths more complicated. We focus in OceanTeacher on the use of meters (as used in the World Ocean Database). These levels are often used in atlases as the depths of the variables to be analyzed.
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