Current measurement problems in a circulation survey

"Operational" or 'circulation" surveys are carried out year-round by the National Ocean Survey on both coasts of the United States, including Alaska. Each survey completely covers a specific area, usually an estuary, for a period ranging from two months to several years, obtaining current measurements along with simultaneous tide, salinity, temperature, and weather data. These measurements are made at selected locations and depths in order to obtain a reasonably complete three-dimensional description of the dynamic properties of the body of water, that can be used for environmental purposes as well as for navigation. Analysis results of these data are also included in the Tide and Tidal Current Tables and Tidal Current Charts published by NOS. Formerly the Coast and Geodetic Survey, the National Ocean Survey has been taking current measurements on a regular basis since 1844 and has used a number of current measuring devices, from the early current pole to the present Aanderaa and TICUS current meters now being used on the West and East Coasts, respectively. NOS is presently reviewing available and prototype current sensors in preparation for an upgrading of its current measurement systems in the near future. In addition to the usual current measurement problems that affect all users (e.g., the effects of noise, mooring motion and drag, uncertain dynamic response characteristics of the sensors, accuracy, etc.), NOS must also cope with instrument errors that interrupt the processing scheme setup for the handling by technicians of the huge quantity of data it receives year-round. For example, in the past if a current sensor was not equipped with an independent interval counter or hour marker, the loss of one or more data points in a current record would create additional hours of work in order to accurately assign time to the data series. Time determination is critical for NOS since accurate tidal current predictions must be made based on these data. Also, frequent errors in the recorded data values, due to bit drop or other electronic or mechanical causes, though correctable using computerised statistical editing techniques, require considerable computer time to do so and occasionally hand editing as well. These problems and others, as well as NOS's on-board and in-house processing schemes, are described.