Digitising tide curves
To avoid having to manually digitise tidal curves to obtain electronic versions, direct input systems have been developed.
Digitising necessarily involves encoding the analogue signal, and this can be done in various ways, one of the safest being to optically code the rotation of a disc fixed to the pulley driven by the float. Another widely used but probably less reliable system uses a potentiometer whose voltage measured at the cursor is converted into frequency.
These systems are then backed up with either a magnetic tape recording (the perforated tape used at one time was abandoned), or transmission of the measurement over the airwaves or via the telephone network to a remote recorder. All these refinements require additional skills, and experience has shown that in the event of a breakdown, the chart recording is needed to recover the data.
The tide curve is usually digitised by recording the height every hour on the hour, a frequency that is usually sufficient for tidal analyses. In centres that process many tide curves, this work is facilitated by the use of a digitising table where an operator enters each height into the computer.
The records must be accompanied by a tide gauge verification sheet that lists, at given times, the heights read by the operator or by the tide staff or the water level indicator tape.
A significant source of error arises if the verification sheets they have not been filled out with due care.
Another problem with manual or semi-automatic digitising of tide curves is the boring and unrewarding nature of this work, which can lead to a lack of motivation, which in turn generates errors.
NUNIEAU software digitises paper tide curves and limnographs. It was developed at CETE Méditerranée/Service Hydraulique.
NUNIEAU is a valuable tool that uses modern image processing technologies. It replaces the conventional digitising tables to considerable benefit. Its many advantages include the ability to obtain water levels at a far higher frequency than once per hour for an equivalent amount of preparation and digitising work.
To find out more:
- Simon B. (2007). La Marée - La marée océanique et côtière. Edition Institut océanographique, 434pp.
Last updated: 12/12/2012