APPENDIX 1: GLOSS REQUIREMENTS FOR GAUGES

The aim of any tide gauge recording should be to operate a gauge which is accurate to better than 1 cm at all times i.e. in all conditions of tide, waves, currents, weather etc. This requires dedicated attention to gauge maintenance and data quality control. In brief, the major requirements for GLOSS stations are (IOC, 1997):

(i) a sampling of sea level, averaged over a period long enough to avoid aliasing from waves, at intervals of typically 6 or 15 minutes, but in all circumstances the minimum sampling interval should be one hour;

(ii) gauge timing be compatible with level accuracy, which means a timing accuracy better than one minute (and in practice to seconds or better with electronic gauges);

(iii) measurements must be made relative to a fixed and permanent local Tide Gauge Bench Mark (TGBM). This should be connected to a number of Auxiliary Marks to guard against its movement or destruction. Connections between the TGBM and the gauge zero should be made to an accuracy of a few millimetres at regular intervals (e.g. annually);

(iv) the readings of individual sea levels should be made with a target accuracy of 10 mm;

(v) gauges should if possible be equipped for averaging and rapid sampling of waves, and should be also equipped for automatic data transmission to data centres in addition to recording on site;

and (vi) sea level measurements should be accompanied by observations of atmospheric pressure, and also winds and other environmental parameters, which are of direct relevance to the sea level data analysis.

Regular (e.g. daily) inspection of data will inform operators when a gauge is malfunctioning, and lead to overall better long term data sets. Data from gauges in polar or other remote locations will inevitably be inspected less frequently, unless satellite data transmission can be installed. Similarly, data from the relatively few gauges recording only on paper charts will be slow to reach centres for quality control; these must be considered priorities for upgrading to modern standards.

Operators of gauges must always be aware of possible systematic jumps in sea level time series when one form of recording is replaced by a 'better' one. All gauges have systematic errors, but those errors will be irrelevant for time series work if the same technique is used throughout. New technology gauges are, by definition, less well understood than old ones, and they must always be operated alongside the older ones until sufficient experience has been obtained.