Contributing data to the PSMSL

The Permanent Service appreciates the contributions from all organisations supplying mean sea level data and does not seek to impose unnecessary conditions upon contributors. Nevertheless a minimum of quality control must be exercised if the data bank is to be an authoritative reference. To this end the PSMSL requests the following information together with each set of monthly and annual mean sea level values supplied:

the units used (metres, rarely feet),
a statement of the datum to which the values refer,
a statement of the measured depth of that datum below the primary tide gauge bench mark (TGBM),
an indication of incomplete or deduced data (see below),
the number of observations per day used to calculate the monthly means,
any information of changes in datums, bench marks or relevant procedures since the previous batch of data,
any information on the availability of more frequent readings (e.g. hourly heights).

Although data will be accepted in any format, mean heights should preferably be in the metric system to the nearest millimetre, and the datum to which the means refer should preferably be the tide gauge zero. Data will be gratefully received in any form (e.g. as paper tabulations or digital formats).

Treatment of incomplete records

One of the most important things for users of the mean sea level data bank to know is the accuracy of the published figures. Details of the treatment of gaps in the tidal record are of particular interest. Therefore, the PSMSL makes the following recommendations:

small gaps in observed tidal records should be interpolated, if possible before computing monthly and annual means,
the interpolation should be performed at an early stage in the processing. One principle to adopt is that of a comparison with the complete records from a nearby station. However we would stress that predicted values are not suitable for interpolation because of meteorological effects,
in cases where interpolation is impossible the monthly mean should be compiled from the incomplete data. Where more than 15 days are missing from a month a mean value should not be computed,
when sending mean values to the PSMSL, authorities are requested to indicate if interpolation has been effected or the exact number of missing days of data. These details should be sent as suffixes after each monthly mean and shown in brackets:
e.g. 2487(9) would mean 9 daily mean values were missing and not interpolated when computing the mean of 2487mm; 913(XX) would mean missing data were interpolated to provide the average of 913mm,
if there are 11 or 12 monthly mean values available then an annual mean should be calculated. If the annual mean is computed by averaging the monthly means, the monthly means must first be weighted. The weight for each month should be the number of days for which readings were available.

Computation of monthly and annual mean values

The attention of data contributors is drawn to four publications entitled 'Manual on Sea Level Measurement and Interpretation' (IOC, 1985, 1994, 2000, and 2006). In addition, the PSMSL will be pleased to assist with advice on methods of data processing and the determination of mean values.

Preservation of original data

Contributors are urged to preserve the original sea level data in some permanent form. The information contained in such basic time series is of great value in many scientific studies, is irreplaceable, and should not be lost to posterity. Where original data are available in digital formats, the PSMSL would be grateful to receive copies.