In order to construct time series of sea level measurements at each station, the monthly and annual means have to be reduced to a common datum. This reduction is performed by the PSMSL making use of the tide gauge datum history provided by the supplying authority. To date, approximately two thirds of the stations in the PSMSL database have had their data adjusted in this way, forming the 'REVISED LOCAL REFERENCE' (or 'RLR') dataset. In general, only RLR data should be used for time series analysis. (See below for further comments on this and a few exceptions.)
The RLR datum at each station is defined to be approximately 7000mm below mean sea level, with this arbitrary choice made many years ago in order to avoid negative numbers in the resulting RLR monthly and annual mean values. The detailed relationships at each site between RLR datum, benchmark heights, tide gauge zero etc. are not normally required by analysts of the dataset, but is available for most station from the individual station pages.
Without the provision of full benchmark datum history information, records generally remain as 'Metric only' in the databank and not as 'RLR'. In general, 'Metric' records should NEVER be used for time series analysis or for the computation of secular trends. Without datum continuity their only use is in studies of the seasonal cycle of mean sea level. If there is any doubt about the datums for a particular record, the PSMSL would be pleased to supply clarification.
There are, however, some 'Metric only' records which almost certainly can be used for time series work, even though the PSMSL does not have full benchmark datum histories. They include, in particular, a number of German 'Metric' records which are measured with respect to Normal Null (NN). Any such information is included in the station comments in the relevant documentation sections of the data sets supplied by the PSMSL. Even though these records are expressed relative to the national levelling systems, they are, in effect, relative to a local level as required for RLR purposes i.e. the records do not (as far as we know) contain datum shifts contributed by re-levelling adjustments. In general, however, measurements relative to national levelling systems may well reflect such adjustments, which explains why the PSMSL has traditionally steered clear of classifying such data as RLR. We have relaxed this classification slightly for a few of the German stations. In the late 1990s and early 2000s, the PSMSL received data from these stations that could be related to a tide gauge benchmark as well as NN. Using these relationships, we have reclassified these entire time series as RLR.
In the past, the PSMSL also included the the Netherlands data in the above category of Metric records acceptable for time series work. These records are expressed relative to the national level system Normaal Amsterdamsch Peil (NAP). However, a recent re-levelling of NAP in 2005 introduced a small datum shift for the tide gauge time series. In order to maintain utility of these long records, we have reclassified most of the Netherlands records as RLR and introduced different RLR factors for the periods before and after 2005. While these records do not meet the strict definition of RLR and may still include prior re-levelling adjustments, we believe this represents the best path forward.