Revised Local Reference (RLR) Definition
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. These include, in particular, all Netherlands 'Metric' data, the records of which are expressed relative to the national level system Normaal Amsterdamsch Peil (NAP). They also include a number of German 'Metric' records which are measured with respect to Normal Null (NN). Any such information is included in the authority comments in the relevant documentation sections of the datasets 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 relevelling 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.
