1 July 2003 THE PERMANENT SERVICE FOR MEAN SEA LEVEL: FURTHER INFORMATION Since 1933, the Permanent Service for Mean Sea Level (PSMSL) has been responsible for the collection, publication, analysis and interpretation of sea level data from the global network of tide gauges. It is based at the Proudman Oceanographic Laboratory, Merseyside, United Kingdom and is a member of the Federation of Astronomical and Geophysical Data Analysis Services (FAGS) established by the International Council for Science (formerly the International Council of Scientific Unions, ICSU). It is supported by FAGS, by the Intergovernmental Oceanographic Commission (IOC) and by the U.K. Natural Environment Research Council. Information on FAGS and its various services, including the PSMSL, can be found in FAGS (1989) and via http://www.kms.dk/fags/. DESCRIPTION OF PSMSL 'RLR' AND 'METRIC' DATASETS The database of the Permanent Service for Mean Sea Level (PSMSL) contains monthly and annual mean values of sea level from almost 2000 tide gauge stations around the world. (Note that, in common with most other climate variables, by 'monthly' we mean CALENDAR monthly means.) The PSMSL receives monthly and annual mean values of sea level from almost 200 national authorities, distributed around the world, responsible for sea level monitoring in each country or region. Data from each station are entered directly as received from the authority into the PSMSL raw data file for that station (usually called the METRIC file in PSMSL publications). The monthly and annual means so entered for any one year are necessarily required to be measured to a common datum, although, at this stage, datum continuity between years is not essential. While the PSMSL makes every attempt to spot inconsistent or erroneous data, the responsibility for the monthly and annual means entered into the METRIC files in this way is entirely that of the supplying authority. A description of data checks routinely made by the PSMSL is given below and in Woodworth, Spencer and Alcock (1990) and IOC (1992). 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. For scientific purposes, the RLR dataset is normally superior to the 'METRIC', although the latter, which contains the total PSMSL data holdings, can also be analysed bearing in mind the above datum continuity considerations. (See below for further comments on METRIC and RLR differences). 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, although they can be made available on request. The contents of the PSMSL dataset used to be described in 'Data Holdings of the PSMSL' printed reports, of which the last was Spencer and Woodworth (1993). Regular updates are now made via the web. Further information about the PSMSL, and about the spatial and temporal distribution of the PSMSL data set, can be found in Woodworth and Player (2003) and earlier journal papers referenced therein. The detailed current contents of the PSMSL databank are described via files accessed from the PSMSL web page: http://www.pol.ac.uk/psmsl/ In particular, this PSMSL home page points to http://www.pol.ac.uk/psmsl/datainfo/ which describes how to access the MSL data sets. This page points to the main PSMSL Data Catalogue which describes how each country is identified by a 3-figure country-code, with the convention for country-codes following that described in earlier PSMSL reports. Iceland is defined as code '010', followed by Jan Mayen ('012') and the Faeroe Islands ('015'), and then progressing around the world coastline in essentially an eastward direction until Greenland ('980'). Data from Antarctica ('A ') are added at the end. As the Catalogue progresses through the various oceans and seas it also includes the nearby island gauges. Separate country-codes are used for countries abutting more than one ocean or sea, e.g. France (Atlantic) and France (Mediterranean). We stress that the term 'country' has no political significance, but is used in a geographical subdivision sense only, usually with one 'country' corresponding to one data supplier. The Catalogue lists the stations within each country and the amount of sea level data available in each case. For each station is shown a 3-figure station code, station name, location (latitude and longitude), authority-code, frequency-code and, if appropriate, the corresponding GLOSS station-code. The authority-code identifies the national authority from which the data have been obtained (see http://www.pol.ac.uk/psmsl/pub/indexa.html for a full list), while the frequency-code specifies the sea level sampling which has been used for the determination of mean sea level. Frequency-codes are defined as follows: Code Meaning Integer n n measurements/day C Integration from continuous recording HL Mean of high and low waters (i.e. Mean Tide Level) On the next line, the complete range of mean sea level data (METRIC data) available for that station is shown together with next, if appropriate, the amount of data with controlled benchmark datum stability in the RLR dataset. Years with incomplete data are shown next. A 'condensed format' version of the Catalogue contains similar information but in a briefer form. For each station is shown a 6-figure station code (the first three characters of which are the country-code), station name, location (latitude and longitude), authority-code, frequency-code and, if appropriate, the corresponding GLOSS station-code. On the same line, the ranges of Metric and RLR data are given, showing number of years and the first and last years of data, and a percentage missing flag which is the percentage of missing years of data between the first and last. The page http://www.pol.ac.uk/psmsl/pub/indexa.html gives the names of the national authorities with which the PSMSL is in contact together with a list of country/station-codes of the stations for which they are responsible. Some of the authorities are flagged as being 'not active'. Stations with 'R' after the country/station-code have data in the PSMSL RLR dataset. For some of the longer sea level records, different authorities may have been responsible for the operation of the tide gauge and the analysis of its data at different times. The authority-codes shown here refer to the authority most recently responsible. A change of authority is indicated within the documentation embedded with the PSMSL data files, while a full history of the operation of a particular tide gauge can be obtained from the PSMSL. FURTHER COMMENTS ON SEVERAL RLR RECORDS FROM A STATION You will see that several stations contain more than one PSMSL RLR record. RLR records are kept separate if, for example, they are from completely separate sets of measurements from different locations in the area of the station. Another reason is if the benchmark datums of the separate records epochs cannot be geodetically connected. A further reason is if one section of data is MSL and a further set is MTL. However, there are a small number of pairs of RLR records which can be combined into composites, with some reservations about care in using them because of, perhaps, long periods of time between them. These include: 170/011 Aberdeen I which is MSL and 170/012 Aberdeen II which is MTL but which are geodetically connected to a common benchmark 270/051 San Stefano and 270/054 Punta della Salute in Venice, which are from separate sites but which are geodetically connected to a common benchmark. 611/011 North Point and 611/010 Quarry Bay in Hong Kong, which are from quite separate sites in areas of reclaimed land but which are geodetically connected to a common, if somewhat distant, benchmark. 690/001 Auckland and 690/002 Auckland II from Auckland, New Zealand, the former being provided by the RNZN Hydrographic Office, although gappy, and the latter being provided by the work of Prof. John Hannah and more complete but not up-to-date. 863/001 Port Stanley and 863/002 Port Stanley II in the Falkland Islands (Malvinas) which are geodetically connected to a common benchmark but are separated by a large period of time In each of these cases, the brief documentation should be consulted with regard to the construction of composite time series. FURTHER COMMENTS ON THE USE OF 'METRIC' DATA Without the provision of full benchmark datum history information, records will remain as 'Metric only' in the databank and not as 'RLR'. It is a good general rule, therefore, that '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 station 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. 'PUBLICATION FORMAT' PRINTOUTS The page http://www.pol.ac.uk/psmsl/datainfo/ gives links to printouts of all the RLR and Metric data records in so-called 'Publication Format'. These are perhaps easier to use than the other data file options for people unfamiliar with programming. The printout for each station starts on a new page with the stations ordered in country-station code order as described above. In the case of RLR records, monthly and annual mean sea level data are presented only for those stations and years for which sufficient datum history is available for the PSMSL to reduce the data to a common datum (see above). The data are shown in 13 columns, preceded by the year. The first 12 data columns labelled I, II,-XII are monthly means; that labelled Y, the yearly mean. Incomplete sets of observations are indicated by characters appended in parenthesis. Thus: - a monthly mean given as 7053(14) signifies 14 daily mean values were missing due to lack of observations; no interpolations were made when computing the mean of 7053 mm. - a monthly mean given as 7033(XX) signifies that missing observations were interpolated before computing the mean of 7033 mm. - an annual mean which is likely to be materially affected by either of the above considerations is accompanied by (XX). The data are preceded by the station name, the station position (latitude and longitude expressed in degrees, minutes and hemisphere), the PSMSL country- code, the PSMSL station-code (expressed within the country), an authority code and frequency code (see above) and, if the station is part of the GLOSS network, the relevant GLOSS station number. In the case of RLR data, there will then be a line indicating the datum being used to define the RLR time series. In the case of Metric data, there will be printed a warning that the values shown are Metric data which should not be employed for multi-year time series analysis. For many stations, a small amount of documentation is stored with the data in the PSMSL data base, and these comments are listed out after the name and address of the most recent authority (see above). Further information on the data from a particular station may be obtained from the PSMSL. Each set of PSMSL data files contain within them documentation warning flags which are used to point to either a suspect station record or suspect station-years. These flags are set on the basis of either the data checks described below or after further detailed scientific analysis. PSMSL DATA CHECKS The following describes the checks made by the PSMSL on data received from national authorities. Details can also be found in PSMSL publications (e.g. Woodworth, Spencer and Alcock, Int.Hyd.Rev., 67(1), 131-146, 1990). In general, in years past the PSMSL has not received copies of original tide gauge hourly height measurements or continuous charts but has accepted monthly and annual mean sea level (MSL) values from national authorities on the understanding that these quantities have been computed accurately. Inevitably, this has always not been the case. The PSMSL has devised a range of tests on the supplied MSL information which guards against gross errors in the dataset such as transcription errors or large unrecorded datum changes. Some of these tests are 'common sense', others are 'statistical': (1) A check is made that the average of the quoted monthly mean values is consistent with the quoted annual mean. (2) 'Common sense' consistency checks are made on the data including checking that the datum information is consistent with previous knowledge. This includes reference to back correspondence and simple time series plotting. (3) A search for outliers is made on data for each calendar month of the year separately, and for the annual means, for all possible time-spans containing at least 20 years of data. A linear fit is made to the time series and any individual monthly mean value more than 4.5 standard deviations from the fitted line is flagged. (4) A search is made for incorrect datum information by performing a set of linear regressions of RLR annual mean values against the supplied datum correction factors ('RLR factors' in PSMSL terminology) in all possible 20-year time-spans. A correctly adjusted RLR time series should be uncorrelated with the RLR factors. (5) A search for jumps in the RLR time series is made for each calendar month of the year separately and for the annual means. The difference between a mean value and the corresponding value for the next year of data is histogrammed and any outlier more than 4.5 standard deviations from the mean-difference is flagged. (6) A test is made for 'upside down' data. In several countries the main research interest is the study of vertical land movements, rather than sea level changes, with the result that MSL data are often quoted as the distance below a benchmark height rather than above it. The most sensitive test to guard against such an error is an inspection of the seasonal cycle which, for 'upside down' data, would appear opposite to that observed in neighbouring records and opposite to oceanographic and meteorological expectations. (7) A set of 'buddy checking' is made in which the RLR data from one station is subtracted from that of a neighbouring station (or 'buddy') which is less than 400km away. Over this short distance most of the MSL variability due to oceanographic and meteorological forcings in the two records should be similar and will cancel out giving a difference time series primarily composed of relative vertical land movements, instrumental and datum errors and any small spatially varying ocean and weather influences. The previous tests are then applied to the difference time series and any discrepancies are flagged. These tests have been applied to the entire dataset and inconsistencies have been referred back to the national authorities, although the reasons for some apparent oddities are no doubt lost in history. RECOMMENDATIONS TO AUTHORITIES CONTRIBUTING DATA TO THE PSMSL a) General 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: i) the units used (metres, rarely feet), ii) a statement of the datum to which the values refer, iii) a statement of the measured depth of that datum below the primary tide gauge bench mark (TGBM), iv) an indication of incomplete or deduced data (see paragraph b), v) the number of observations per day used to calculate the monthly means, vi) any information of changes in datums, bench marks or relevant procedures since the previous batch of data, vii) 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 in letters or via email). b) 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: i) small gaps in observed tidal records should be interpolated, if possible before computing monthly and annual means, ii) 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, iii) 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, iv) 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, v) 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. c) Computation of monthly and annual mean values The attention of data contributors is drawn to three IOC publications entitled 'Manual on Sea Level Measurement and Interpretation' which can be downloaded from http://www.pol.ac.uk/psmsl/manuals/ The third one is especially worth considering with regard to monthly mean computation. The PSMSL will be pleased to assist with advice on methods of data processing and the determination of mean values. d) Preservation of original data Contributors are urged to preserve the original sea level data in 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 computer compatible form, the PSMSL would be grateful to receive copies (and in the case of GLOSS stations, it is a requirement that any such data are provided to GLOSS Archiving Centres, of which the PSMSL is one, see below). Such global databanking of higher frequency information will evolve considerably over the next few years as a result of GLOSS and related activities. LINKAGE TO THE GLOSS PROGRAMME The Global Sea Level Observing System (GLOSS) is a programme coordinated by the Joint Technical Commission for Oceanography and Marine Meteorology (JCOMM) of the Intergovernmental Oceanographic Commission (IOC) and the World Meteorological Organisation (WMO). It aims for the establishment of global and regional sea level networks for oceanographic, climate change and coastal research purposes. The main component of GLOSS is the 'Global Core Network' (GCN) for long term climate change and oceanographic sea level monitoring. For full details, see the page http://www.pol.ac.uk/psmsl/programmes/gloss.info.html. The development of GLOSS can be traced through the various Implementation Plans for the programme and through related documents (IOC, 1990; Woodworth and Player, 2003). The most recent Implementation Plan (Woodworth, 1998) can be downloaded from the above page. The PSMSL took a major lead in the planning of GLOSS which in the long term will result in a significant improvement in the quantity and quality of data delivered to the PSMSL. Further information on the development of GLOSS from a PSMSL perspective can be found via the above web pages and Woodworth (1998) and Woodworth and Player (2003), while the PSMSL provides details of current status of the programme via its web pages. The PSMSL has played a particularly important part in GLOSS's provision of training courses and training materials with courses held at the PSMSL, and with PSMSL-related scientists having taken part in courses overseas. Recent years have seen major efforts to collect higher frequency (typically hourly) sea level data as well as MSL information. In collaboration with the British Oceanographic Data Centre and the University of Hawaii Sea Level Center, the PSMSL also functions as a 'GLOSS Archiving Centre' for higher frequency sea level data. See the above-mentioned GLOSS web page and the page http://www.pol.ac.uk/psmsl/highfreq.html for further details. References ---------- FAGS, 1989. Federation of Astronomical and Geophysical Data Analysis Services (FAGS). Chronique de l'UGGI, No.194, 1-67. IOC, 1990. Global Sea Level Observing System (GLOSS)implementation plan. Intergovernmental Oceanographic Commission, Technical Series, No.35, 90pp. IOC, 1992. Joint IAPSO-IOC workshop on sea level measurements and quality control. Intergovernmental Oceanographic Commission, Workshop Report, No.81, 167pp. Spencer, N.E. and Woodworth, P.L. 1993. Data holdings of the Permanent Service for Mean Sea Level (November 1993). Bidston, Birkenhead: Permanent Service for Mean Sea Level. 81pp. Woodworth, P.L., Spencer, N.E. and Alcock, G.A. 1990. On the availability of European mean sea level data. International Hydrographic Review, 67(1), 131-146. Woodworth, P.L. (ed.) 1998. Global Sea Level Observing System (GLOSS) Implementation Plan 1997. Intergovernmental Oceanographic Commission, Technical Series, No. 50, 91pp. & Annexes. Woodworth, P.L. and Player, R. 2003. The Permanent Service for Mean Sea Level: an update to the 21st century. Journal of Coastal Research, 19, 287-295.