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The annual mean sea level anomalies presented for a given year are calculated by subtracting the long term annual mean at each station from annual means.
The long term mean for a station used is the arithmetic mean of all annual means available over the chosen baseline period: 1960-1990.
If the 'remove trend' option is selected, the anomalies used are the difference from a linear least-squares trend line fitted to all available data over the baseline period. However, if less than 70% of annual means are present over the period, the station is ignored and will not appear in the maps. (See the derived trends page for more information.)
In all cases:
Should you wish to further investigate the anomalies at a particular site, we would invite you to download the RLR data and examine the relevant station documentation.