Methods used for calculating sea level anomalies

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:

Only RLR data are used, as Metric data have no long term datum control. See our help file for an explanation of these terms.
Anomalies are not calculated for stations which have been marked with a quality control flag.
Annual Data marked with a quality control flag are ignored, and are treated as missing.

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.

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Methods used for calculating sea level anomalies