03 Feb 2016
As part of the effort to more clearly identify mean tide level (MTL) data in mean sea level (MSL) data sets (see associated news item), the PSMSL has introduced a new flag that may appear on individual monthly or annual mean values in a tide gauge record. The end effect is that users should examine their routines to determine if the new flag will be handled properly.
To identify those values which are MTL in an otherwise MSL record, we have introduced a new flag, "010". If this flag is present in a metric record, it indicates that the value is MTL. In a RLR record, an estimate of the annual average of the MTL-MSL may have been applied to the data to make it comparable with the rest of the MSL values.
In some cases, both the MTL flag, "010", and the 'flag for attention', "001", may be set, and will appear as "011". The 'flag for attention' is set if either we do not have an estimate of MTL-MSL or we think that the uncertainty of the estimate is greater than 1 cm. In these cases, we suggest that one not use these values in estimating long-term trends.
As a general note, our flags are based upon the octal representation used for UNIX permission flags. For example, in the future we could introduce additional flag in the future, say "002". The value "003" would indicate both that the value was ‘flagged for attention’ and that the new flag was true. This system allows us to introduce nine distinct flags: 001, 002, 004, 010, 020, 040, 100, 200, and 400.