Observed hourly heights, daily means and monthly instantaneous extremes form the basis of ISDM digital archives.  Observed tidal highs and lows are also available for a limited number of years.  These data undergo two levels of quality control and are adjusted if there is a change in the reference datum at the station.

Original analog charts are stored on microfilm, some of which date back to the turn of the century and have not all been digitized.

Many water level gauges record in digital format at 15-minute sampling intervals while other gauges record in analogue form on a continuous strip chart which is then digitized to hourly intervals.  Hourly levels and the monthly instantaneous extreme levels are taken directly from the digital readings without averaging or interpolation.  For chart records, the monthly instantaneous extreme levels are taken directly from the chart. 15-minute data do not carry historical datum corrections, are not quality controlled, and hence are not made generally available.

For some very early records, the instantaneous monthly extreme level is known but the exact time and/or day is not.  In these instances the appropriate field(s) is filled with asterisks.  Also, in some instances, the extreme daily mean will exceed the value of the recorded all time instantaneous extreme, since for very early records, daily means were recorded for a station, but instantaneous extremes were not.

Daily and monthly means are calculated from the hourly levels.   The daily mean is not computed if the number of observations is less than 16 per day for water level stations and 22 per day for tidal stations.  The monthly mean is also computed from the hourly levels in each month, but only for months with at least 20 days of data.  The yearly mean is computed from all the daily means available in each year, provided that the number of days is at least equal to 180.

A national database of harmonic constituents for tidal stations is also maintained at ISDM in co-operation with the Canadian Hydrographic Service (CHS).