Some notes on the South African Tidal Chart Datum
In South Africa, the Surveyor General specifies a Land Leveling Datum at various locations from where height above and below sea level is measured. Ocean tide heights are also measured from a specific reference level - known as the Chart Datum - which is in turn specified relative to the aforementioned Land Leveling Datum. The Chart Datum is calculated and published by the Hydrographic Office of the South African Navy.
The table below lists the offsets of the Chart Datum relative to the Land Leveling Data at the specified locations. Measurements are in meters.
|Port||1 Jan 2003 onwards|
|Granger Bay (Cape Town)||-0.825|
|False Bay (Simon's Town)||-0.843|
The current Chart Datum is based on what is referred to as the Lowest Astronomical Tide (LAT). This is the lowest tide level that can be predicted to occur under average conditions in a 19 year cycle at a given location.
It is said that the mean spring high and low tide (MHWS, MLWS) as well as the mean neap high and low tide (MHWN, MLWN) values vary from year to year with a cycle of approximately 18.61 years. This is good since in South Africa observation has now been conducted for over 20 years and it was hence possible to determine the aforementioned LAT - the lowest predicted tide within a 19 year cycle.
So, the following equation holds: Chart Datum at any location = LAT at that location. The Chart Datum or LAT at a given location will be 0.0 meters with reference to measuring tidal heights.