ERASED CITY - Technical challenge : water levels
The drawing of the mouth of the revealed river Walbrook meeting the Thames in section helped to determine how to set the different water levels by analysing their relationship to the Ordnance Datum (Newlyn) in order to use the tidal forces in the design process:
ODN:
In the United Kingdom, the Ordnance Datum (OD) is a datum of the land levelling or vertical coordinate system used by the National Mapping Agency (Ordnance Survey-OS) as the basis for deriving altitudes on maps and setting up navigational charts.
The ODN refers all chart datum levels to a common, national horizontal plane and allows the levels to be checked. The datum was established at the level of the average value of Mean Sea Level (MSL) at Newlyn in Cornwall for a six-year period between 1915 and 1921. (Prior to 1921, OD was taken from the level of the Victoria Dock, Liverpool) Nowadays, the ODN consists of the original tide gauge datum and a TRF (Terrestrial Reference Frame) observed by spirit levelling between 200 fundamental bench marks (fixed reference points) across Britain. The TRF is densified by more than half a million lower-accuracy bench marks.
In case of the Thames, the true depth of water can be roughly determined by adding the tidal height to the charted depth below ODN.
Chart Datum:
In order to describe a point on the Earth’s surface in latitude and longitude or another coordinate system, one refers to horizontal datums; to measure elevations or depths, one uses vertical datums.
To determine the manner in which tidal levels vary along any given stretch of coastline, it is necessary to refer all levels to a common horizontal plane. In general, the highest and lowest predicted tides that can occur are called Highest Astronomical Tide (HAT) and Lowest Astronomical Tide (LAT). These levels are the highest and lowest (not to be confound with extreme) levels which can be predicted to occur under average meteorological conditions, and under any combination of astronomical conditions. ( HAT and LAT are not the extreme levels which can be reached, as Storm Surges (wind-induced long period waves causing higher and lower-than-predicted levels to occur) and pressure effects can significantly alter the times and / or heights of the observed tide.) By international agreement the Chart Datum is defined as a level so low that the tide will not frequently fall below it. In the United Kingdom, this level is normally approximately the level of LAT.
As the slope of the River Thames makes it difficult to use L.A.T., a more specific step datum is set up to measure the tides at different sections along the river. The difference between each step is rather small (usually 10-15 cm) and are on average 2-2.5km apart. Since the Chart Datum varies in different locations and is not suitable as a national reference plane. Hence, the chart datum steps on the Thames are referenced to the UK vertical land datum called ODN (Ordnance Datum Newlyn).