Chart datum is the level that depths on a chart are measured from, set at about the lowest level the tide normally falls to. The real depth is usually the charted depth plus the height of the tide at that moment.
Depths on a nautical chart are called soundings. On New Zealand and most modern charts they are in metres, and a small subscript digit gives tenths, so a sounding written as 4 with a small 2 means 4.2 metres.
The depths are measured from chart datum, a level set close to the lowest astronomical tide, roughly the lowest the tide will fall under average weather. Choosing a low datum means the charted figure is a cautious one: most of the time there is more water than charted, not less.
To find the depth at a given time:
- read the charted depth at the spot;
- find the height of tide above chart datum at that time, from the tide tables for the nearest port (the rule of twelfths gives a rough figure between high and low water);
- add the two together.
Some areas uncover at low water. Their drying heights are shown underlined, and are heights above chart datum. A bank with a drying height of 1.5 metres has no water over it until the tide has risen 1.5 metres, and then only a little.
Other features of the chart to know:
- depth contours join points of equal depth, and shallow areas are tinted blue;
- heights of land and clearances under bridges and power cables are measured from a high-water level, not from chart datum; the chart's notes say which;
- the type of seabed (mud, sand, rock) is printed in abbreviations, which helps when anchoring.
Safety notes: allow a margin for under-keel clearance, more in waves, which make the boat rise and fall. Strong offshore winds and high barometric pressure can make the tide lower than predicted. Know your boat's draught, and know whether your depth sounder shows depth below the surface, below the transducer or below the keel.
More in the book: Chapter 14, Buoys, charts and tides