COLOUR GUIDE

For many years I have used a set of Admiralty Leisure Folio charts, which cover the entire Bristol Channel (right) in convenient sections. Leisure Folio charts are perfect for use aboard small boats because they are a more user-friendly to handle than a full-sized chart. ey are produced to exactly the same quality and contain all of the information found on the larger charts, but with a Leisure Folio you get a dozen or more charts that are double-side printed, and even include a handy sheet of symbols and distress signals. e dominant colour on most charts is mustard yellow, which indicates land above the high-water mark (see below), meaning it always remains dry. e yellow is bordered by green, which is the area affected by tides, known as the inter-tidal zone. erefore, the line between yellow and green is the high-water mark, and the line between green and blue being the low water mark. is blue area is the area from low water at chart datum (detailed later) down to a depth of five metres. At the five-metre sounding, the colour changes to a lighter blue marking the transition from a depth of five metres down to 10 metres, at which point the colour changes again to white (see left), indicating depths greater than 10 metres.

All depths, heights and tidal ranges printed on the chart are reduced to what is known as chart datum. is chart datum is the depth at low water on the highest astronomically predictable spring tides and is the maximum extent at which the tide could be expected to fall at low water on the biggest calculable spring tide. erefore, at low water on a normal spring tide, you could expect the tide to ebb back to slightly before the extreme limit illustrated on a chart.