On line theory basic chart work lesson for section 2 question 1a,1b and 1c.

Chart depths datum and contour lines.

A contour line is a line that follows a set depth on the chart as in A on the right. Everywhere this line is the depth will be about 10m It is not always accurate but is generally a good guide.

The sea in tidal areas rises and falls this caused by the action of the sun and moon pulling on the surface of the earth. So sometimes land is covered by the sea and other times it is not.
This is the area in green on the chartlet and is covered with water when the tide is high usually a spring tide (more about this later) and uncovered when the tide is low.

In other words the green part covers and uncovers with water depending on the height of the tide.

The dark blue area is between the 0m depth contour and 10m contour the 0m contour is called chart datum and information on what this is based on can be found on the chart information which we will come to later.

The light blue area is between the 10m contour and the 20m contour.

The buff coloured area is always land.

On this chart the first white area is between 20m and 50m.

Outside the 50m contour the depth will be over 50m unless otherwise specified.