Chart Scale

Charts vary according to the area that they cover and hence the detail they contain.

The scale of a chart is given as a ratio; one unit of length on the chart represents very many units of length on the Earth's surface. So a chart scale of 1:1 000 000 means that 1 inch or 1 centimetre on the chart represents 1 000 000 inches or centimetres on the Earth's surface.

The scale chosen for use in any particular circumstance depends on the detail required. You could not enter a harbour using a 1:1 000000 scale chart; you would probably use a chart with a scale of 1:10000 or 1:5 000.

• A small-scale chart covers a large area and has a high-scale number (say 1:1 000 000) (one divided by one million is a small number).

• A large-scale chart covers a small area and has a low-scale number (say 1:5 000) (one divided by five thousand is a larger number and therefore larger scale).

Large scale

Chart scale

Large scale

Chart scale

Ideally, you would have a small-scale chart of your cruising area and large-scale charts containing more detail for the harbours and anchorages. In fact, you may need charts of an intermediate scale as well. I, personally, don't buy charts by scale. I look at individual charts and their detail and buy what I want to suit my needs.