Write an article based on this "Follow dotted contour lines to monitor similar water depths. Look for light blue areas to identify shallow water. Find yellow or tan areas to identify land and sandbars."
Contour lines are there to give you an idea of what is happening beneath the water's surface. Contour lines, often red, connect similar water depth listings. This has the effect of dividing areas into shallower and deeper waters. Use the contour lines as a reference for the overall water depth in an area. For example, contour lines may connect points listed as 20ft (240in) deep. All of the points inside those lines are even shallower than that. On nautical charts, white space is normally water, so the blue areas will stand out more. Shoals and other shallow areas are blue. These spots tend to be no more than 18ft (5.5m) deep, although this varies from chart to chart. Shallow areas are often marked off by contour lines. The exact depth of shallow areas depends on the chart. Some charts color in areas under 3 fathoms, while others mark spots less than 1 fathom deep. Dry land will usually be inside a contour line, but not always. Pay particular attention to any tan spots in the middle of water channels. Sometimes these spots are submerged and can damage your boat, especially in shallow water. Land is always a light color to distinguish it from water and other navigational aids.