History of Navigation: Using a Chart

A nautical chart has a description that starts with its name and perhaps a catalog number. It includes the type of projection, the scale of the chart, such as 1:40,000, and the chart datum, typically at a very low tide (mean lower low water). Elevations are often given from a mean high water. Most charts show true north on the top, with a compass rose that tells the location of true north, magnetic north, the variation between the two, and the change of variation each year, as the magnetic north pole moves.

Loran Lines on Penobscot Bay Chart

Symbols on charts have meanings, listed in NOAA’s Chart Number 1. These include aids to navigation like buoys (lit, unlit, bells, gongs, etc.), lighthouses, and day beacons as well as natural features such as rocks and reefs; and man-made structures such as wrecks, water tanks, chimneys, and smokestacks. The bottom is shown topographically, with lines often at 18 feet deep, 30 feet, 60 feet, and 120 feet. These numbers are used because they are equivalent to 3, 5, 10, and 20 fathoms. Some charts published today replace feet and fathoms with meters.

Though most charts do provide a scale of nautical miles and yards, navigators often use the latitude markings on the chart’s side instead, as one minute of latitude equals one nautical mile.

Some small scale charts show a grid in colored lines used with long range radio navigation equipment, like loran or Omega.