Source: Mariners Learning System, by Captain Bob Figular

The nautical chart shows channels, depth of water buoys, lights, lighthouses, prominent landmarks, rocks, reefs, sandbars, and much more useful information for the safe piloting of the boat. The chart is the most essential part of all piloting equipment. Here are some basic facts to know about charts:

Charts are oriented with north at the top.

• The frame of reference for all chart construction is the system of latitude and longitude.
• Any location on a chart can be expressed in terms of latitude or longitude.
• The latitude scale runs along both sides of the chart.
• The longitude scale runs across the top and bottom of the chart.
• Latitude lines are reference points in a north and south direction with the equator as their zero reference point.
• Longitude lines are the east and west reference points with the prime meridian as their zero reference point.

Nearly all charts employ color to distinguish various categories of information such as shoal water, deepwater, and land areas. Color is also used with AtoN to make them easier to locate and interpret.

Nautical purple ink (magenta) is used for most information since it is easier to read under red light normally used for navigating at night.

Lettering on a chart provides valuable information. Slanted Roman lettering on the chart is used to label all information that is affected by tidal change or current (with the exception of bottom soundings). All descriptive lettering for floating AtoN is found in slanted lettering.

Vertical Roman lettering on the chart is used to label all information that is not affected by the tidal changes or current. Fixed aids such as lighthouses and ranges are found in vertical lettering.

A chart is only as accurate as the survey on which it is based. Major disturbances, such as hurricanes and earthquakes, cause sudden and extensive changes in the bottom contour. Even everyday forces of wind and waves cause changes in channels and shoals. The prudent boat operator must be alert to the possibilities of changes in conditions and inaccuracies of charted information.

Compromise is sometimes necessary in chart production as various factors may prevent the presentation of all data that has been collected for a given area. The information shown must be presented so that it can be understood with ease and certainty.

The source and date of the chart are generally given in the title along with the changes that have taken place since the date of the survey. The earlier surveys often were made under circumstances that precluded great accuracy of detail. Until a chart based on such a survey is tested, it should be regarded with caution. Except in well-frequented waters, few surveys have been so thorough as to make certain that all dangers have been found.