2.1 Nautical Charts: Reading, Scale, Lat/Long & Chart No. 1

Nautical Charts: Reading, Scale, Latitude/Longitude & Chart No. 1

Quick Answer: A nautical chart is a scaled overhead picture of the water. Positions are given in latitude and longitude, distance is measured on the latitude scale because one minute of latitude equals one nautical mile, and the title block tells you the units and datum for the printed depths. Chart No. 1 is the booklet that explains what every symbol means.

Nearly every Navigation General question that involves position, distance, or symbols starts with the chart. You must be fluent in reading one before the plotting problems in the Chart Plot module make sense.

Latitude and Longitude

Latitude measures how far north or south of the equator you are, from 0 degrees at the equator to 90 degrees at the poles. Lines of latitude (parallels) run east-west, and their scale is printed on the left and right edges of the chart. Longitude measures how far east or west of the prime meridian (Greenwich) you are, from 0 to 180 degrees. Lines of longitude (meridians) run north-south, and their scale is printed along the top and bottom of the chart.

A position is written latitude first: 41 degrees 15.0 minutes N, 072 degrees 30.0 minutes W. Each degree is divided into 60 minutes, and each minute into tenths (or into 60 seconds on older charts). To plot that position, set one pair of dividers to 41 degrees 15.0 minutes on a side scale, walk a parallel ruler across to carry that latitude line, then set dividers to 072 degrees 30.0 minutes on the top or bottom scale and mark where the two meet. Reading a position off the chart is the same process in reverse.

Why Distance Comes From the Latitude Scale

Because one minute of latitude equals exactly one nautical mile, the latitude scale doubles as your distance scale. To measure the distance between two points, set your dividers on the two points, carry them to the latitude scale at roughly the same latitude as your track, and read the number of minutes spanned. Twelve minutes of latitude spanned equals twelve nautical miles.

The single most common chart trap on the exam: never measure distance on the longitude scale. On a Mercator projection (the standard for coastal charts) the meridians are drawn as parallel vertical lines, which stretches the longitude scale more and more toward the poles. A minute of longitude therefore does not equal a nautical mile except at the equator. Latitude minutes, by contrast, stay true to distance everywhere on the chart. The Mercator projection is popular precisely because a straight line drawn on it is a rhumb line (a line of constant compass direction), which is exactly what you steer. (Great-circle charts use a gnomonic projection instead, on which the shortest route is a straight line but the rhumb line curves — you will not use those for coastal OUPV work.)

Chart Scale

Scale is the ratio between a distance on the chart and the real distance it represents, written like 1:40,000 (one unit on the chart equals 40,000 of the same units on the water).

Remember it as "large scale, large detail." A 1:20,000 harbor chart is large scale: it covers a small area but shows individual buoys, wharves, and soundings. A 1:200,000 coastal chart is small scale: it covers a whole coastline but omits fine detail. Always pilot on the largest-scale chart available for your area, because it shows the most hazards.

Soundings, Datum, and the Title Block

The printed depths on a chart are soundings. Before you trust a single number, read the title block, which states the unit: feet, fathoms, or meters (one fathom equals six feet). A sounding of "4½" means 4.5 feet, 4.5 fathoms (27 feet), or 4.5 meters depending entirely on that title block.

U.S. chart soundings are referenced to a low-water datum, normally Mean Lower Low Water (MLLW) — the average of the lower low water heights. Because the datum is a low-water average, the actual depth is usually a little more than charted, but during a strong minus tide it can be less. Heights of bridges and overhead cables use a different high-water datum (Mean High Water), giving you conservative clearance.

Chart No. 1

Chart No. 1 is not a chart at all — it is the booklet U.S. Chart No. 1: Symbols, Abbreviations and Terms. It decodes everything you see: buoy and beacon shapes, light characteristics, the letters for bottom characteristics (S sand, M mud, Rk rock, Co coral, Sh shells), colors, and the meaning of dashed and dotted lines. When a question shows an unfamiliar abbreviation, Chart No. 1 is the authority.

Also on the chart: a source or reliability diagram, the edition number and date (older surveys are less reliable, and shoals can move), and one or more compass roses used to lay off courses and bearings. Keeping charts corrected from the Notice to Mariners is a legal expectation of good seamanship — an aid may have been moved, discontinued, or replaced since the chart was printed.