Navigation Tips by Andy Cascardi Bennet Brothers Yachts Most frequent travelers of our waterways are to some degree familiar with nautical charts. Charts can be broadly categorized into two groups: paper “hard copy” charts, and electronic charts. A nautical chart, according to The American Practical Navigator, is a representation of the spherical surface of the earth on a plane surface. Simply stated, it is a map of the coastal bays, harbors and rivers and the associated landmasses, or coastal oceans for use in marine navigation. There is a vast amount of information on a nautical chart, which has been drawn from a number of government and civilian sources at great expense to the taxpayer. The result of this collective effort is a spectacular tool for the navigator, one that is nearly a work of art. This article will address paper charts and charts in general; much of the information applies to electronic charts as well. If you have a NOAA paper chart handy, take a look at it. Each one has a title and number. The title is presented in an organized form, telling you what part of the earth is represented on the chart; for example, “United States-East Coast, Chesapeake Bay, Mobjack Bay and York River Entrance.” Beneath that information, the type of chart - Mercator Projection, and the scale. The scale of a chart is expressed in a ratio, also known as a fractional scale; for example, 1:20,000,1:40,000, etc. On a chart of 1:20,000, one inch on the chart represents 20,000 inches on the earth's surface. Most recreational boaters will use charts ranging in scale from 1:2,500 to 1:150,000. For those readers interested in details, the chart's scale is accurate at specific latitude that is identified in the chart title. Also identified in the title and on the white borders of the chart is the datum - another term for benchmark - for water depths, commonly known as soundings. For almost all U.S charts today, the datum for soundings is mean lower low water (MLLW). MLLW is defined as the average of the lowest low water height of each tidal day over an established time period called the National Tidal Datum Epoch. More nautical trivia for details folks: the present tidal epoch is the 19-year period from 1960 to 1978. For a chart to be at all useful as a navigational tool, one must have a grasp of some basic information. Hopefully those of you with an understanding of navigation beyond basic the basic level will be patient with me while I explain some charting basics. Charts have black lines on them representing lines of latitude and longitude, called parallels and meridians, respectively. These lines, usually expressed in degrees, minutes (60 minutes per degree), and tenths of minutes, create a grid system to enable a mariner to identify numerically a point on the earth. Wilmington, for example, is in approximate position 34 degrees, 14 minutes north latitude, 077 degrees, and 58 minutes west longitude. Note that degrees of longitude are always expressed in three digits, because longitude is measured 0-180 degrees, east and west, where latitude is measures 0-90 degrees, north and south. On most charts longitude lines run from the top to the bottom of the chart, and parallels of latitude run from side to side. True (not magnetic) north is the top of the chart, east on the right, south on the bottom, and west on the left. Lines of latitude and longitude are numbered in black print on the perimeter of the chart. Now to something one must know to use a chart: to measure distance on a chart, measure minutes of LATITUDE or use the dedicated linear nautical mile scale. One minute of latitude is equal to one nautical mile, no matter where you are on earth. One minute of longitude is one nautical mile only at the equator; everywhere else it is less then one mile, decreasing as one approaches either pole. The white portions of the chart represent the deep water, blue represents shallow water, green represents wetland, and brown represents land. The numbers in black print all over the water areas are soundings, except those that are in quotation marks; those in quotes are the numbers found on lights, buoys, and day beacons. The seemingly random lines drawn on the chart's water area are actually lines connecting equal depths - usually drawn on large-scale charts at 6, 12, and 18 ft. curves. The soundings can be in one of three different units of measure: feet, meters, or fathoms. A fathom is six feet. Remember the datum for soundings is mean lower low water. Can there be less water then the soundings indicate? Yes, as MLLW is a “mean,” having been developed from a range of data above and below, by definition. This disparity will always exist during times of spring tide, which occur twice each month. Read the depths on charts with caution! I am sure many of you have wondered about the accuracy of charts. NOAA does have a series of standards for accuracy; surveyed features are plotted to an accuracy of approximately one millimeter (mm). What that means is, on a 1:40,000 scale chart, one mm of charting error equals 40 meters on the earth … approximately half a football field. A sharp pencil line is .5 mm, or 20 meters. When was the last time a survey was conducted to confirm soundings? NOAA has a National Survey Plan that prioritizes survey needs throughout our coastal waters. Most of the area in which we boat here on the Carolina coast is designated priority three or four - the two lowest! Excepting in the approaches to our deep draft ports, don't expect a survey soon, due to federal budget constraints. Another “what if” is created by the advent of GPS; a submerged obstruction may have been charted during a 1980 survey, on a chart that is now largely developed from GPS derived data. How accurately is that obstruction charted today? In spite of the cautions I have addressed, our charts are justifiably recognized as the world's standard. Charts display compass roses, spoil areas, isogonic lines, federal regulatory notes, dredged channel project data, horizontal datum information, an understanding of which is part of the making of a capable navigator. If the art of navigation piques your interest, contact me at andyc@bbyachts.com. I would be pleased to meet with a group of boaters interested in developing finer navigation skills, from basic piloting to celestial navigation. In future issues of Cape Fear Coast Pilot, I will discuss GPS, LORAN, tides and currents, Rules of the Road, compass error, set and drift calculation, danger bearings, and maybe a touch of celestial navigation. Safe boating to all!