Sunday, June 21, 2009

How to Read a Nautical Chart

To operate your boat safely, you should carry paper nautical charts on your boat. Becoming familiar with nautical chart basics will form a foundation for knowing how to read the chart symbols that show channels, water depth, buoys and lights, landmarks, obstructions, and other important information that will ensure you safe passage.

The general information block of the chart shows the chart title, usually the name of the navigable water in the covered area, the type of projection and the unit of measurement (Soundings in Feet or Fathoms). If the unit of measurement is fathoms, one fathom equals six feet.

The notes contained in the general information block give the meaning of abbreviations used on the chart, special caution notes, and reference anchorage areas. Reading these will provide important information about the waterways you navigate not found elsewhere on the chart.

Having a variety of charts will serve you well. Depending upon the location you will be navigating, different charts will be necessary because they are produced in different scales, or ratios (type of projection). Sailing charts are used for open ocean navigation. General charts are used for coastal navigation in sight of land. Coastal charts zoom in on one particular portion of a larger area and are used for navigating bays, harbors, or inland waterways. Harbor charts are used in harbors, anchorages, and small waterways. Small craft charts are special editions of conventional charts printed on lighter paper so they can be folded and stowed on your vessel.

Nautical charts can pinpoint your location using lines of latitude and longitude. The latitude scale runs vertically along both sides of the chart indicating North and South with the equator as the zero point, the longitude scale runs horizontally on the top and bottom of the chart, and indicates East and West with the Prime Meridian as the zero point.

The chart number is the number assigned to the chart located in the lower right hand corner. Use this to locate charts online and to make purchases. The edition number is located in the lower left hand corner and indicates when the chart was last updated. Corrections published in the Notice to Mariners that occur after the publish date will need to be entered by hand.

One of the most important functions of a nautical chart is to show the depth and bottom characteristics through numbers, color codes and underwater contour lines. The numbers indicate soundings and show the depth in that area at low tide.

Soundings in white indicate deep water, which is why channels and open water are typically white. Shoal water, or shallow water, is indicated by blue on the chart and should be approached with caution using a depth finder. Fathom curves are the wavy lines, and they provide a profile of the bottom.

Nautical charts have one or more compass roses printed on them. A compass rose is used to measure directions using true or magnetic bearing. True direction is printed around the outside, while magnetic is printed around the inside. Variation is the difference between true and magnetic north for the covered area. It is printed with annual change in the center of the compass rose.

The compass rose is used to plot a course when navigating using direction bearings. The last section of the chart to note is the distance scale. This is a tool used to measure distance of a specific course drawn on the chart in nautical miles, yards, or meters. The scale is usually printed at the top and bottom of the chart. The latitude and longitude scale can also be used to measure distance.