Thursday, September 11, 2014


Some Basic Chart Knowledge

If you are sailing in an area that you are not familiar with, it is good advice to have a NOAA chart for the area.  And, to have some basic knowledge regarding what you see on the chart.

Just for starters: tan is land; light green is a tide flat (land at low tide, water at high tide); light blue is for shallow water; and white is for deeper water.  How deep the water is will be shown by numbers printed on the water.  Soundings (water depths) will be in Feet for fresh water areas; Fathoms (6 feet) for salt water areas; and Meters (metric) on nearly all charts of foreign waters.  The Soundings are identified on the chart at the lower right corner and the middle of the left top.

There will be a “distance scale” that shows distances that you can use a divider to figure out how far it is from one place to another.

The Latitude Scale along the left and right margin of the chart show the degree latitude north or south of the equator.  The earth is divided into 90 equal parallels which equal one degree north or south of the equator.  When you look at the latitude scale, you will see the degrees and minutes (as well as tenths of a minute).  One minute of latitude is exactly the same distance as one nautical mile (a nautical mile equals a minute of arc).  So, you can use the latitude scale instead of the distance scale if the latitude scale is closer to the part of the chart you are working on.

You’ll find Anacortes, WA lies right on 48 degrees 30 minutes north latitude.  And that Bellingham straddles 48 degrees 45 minutes of north latitude.

Don’t confuse the latitude scale with the longitude scale found at the top and bottom margins of the chart.  Longitude is the east or west distance from the Prime Meridian (0 degrees – which crosses through the observatory at Greenwich, England.  These meridians connect the geographic north and south poles.  Longitude is measured in degrees and minutes… but because all of the meridians taper to the poles, the distance is much greater at the equator, and smaller as you get closer to the poles.  If you take NOAA Chart 18421 and measure the length of a longitude minute at the bottom of the chart, you will find that it is only seven tenths of a minute of latitude (or about 600 yards shorter).

In the United States the compass rose shows compass points.  There are three rings on the compass rose.  The outer ring shows a compass that points to True North (the geographic north pole).  The next inner ring shows a compass that points to the magnetic north pole.  The magnetic north pole wobbles around the Arctic Ocean – and is presently near the northern coast of Canada.  Because of this, if you are in the middle of the United States, these two rings are about the same… but as you travel to the east or west coast, the variance between True North and magnetic north increases.  In the San Juan Islands, the variance is about 18 degrees.

The third ring (most inner ring) shows what sailors refer to as points on the compass.  There are eight points for each 90 degrees.

Nearly all charts show the aids to navigation (or buoys, lighthouses, and day shapes).  If the aid to navigation has a light, the aid is highlighted by a purple exclamation mark on its side or upside down.  There is lots of information for each aid.  Is the light white (or another color)?  Is there numbers or letters painted on the aid?  Is there a sound device (bell, horn, etc.)?  And so on…

If you are not familiar with charts and the information found on the chart, take a look at John Rousmaniere’s book “The Annapolis Book of Seamanship” and NOAA Chart #1 (a book all about chart symbols).