Using Your Chart - A Few Tips

A Few Tips To Help You Use Your Charts

Knowing how to read a chart and how to use the chart for navigation (especially Dead-Reckoning) is an important seamanship skill.  A few of the key parts of the chart that you should recognize immediately are: the Compass Rose, the Latitude Scale, what units the Soundings are in, Scale and Linear Measurements, and Cautions, Notes, and Warnings

The Compass Rose shows you two rings with degree markings around the rings.  The outer ring (with the Star at the top of the circle) shows you True North and all of the points relative to True North.  On charts, True North points to the geographic North Pole and is straight up the chart.  The inner ring shows you Magnetic North (the little arrow) and all of the compass points relative to Magnetic North.  On most charts there is a difference between True and Magnetic.  This difference is called Variance.  In the “old days”, navigators would adjust compass information into True information.  That is complicated and can easily lead to errors.  Most of us now use the inner Magnetic circle because we can directly use those numbers with our compass or GPS information.

The Latitude Scale along the left and right sides of the chart show (in Degrees, Minutes, and tenths of minutes) how far north or south of the equator you are.  The reason that this scale can be so important is that the navigation saying, “A Mile per Minute”.  Each dark or light section on the Latitude Scale represents one nautical mile.  If you are working on a part of the chart that does not show the Scale and Linear Measures, you can easily use the Latitude Scale to determine distances

The depth of the water is shown across the chart in many places with little numbers.  It is important to know what these numbers represent.  For fresh water charts (like Lake Pend Oreille, the Great Lakes, and many Rivers) the soundings are in Feet.  For salt water charts (like 18421 or the San Juan Islands) the soundings are in Fathoms.  One Fathom equals six Feet.  For Canadian charts (and most of the rest of the world as well) the soundings are in Meters (Metric).  When you move from one chart to another make sure that you take note of the soundings.  This is particularly important if you are sailing from the San Juan Islands north up the Strait of Georgia.  The San Juan Islands’ chart shows soundings in Fathoms; the Strait of Georgia is in Canada, so the soundings are in Meters.  There is a big difference between 5Fathoms and 5 Meters.  5 Fathoms is 30 feet.  5 Meters is about 15 feet (half as deep).

Most charts have typed information in the form of Cautions, Notes, and Warnings regarding conditions relating to the chart.  Chart 18421 of the San Juan Islands has many of these; including one warning along the West shore of Whidbey Island near the bottom of the chart that says:  “Naval Air Station small arms range operates 7 days a week.  Red flashing light and flags are displayed during live fire exercises.  Use caution when transiting near the zone.”  So, read all of the “fine print” on the chart.  BTW – the chart for Lake Pend Oreille has a warning near Oden Bay regarding high voltage lines crossing above the lake.

There is lots of other information on charts.  But these five things are excellent starting points.

Barnacle Bill Holcomb

barnaclebillholcomb@gmail.com