Using Your Chart's Compass Rose

Cruising in unfamiliar waters is fun and (sometimes) exciting.  Cruising in unfamiliar waters without a chart and the equipment to properly navigate might become way too exciting.
First of all, a map is not a chart.  Maps show a lot of detail regarding the land, streets and roads, cities, etc... but precious little information regarding bodies of water.  Most maps only show the general boarders of lakes, rivers and bays.  The assumption is that the edges of the map point toward the North Pole at the top of the map.  This assumption is often incorrect since the map maker has to fit the subject area onto a given size of paper.
Charts, on the other hand have a great deal of information regarding the water.  Soundings (the water's depth) are included in many places.  There is a latitude scale along the left and right edges of the chart and a longitude scale along the top and bottom edges.  Shallow water is shown in light blue.  Deep water is shown in white.  And, the geographic north pole is found by following one of the side edges.

And there are many other useful to navigation pieces of information shown on charts.
One of the most useful items of information is the compass rose.

The compass rose has several features that navigators will find useful and downright necessary to proper navigation.  Notice there are three rings that encircle the cross in the middle of the rose.  Each ring has specific information.  Around the middle is also information regarding the amount of compass variance and how much the variance changes each year.  This variance is an indication for how far off True North (the geographic North Pole) is from the position of the Magnetic North Pole.  In the Pacific Northwest, the variance is around 20 degrees to the east of true north because the magnetic north pole is actually on an island near the Canadian north coast.
So, let's think about that for a minute.  The north that your compass on your boat shows is magnetic north.  If you are using a map to find your route from one place to another and assume that north is found by extending either the left or right side of the map, you will be steering a route that is 20 degrees incorrect.  You might think you are heading north... but you will actually be heading 20 degrees east of north on the map.  On Lake Pend Oreille, True north from Cape Horn would take you to Garfield Bay on the west side of the lake.  Magnetic north would take you to Whiskey Rock on the east side of the lake.
Your chart's compass rose though shows both True and Magnetic north.  True North is indicated by the star (Polaris, the North Star) on the outer ring.  So, we call the information on the outer ring true readings.  Magnetic North is shown by the arrow on the next ring in.  The magnetic ring will give you course information that coincides with your compass (magnetic readings).  Traveling north using your compass will coincide with the north arrow on the second ring in. 
My feeling is that since I steer using compass courses, I am going to always use the magnetic ring on the compass rose to determine courses and do my navigation.
The third ring in shows "compass points" from the old seafaring days.
Take a close look at the magnetic circle on your chart.  Magnetic north is shown by the arrow at 0 degrees.  East is at 90 degrees.  South is at 180 degrees.  And West is shown at 270 degrees.  Between zero and 30 degrees you will see two longer lines with four shorter lines between longer lines.  The shorter lines indicate 2 degrees.  The longer lines indicate 10 degree increments.  So, a course of 14 degrees would be seen on the second short line following the first long line to the right of zero.
One of the interesting things that you can easily do with a compass rose is to determine reciprocal courses... courses in exactly the opposing direction from the one you are traveling.  The reciprocal of 90 is 270.  To find a reciprocal, find the course you are on and draw a line through the middle cross to the opposite side of the circle.  The reciprocal of 20 is 200 .... and so on.
Have fun with your navigating.

 Barnacle Bill Holcomb
509 9933214
barnaclebillholcomb@gmail.com