Figuring distances on charts

This is the next post in this series of on-the-go navigation.   Previously we prepared a chart with magnetic variation lines.   You can see it in this post.

One of the most useful aspects of these prepared charts for on-the-go navigation is the ability to estimate distances to travel.    Most compasses come with a string lanyard.   If you *don't* have a string lanyard on your compass, put one on.   For me, I put on a spare pair of boot laces, so if I break a bootlace, I have a ready swap.

If you want to figure out the distance of a path, lay out the string along the path you want to take.   In the figure below, I show such a path laid out on a chart of the Boston Harbor.   In this case, I'm trying to estimate how long it takes to go from Deer Island Pier in Winthrop to Little Brewster Island, which is the site of the Boston Lighthouse.   It also takes the paddler through part of the original passage out of the Harbor.

In order to find this out, take the string from the lanyard and snake it through the path you want to take. You may have to lay it out into segments by holding the string taught, but keep the overall length of string as an accurate representation of the path.

String snaked around the desired route to be taken.   In this case, it's from Deer Island Pier in Winthrop to Little Brewster Island. 

Now that you have the path laid out, hold the two ends of the string taut and extend the string straight and perpendicular to the lines spaced at intervals of one nautical mile.   This is shown below.   The human eye is fairly good at visually subdividing intervals.   In this case, the length of the stretched out line is 4.6 nautical miles. 

Hold the string taut and perpendicular to the magnetic declination lines spaced out by one nautical mile.   In the case of the path from Deer Island Pier to Little Brewster Island, this turns out to be 4.6 nautical miles.

As a final step, you probably are the most interested in how long a paddle it will be.   Most people figure on 3 knots (3 nautical miles per hour).  4.6 nautical miles translates into about an hour and thirty minutes, give or take.   

Next:  finding bearings on a chart or map.