navigation - page 7

6.7 Instruments for Plotting

An increasing number of mariners today rely on automated tools such as GPS for navigation and getting from one point to another. Indeed, a good marine GPS will not only replicate a chart's details, but can aid in determining course, distance, elapsed time, and position. Nevertheless, a user of any form of technology should know something about what's happening "under the hood." In that spirit, this section introduces some of the hand tools used to manually perform routine navigational tasks using a nautical chart.

Parallel Rule

One of the tools most commonly associated with navigation is the parallel ruler (photo 6.7.1). A parallel ruler is most commonly used to determine the heading on the compass rose from a line of direction from a chart (or vice versa). The "arms" connecting the upper and lower part of the ruler allow it to be moved across the chart without changing its orientation; this is called "walking" the ruler.

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Photo 6.7.1

Dividers  
The second tool most commonly associated with plotting and navigation is a pair of dividers (photo 6.7.2). Dividers are used to mark the distance between two points on the chart or to mark points on the chart at which to place a ruler, parallel ruler, or roll plotter.


Note that the dividers have two points at the end. Replacing one point with a pencil or lead marker turns this into a compass. 

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Photo 6.7.2