Sunday, March 22, 2009

Basic Chart Plotting

Plotting Directions
Once you have determined a direction by compass, pelorus, or some other means, you have to plot it on your chart. There are several ways and instruments for doing this, and they are the same tools used for determining direction from a chart.

Course Plotters
These are clear plastic, usually rectangular, that have one or more semi-circular angular scales marked on them. The center of the scales is at or near the center of one of the longer sides of the plotter and usually has a small circle or bull's eye. Plotters normally have two main scales, one from 000° to 180° and the other from 180° to 360° each calibrated in degrees. There can also be smaller auxiliary scales that are offset 90 degrees from the main scales. Lines are marked on the plotter parallel to the longer sides.

How to use Course Plotters
To determine the direction of a course or bearing from a given point. Place the plotter on the chart so that one of its longer sides is along your course or bearing line, and slide the plotter until the bull's eye is over a meridian (longitude lines running north / south). Read the true direction on the scale where it is intersected by the meridian. Easterly courses are read on the scale that reads from 000° to 180° and westerly courses on the other main scale. If it is more convenient you, find your plotted course or bearing with one of the plotter's marked parallel lines rather than its edge. It is not a must that you actually draw in the line connecting the two points, the plotter can be aligned using only the two points concerned but you will usually find it easier and safer to draw in the connecting line.

When the direction to be measured is within 20 degrees or so of due north or south, it might be harder to reach a meridian by sliding the course plotter across the chart. The small inner auxiliary scales have been included on the plotter for these cases. Slide the plotter until the bull's eye intersects a parallel of latitude (east / west) line. The intersection of this line using the proper auxiliary scale indicates the direction of the course or bearings. To plot a specified direction course or bearing from a given point. Put a pencil on your starting point, keep one of the longer edges of the course plotter snug against your pencil, and slide the plotter around until the center bull's eye and the desired mark on the appropriate main scale both lie along the same meridian. With the plotter positioned draw your direction from your starting point.

You can also first position the plotter using the bull's eye and scale markings without using the origin point, then slide the plotter up or down the meridian until one of the longer edges is over your starting point and then draw in your direction line. For directions near north or south, use one of the small auxiliary scales on a parallel of latitude. To extend a line that will be longer than the length of the course plotter. Place your dividers, opened to three or four inches, tightly against the edge of the plotter and then slide the plotter along using the divider points as guides. Draw in the extension of the course or bearing line after the plotter has been advanced. To draw a new line parallel to an existing course or bearing line. Use the parallel lines marked on the course plotter as guides.

Course Protractors
Some people like to use a course protractor as their main plotting tool. This instrument is not as easy to use as the course plotter, but you can get the same results. To measure the direction of a course or bearing. Place the center of the course protractor on the chart exactly over the your starting point, such as your boat's position or an aid to navigation. Then swing the protractor's arm around to the nearest compass rose on the chart, making the upper edge of the arm, which is in line with the center of the compass part of the course protractor pass directly over the center of the compass rose. Holding the course protractor arm in this position, turn the compass part of the protractor around until the arm's upper edge cuts across the same degree marking of the protractor compass as it does at the compass rose. The compass and the rose are now parallel. Holding the protractor compass tight against the chart, move the protractor arm around until its edge cuts across the second point of your course or bearing. You can now read the direction in degrees directly from the protractor compass scale. To lay off a line in a given direction from the given point. Line up the protractor rose with the chart's compass rose. Then rotate the arm until the desired direction is shown on the compass scale, and draw in the line, extend the line back to the starting point.

Parallel Rulers
One of the traditional instrument for measuring and plotting directions on charts is a set of parallel rulers. Parallel rulers can be made of black clear transparent plastic. The two rulers are connected by linkages that keep their edges parallel. To measure the direction of a line, line up one ruler with the desired objects on the chart and then walk the pair across the chart to the nearest compass rose by alternately holding one ruler and moving the other. To plot a line of direction, reverse the process, start at the compass rose and walk to the desired origin point. Make sure you push down slightly so the rulers do not slip.

Drawing Triangles
You can also use a pair of ordinary plastic drawing triangels for transferring a direction from one part of a chart to another, but only for short distances. The two triangles need not be similar in size or shape. Place the two longest sides together, and line up one of the other sides of one triangle with the course or bearing line, or with your desired direction at the compass rose. Hold the other triangle firmly in place as a base, and slide the first one along its edge carrying the specified line to a new position while maintaining its direction. If you have to, alternately slide and hold the triangles for moving longer distances.

Distance is measured on a chart with a pair of dividers. Open the two arms and the friction at the pivot is good enough to hold the separation between the points. Most dividers have some means for adjusting this friction, it should be enough to hold the arms in place, but not so much as to make opening or closing hard. A special type of dividers has a center cross piece which can be rotated by a knurled knob to set and maintain the opening between the arms, the distance between the points cannot accidentally change. These type of dividers are useful if kept set to some standard distance, such as one mile to the scale of the chart you are using.

To measure distance with dividers, first open them to the distance between two points on the chart, then transfer them without change to the chart's latitude scale. Note that the zero point on this scale is not at the left-hand end, but is one basic unit up the scale. This unit to the left of zero is more finely divided than are the remaining basic units. To measure any distance, set the right hand point of the dividers on the basic unit mark so that the left hand point falls somewhere on the divided unit.

If the distance on the chart cannot be spanned with the dividers opened wide, 60 degrees is the maximum practical opening, set them at a convenient opening for a whole number of units on the scale or latitude subdivisions, step this off the number of times, then measure the odd remainder. The total distance is then the sum of the parts stepped off and measured separately. To mark off a desired distance on the chart, set the right point of the dividers on the nearest lower whole number of units, and the left point on the remaining fractional part of a unit measured leftward from zero on the scale. The dividers are now properly set for the specified distance at the scale of the chart being used and can be applied to the chart. If the distance is too great for one setting of the dividers, step it off in increments.