Plotting Procedures (Part Two)

Plotting Course From Given Position

A course from a given position can be plotted quickly in the following manner. Place the point of a pencil on the position and slide the plotter along this point, rotating it as necessary, until the center hole and the figure on the protractor representing the desired direction are lined up with the same meridian. Hold the plotter in place and draw the line along the straight edge. [Figure 4-7]

Figure 4-7. Plotting course from given direction.

Figure 4-7. Plotting course from given direction. [click image to enlarge]

Measuring Distance

One of the disadvantages of the Mercator chart is the lack of a constant scale. If the two points between which the distance is to be measured are approximately in a northsouth direction, and the total distance between them can be spanned, the distance can be measured on the latitude scale opposite the midpoint. However, the total distance between any two points that do not lie approximately north or south of each other should not be spanned unless the distance is short.


In the measurement of long distances, select a midlatitude lying approximately halfway between the latitudes of the two points. By using dividers set to a convenient, reasonably short distance, such as 60 NM picked off at the midlatitude scale, determine an approximate distance by marking off units along the line to be measured. [Figure 4-8]

Figure 4-8. Midlatitude scale.

Figure 4-8. Midlatitude scale. [click image to enlarge]

The scale at the midlatitude is accurate enough if the course line does not cover more than 5° of latitude (somewhat less in high latitudes). If the course line exceeds this amount or if it crosses the equator, divide it into two or more legs and measure the length of each leg with the scale of its own midlatitude.

Plotting Procedures for Lambert Conformal and Gnomonic Charts

Plotting Positions

On a Lambert conformal chart, the meridians are not parallel as on a Mercator chart. Therefore, plotting a position by the method described under Mercator charts may not be accurate. On small scale charts, or where there is marked convergence, the plotter should intersect two graduated parallels of latitude at the desired longitude rather than parallel to the meridian. Then, mark off the desired latitude with dividers. On a large scale chart, the meridians are so nearly parallel that this precaution is unnecessary. The scale on all parts of a Lambert conformal chart is essentially constant. Therefore, it is not absolutely necessary to pick off minutes of latitude near any particular parallel except in the most precise work.


Plotting and Measuring Courses

Any straight line plotted on a Lambert conformal chart is approximately an arc of a great circle. On long distance flights, this feature is advantageous since the great circle course line can be plotted as easily as a rhumb line on a Mercator chart.

Figure 4-9. Use midmeridian to measure course on a Lambert conformal.

Figure 4-9. Use midmeridian to measure course on a Lambert conformal. [click image to enlarge]

However, for shorter distances where the difference between the great circle and rhumb line is negligible, the rhumb line is more desirable because a constant heading can be held. For such distances, the approximate direction of the rhumb line course can be found by measuring the great circle course at midmeridian. [Figure 4-9] In this case, the track is not quite the same as that indicated by the course line drawn on the chart, since the actual track (a rhumb line) appears as a curve convex to the equator on a Lambert conformal chart, while the course line (approximately a great circle) appears as a straight line. Near midmeridian, the two have approximately the same direction (except for very long distances) along an oblique course line. [Figure 4-10]

Figure 4-10. At meridian, rhumb line and great circle have approximately the same direction.

Figure 4-10. At meridian, rhumb line and great circle have approximately the same direction. [click image to enlarge]

For long distances involving great circle courses, it is not possible to change heading continually, as is necessary when following a great circle exactly, and it is customary to divide the great circle into a series of legs, each covering about 5° of longitude. The direction of the rhumb line connecting the ends of each leg is found at its midmeridian.

Measuring Distance

As previously stated, the scale on a Lambert conformal chart is practically constant, making it possible to use any part of a meridian graduated in minutes of latitude to measure NM.


Plotting on a Gnomonic Chart

Gnomonic charts are used mainly for planning great circle routes. Since any straight line on a gnomonic chart is an arc of a great circle, a straight line drawn from the point of departure to destination gives a great circle route. Once obtained, this great circle route is transferred to a Mercator chart by breaking the route into segments. [Figure 4-11]

Figure 4-11. Transferring great circle route from gnomonic to mercator chart.

Figure 4-11. Transferring great circle route from gnomonic to mercator chart. [click image to enlarge]

Plotting Hints

The following suggestions should prove helpful in developing good plotting procedures:

  1. Measure all directions and distances carefully. Doublecheck all measurements, computations, and positions.
  2. Avoid plotting unnecessary lines. If a line serves no purpose, erase it.
  3. Keep plotting equipment in good working order. If the plotter is broken, replace it. Keep sharp points on dividers. Use a sharp pencil and an eraser that does not smudge.
  4. Draw light lines at first, as they may have to be erased. When the line has been checked and proven to be correct, then darken it if desired.
  5. Label lines and points immediately after they are drawn. Use standard labels and symbols. Letter the labels legibly. Be neat and exact.