Chart work should be an accurate and graphic picture of the progress of the aircraft from departure to destination and, with the log, should serve as a complete record of the flight. Thus, it also follows that the navigator must be familiar with and use accepted standard symbols and labels on charts. [Figure 4-1] See Appendix A for additional chart and navigation symbols.
Several terms have been mentioned in earlier portions of this handbook. Precise definitions of these terms must now be understood before the mechanics of chart work are learned.
- True Course (TC)—the intended horizontal direction of travel over the surface of the earth, expressed as an angle measured clockwise from true north (000°) through 360°.
- Course Line—the horizontal component of the intended path of the aircraft comprising both direction and magnitude or distance.
- Track—the horizontal component of the actual path of the aircraft over the surface of the earth track may, but very seldom does, coincide with the TC or intended path of the aircraft. The difference between the two is caused by an inability to predict perfectly all inflight conditions.
- True Heading (TH)—the horizontal direction in which an aircraft is pointed. More precisely, it is the angle measured clockwise from true north through 360° to the longitudinal axis of the aircraft. The difference between track and TH is caused by wind and is called drift.
- Groundspeed (GS)—the speed of the aircraft over the ground. It may be expressed in nautical miles (NM), statute miles (SM), or kilometers (km) per hour, but, a navigator uses NM per hour (knots).
- True Airspeed (TAS)—the rate of motion of an aircraft relative to the airmass surrounding it. Since the airmass is usually in motion in relation to the ground, airspeed and GS seldom are the same.
- Dead Reckoning Position (DR Position)—a point in relation to the earth established by keeping an accurate account of time, GS, and track since the last known position. It may also be defined as the position obtained by applying wind effect to the TH and TAS of the aircraft.
- Fix—a position determined from terrestrial, electronic, or astronomical data.
- Air Position (AP)—the location of the aircraft in relation to the airmass surrounding it. TH and TAS are the components of the vector used to establish an AP.
- Most Probable Position (MPP)—a position determined with partial reference to a DR position and partial reference to a fixing aid.
A fine-tipped pencil, a good pair of dividers, and a plotter are imperative for accurate chart work.
Dividers should be manipulated with one hand, leaving the other free to use the plotter, pencil, or chart as necessary. Some navigation dividers have a tension screw that can be adjusted to prevent the dividers from becoming either too stiff or too loose for convenient use. Adjust the points of the dividers to approximately equal length. A small screwdriver, required for these adjustments, should be a part of the navigator’s equipment.
A common plotter is shown in Figure 4-2. This plotter is a semicircular protractor with a straight edge attached to it. A small hole at the base of the protractor portion indicates the center of the arc of the angular scale. Two complete scales cover the outer edge of the protractor and are graduated in degrees. An abbreviated inner scale measures the angle from the vertical. [Figures 4-3 and 4-4] The angle measured is the angle between the meridian and the straight line. The outer scale is used to read all angles between north through east to south, and the inner scale is used to read all angles between south through west to north.
Plotting Procedures for Mercator Charts
Many charts and plotting sheets are printed on the Mercator projection. Before starting any plot, note the scale and projection of the chart and check the date to make sure that it is the latest edition. The latitude scale is used to represent NM. The longitude scale should never be used to measure distance. Some charts carry a linear scale in the margin, and, where present, it indicates that the same scale may be used anywhere on the chart.
On most Mercator charts, the spacing between meridians and parallels is widely spaced, necessitating the use of dividers. There are several methods by which positions can be plotted on Mercator charts. [Figure 4-5] Place the straight edge of the plotter in a vertical position at the desired longitude. Set the dividers to the desired number of minutes of latitude. Hold one point against the straight edge on the parallel of latitude corresponding to the whole degree of latitude given. Let the other point also rest against the straight edge and lightly prick the chart. This marks the desired position. In measuring the latitude and longitude of a position already plotted, reverse the procedure.
Plotting and Measuring Courses:
Step 1—Plot departure and destination on the chart. [Figure 4-6]
Step 2—Draw the course line between the two points. If they are close together, the straight edge of the plotter can be used. If they are far apart, two plotters can be used together or a longer straight edge can be used. If none of these methods is adequate, fold the edge of the charts so that the fold connects the departure and destination points, and make a series of pencil marks along the edge. A plotter or straight edge can then be used to connect the points where the chart is unfolded. After the course line has been plotted, the next step is to determine its direction.
Step 3—Place the points of the dividers, or a pencil, anywhere along the line to be measured.
Step 4—Place the plotter against the dividers.
Step 5—Slide the plotter until the center hole is over the midmeridian. Make a mental estimate of the approximate direction to avoid obtaining a reciprocal course.
Step 6—Read TC on the protractor at the meridian. Using the midmeridian gives an average TC for the leg.