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For example, the division is at 70o of latitude for the JN series at 80o of latitude for the Operational
Navigation Chart (ONC) series charts.
Section 14C— USAF Grid Overlay
14.6. Basics. The graticule of the USAF Grid Overlay eliminates the problem of converging meridians
(Figure 14.2). It is a square grid and, though its meridians are aligned with grid north (GN) along the
Greenwich meridian, they do not converge at GN. While the USAF Grid Overlay can be superimposed
on any projection, it is most commonly used with the polar stereographic (for flights in polar areas) and
the Lambert conformal (for flights in subpolar areas). This is because a straight line on these projections
approximates a great circle. As the great circle course crosses the true meridians, its true direction
changes but its grid direction remains constant (Figures 14.3 and 14.4). All grid meridians are parallel to
the Greenwich meridian and TN along the Greenwich meridian is the direction of GN over the entire
chart.
14.7. Relationship of Grid North (GN) to True North (TN). Because grid meridians are parallel to the
Greenwich meridian, the aircraft longitude and the convergence factor (CF) of the chart govern the angle
between GN and TN.
298 AFPAM11-216 1 MARCH 2001
Figure 14.2. USAF Grid Overlay.
Figure 14.3. Great Circle True Direction Changes.
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Figure 14.4. Great Circle True Direction Is Constant.
14.7.1. CF of 1.0. Figure 14.5 shows that charts having CFs of 1.0 display GN to TN relationship as a
direct function of longitude. In the Northern Hemisphere at 30o W, GN is 30o W of TN; at 60o W, GN is
60o W of TN. Similarly, at 130o E longitude, GN is 130o E of TN. In the Southern Hemisphere, the
direction of GN with respect to TN is exactly opposite.
Figure 14.5. Correction for Moon's Parallax.
14.7.2. CF of Less than 1.0. Figure 14.6 shows a chart with a CF of less than 1.0 with a USAF Grid
Overlay superimposed on it. The relationship between GN and TN on this chart is determined in the
same manner as on charts with a CF of 1.0. On charts with a CF of less than 1.0, the value of the
convergence angle at a given longitude is always smaller than the value of longitude and is equal to the
CF times the aircraft longitude.
300 AFPAM11-216 1 MARCH 2001
Figure 14.6. Grid Overlay Superimposed on Lambert Conformal (Convergence Factor 0.785).
14.8. Relationship of Grid Direction to True Direction. Use the following formulas to determine grid
direction.
In the Northern Hemisphere:
Grid direction = true direction + west longitude x convergence factor
Grid direction = true direction – east longitude x convergence factor
In the Southern Hemisphere:
Grid direction = true direction – west longitude x convergence factor
Grid direction = true direction + east longitude x convergence factor
14.9. Polar Angle. Polar angle is used to relate true direction to grid direction. Polar angle is measured
clockwise through 360o from GN to TN. It is simple to convert from one directional reference to the
other by use of the formula grid direction = true direction + polar angle. To determine polar angle from
convergence angle (CA), apply the following formulas.
In the Northwest and Southeast Quadrants, Polar angle = convergence angle
In the Northeast and Southwest Quadrants, Polar angle = 360o – convergence angle
14.10. Chart Transition. Since the relationship of the true meridians and the grid overlay on subpolar
charts differs from that on polar charts because of different CFs, the overlays do not match when a
transition is made from one chart to the other. Therefore, the grid course (GC) of a route on a subpolar
chart will be different than the GC of the same route on a polar chart. The chart transition problem is
best solved during flight planning:
14.10.1. Select a transition point common to both charts.
14.10.2. Measure the subpolar GC and the polar GC.
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14.10.3. Compute the difference between the GCs obtained in 14.10.2. This is the amount the compass
pointer must be changed at the transition point. NOTE: If the GC on the first chart is smaller than the
GC on the second chart, add the GC difference to the directional gyro (DG) reading and reposition the
DG pointer; if the GC on the first chart is larger, subtract the GC difference.
EXAMPLE: Chart transition from a subpolar to a polar chart. GC on subpolar chart is 316o. GC on
polar chart is 308o. GC difference is 8o. Gyro reading (grid heading [GH]) is 320o. The transition is
from a larger GC to a smaller GC; therefore, the GC difference (8o) is subtracted from the GH value
read from the DG (320o). The DG pointer is then repositioned to the new GH (312o).
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