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1.22. Choice of Projection. The ideal chart projection would portray the features of the earth in their
true relationship to each other; that is, directions would be true and distances would be represented at a
constant scale over the entire chart. This would result in equality of area and true shape throughout the
chart. Such a relationship can only be represented on a globe. On a flat chart, it is impossible to preserve
constant scale and true direction in all directions at all points, nor can both relative size and shape of the
geographic features be accurately portrayed throughout the chart. The characteristics most commonly
desired in a chart projection are conformality, constant scale, great circles as straight lines, rhumb lines
as straight lines, true azimuth, and geographic position easily located.
1.22.1. Conformality. Conformality is very important for air navigation charts. For any projection to be
conformal, the scale at any point must be independent of azimuth. This does not imply, however, that the
scale at two points at different latitudes will be equal. It means the scale at any given point will, for a
short distance, be equal in all directions.
1.22.1.1. For conformality, the outline of areas on the chart must conform in shape to the feature being
portrayed. This condition applies only to small and relatively small areas; large land masses must
necessarily reflect any distortion inherent in the projection.
1.22.1.2. Finally, since the meridians and parallels of earth intersect at right angles, the longitude and
latitude lines on all conformal projections must exhibit this same perpendicularly. This characteristic
facilitates the plotting of points by geographic coordinates.
1.22.2. Constant Scale. The property of constant scale throughout the entire chart is highly desirable but
impossible to obtain, as it would require the scale to be the same at all points and in all directions
throughout the chart.
1.22.3. Straight Line. The rhumb line and the great circle are the two curves that a navigator might wish
to have represented on a map as straight lines. The only projection that shows all rhumb lines as straight
lines is the Mercator. The only projection that shows all great circles as straight lines is the gnomonic
projection. However, this is not a conformal projection and cannot be used directly for obtaining
direction or distance. No conformal chart will represent all great circles as straight lines.
1.22.4. True Azimuth. It would be extremely desirable to have a projection that showed directions or
azimuths as true throughout the chart. This would be particularly important to the navigator, who must
determine from the chart the heading to be flown. There is no chart projection representing true great
circle direction along a straight line from all points to all other points.
1.22.5. Coordinates Easy to Locate. The geographic latitudes and longitudes of places should be easily
found or plotted on the map when the latitudes and longitudes are known.
38 AFPAM11-216 1 MARCH 2001
Section 1F— Classification of Projections
1.23. Introduction. Chart projections may be classified in many ways. In this pamphlet, the various
projections are divided into three classes according to the type of developable surface to which the
projections are related. These classes are azimuthal, cylindrical, and conical.
1.24. Azimuthal Projections. An azimuthal or zenithal projection is one in which points on the earth are
transferred directly to a plane tangent to the earth. According to the positioning of the plane and the
point of projection, various geometric projections may be derived. If the origin of the projecting rays
(point of projection) is the center of the sphere, a gnomonic projection results. If it is located on the
surface of the earth opposite the point of the tangent plane, the projection is a stereographic, and if it is at
infinity, an orthographic projection results. Figure 1.20 shows these various points of projection.
Figure 1.20. Azimuthal Projections.
1.24.1. Gnomonic Projection. All gnomonic projections are direct perspective projections. Since the
plane of every great circle cuts through the center of the sphere, the point of projection is in the plane of
every great circle. This property then becomes the most important and useful characteristic of the
gnomonic projection. Each and every great circle is represented by a straight line on the projection. A
complete hemisphere cannot be projected onto this plane because points 90o from the center of the map
project lines parallel to the plane of projection. Because the gnomonic is nonconformal, shapes or land
masses are distorted, and measured angles are not true. At only one point, the center of the projection,
　
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