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Since the demand for variety in charts is so great and the properties of the projections vary greatly, there
is no one projection satisfying all navigation needs. The projection that most nearly answers all of the
navigator's problems is the Lambert conformal, and this projection is the one most widely used for
aeronautical charts. An aeronautical chart of some projection and scale can be obtained for any portion
of the earth.
1.27.2. Datums. Maps made by a given country traditionally use the datum created by that country.
There may be as many as a thousand of these various datums in use throughout the world. Inherent
problems result from over a hundred countries using widely different methods and standards to measure
coordinate systems. When added to the effects of local variations in topography and the gravity field,
systems are created that differ substantially from each other. These individualized datums are classified
as local or regional.
1.27.2.1. The Department of Defense adopted a datum in 1987 called World Geodetic System 84 (WGS
84). This global datum is a system that models the entire planet, instead of one small piece. WGS 84 is
used by NIMA for production of almost all new maps and charts. The purpose of such a system is to
minimize the confusion created by the proliferation of local datums. As long as all coordinates are stated
in WGS 84, combat interoperability problems will be minimized. In addition, WGS 84 positions may be
computed from Global Positioning System (GPS) equipment to an extreme level of precision by NIMA
surveyors, well under half a meter anywhere in the world. Widespread use of WGS 84 will virtually
eliminate problems due to different datums.
1.27.2.2. It is important to realize that every coordinate is related to a specific datum. A latitude and
longitude extracted from a WGS 84 chart is still a WGS 84 coordinate, and an MGRS point pulled from
that same chart is also WGS 84. However, a ground survey of that same point could have established a
local datum coordinate that is different from the map derived one by as much as a half mile. Always use
the same datum throughout your mission, or serious positional errors are possible.
1.28. Scale. Obviously, charts are much smaller than the area they represent. The ratio between any
given unit of length on a chart and the true distance it represents on the earth is the scale of the chart.
The scale will vary, and may vary greatly from one part of the chart to another. Charts are made to
various scales for different purposes. If a chart is to show the whole world and yet not be too large, it
must be drawn to small scale. If a chart is to show much detail, it must be drawn to a large scale; then it
shows a smaller area than does a chart of the same size drawn to a small scale. Remember: large area,
small scale; small area, large scale.
1.28.1. Aeronautical Chart. The scale of a chart may be given by a simple statement, such as "1 inch
equals 10 miles." This means a distance of 10 miles on the earth's surface is shown 1 inch long on the
chart. On aeronautical charts, the scale is indicated in one of two ways: representative fraction or graphic
scale.
1.28.2. Representative Fraction. The scale may be given as a representative fraction, such as 1:500,000
or 1/500,000. This means one of any unit on the chart represents 500,000 of the same unit on the earth.
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For example, 1 inch on the chart represents 500,000 inches on the earth. A representative fraction can be
converted into a statement of miles to the inch. Thus, if the scale is 1:1,000,000, 1 inch on the chart
stands for 1,000,000 inches or 1,000,000 divided by (6076 x 12) equaling about 13.7 NM. Similarly, if
the scale is 1:500,000, 1 inch on the chart represents about 6.86 NM. Thus, the larger the denominator of
the representative fraction, the smaller the scale.
1.28.3. Graphic Scale. The graphic scale may be shown by a graduated line. It usually is found printed
along the border of a chart. Take a measurement on the chart and compare it with the graphic scale of
miles. The number of miles the measurement represents on the earth may be read directly from the
graphic scale on the chart. The distance between parallels of latitude also provides a convenient scale for
distance measurement. One degree of latitude always equals 60 NM and 1 minute of latitude always
equals 1 NM.
1.29. DoD Charts and Flight Information Publications. The DoD NIMA publishes the National
Imagery and Mapping Agency Catalog of Maps, Charts and Related Products, which contains the latest
indexes of navigational and planning charts. This catalog also contains the most current list of Flight
Information Publications (FLIP) used by aircrews. This publication is available to aircrews in base
　
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