annotated with various names, numbers, boundaries, and reference grids. Often, those using
charts assume the misrepresentation which is inherent in charts is due to errors on the part of the
cartographer resulting in a distrust of charts. Such mistrust is not warranted, it simply reflects a
lack of understanding of the schematic nature of aeronautical charts.
The discrepancies that exist between the real world and its representation on aeronautical charts
are largely the results of limitations imposed by scale. To portray the world on the TPC, the
cartographer must shrink the charted area to one half-millionth of its true size. If the
cartographer were simply to reduce all features of the earth's surface equally to 1:500,000 scale,
only the largest features would be visible. Roads, small towns, towers, power lines, streams,
railroads, bridges, and many other features would be too small to see. The aeronautical chart
would look like the familiar photographs of the earth taken from orbiting satellites.
Consequently, the scale factor of an aeronautical chart refers primarily to the location of one
object to another. For example, a tower one mile north of a primary road will have that distance
accurately displayed on the chart.
However, the sizes of symbols do not always conform to the true scale. For example, the width
of a two-lane road on the TPC is shown at about 20 times its true scale, otherwise (if drafted
exactly to half-millionth scale) the charted road would be only about 0.0005 inch wide. The
symbol for a lookout tower is about 1/2 nautical mile in diameter to scale on the TPC even
though its counterpart in the real world is much smaller. A Tactical Air Navigation (TACAN)
station is almost a mile wide as portrayed at TPC scale. The open circle denoting a minor airport
has a diameter of 4000 feet to scale on the TPC regardless of the actual size of the field.
Standard symbols are used for easy identification of information portrayed on aeronautical charts.
The variance in symbols between various charts projections is slight. Standard symbols are easy
to identify. Chart legends (Figures 1-4, 1-5, and 1-6) explain the meaning of the relief, culture,
hydrography, vegetation, and aeronautical symbols. You must be completely familiar with all
symbols contained in the ONC and TPC legends.
Chart relief shows the physical feature related to the differences in elevation of the land surface.
These include features such as mountains, hills, plateaus, plains, depressions, etc. Standard
symbols and shading techniques are used in relief portrayal on charts. These include contours,
spot elevations, variations in tint, and shading to represent "shadows."
The highest terrain on each ONC/TPC chart is identified by location (latitude and longitude) and
elevation above mean sea level (MSL) in the chart legend.
The relief of the terrain is emphasized on charts by a system of gradient tints. They are used to
designate areas within certain elevation ranges by different color tints. Green color indicates flat
or relatively level terrain regardless of altitude above sea level. The green color does not indicate
vegetation on either chart!
CHART LEGEND REVIEW