Relief (Hypsography)

CHAPTER ONE

T-6A NAVIGATION

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.

Relief (Hypsography)

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!

1-8

CHART LEGEND REVIEW