DETECTION TECHNIQUES
When the archeologist relies on ground reconnaissance for the detection
of archeological sites, he is limited to sites which are (1) small enough to
comprehend on the ground from visible remains, (2) accessible within practical
and economical limits, (3) still visible in spite of modern-day cultivation and
construction, and (4) recognizable, even though the erosional effects of nature
may have been operating over a long period of time.
Fortunately, aerial discovery techniques are not as severely limited by
the foregoing conditions. Evidence of past landscapes which are too large to
be comprehended from the ground, or which may have been incorporated into the
present landscape and thus gone unrecognized, are often detectable on some
form of aerial imagery. And the advantage of a greater range and vertical
viewpoint, as depicted on an aerial view, helps in understanding the pattern
of things that are seen but not understood on the ground.
For example, subtle suggestions of buried landscapes are sometimes revealed
on conventional aerial photographs by shadow patterns, variations in soil color
ation, or differences in the height, density, or color of the plants that grow
above the buried features. And some remote sensors operate outside the visible
portion of the electromagnetic spectrum, thus providing the archeologist with
"another eye into the past." For example, thermal infrared scanners react to
emitted, rather than reflected energy. These images record variations in heat
waves emitted by the earth's surface. The structure of rock and soil beneath
the surface affects the emissivity of these heat waves, and may be revealed on
the infrared image.
Shadow Marks
Shadow marks are site indicators produced by the sun's rays falling ob
liquely on minor terrain configurations or irregularities. Such surface irregu
larities may have been caused by soil accumulations on the ground, by mounds
of earth that resulted from older structures, or by buried remains of archeo
logical value. Old earthworks, unnoticed banks and ditches, and other charac
teristics of previous landscapes may sometimes be discernible from shadow marks
on aerial photographs—even though such features are virtually invisible to a
ground observer.
Shadow marks dénote variations in surface relief through contrasting tones
of shadows, normal photographic tones, and highlighted areas. Since an oblique
sun angle is ordinarily required to produce good shadow marks, photographic
flights should be planned for early mornings or late afternoons, and preferably
during the winter months in regions of higher latitude.
The direction as well as the altitude of the sun at the time of photography
are important for producing diagnostic shadow marks. When the sun's rays are
parallel to a bank or ditch, the tone on the photograph may be perfectly uniform
and non-revealing. For the best contrast between light and shadow, the sun's
rays should form an angle as close to 90 degrees as possible with suspected
linear relief features. In planning exploratory flights, it may therefore be
necessary to photograph an area from two or more flight-line orientations.
