Geology Reference
In-Depth Information
TABLE 2.2 Aspects of Features That Aid in Their Recognition on Aerial Photographs
(modified after Avery & Berlin, 1992)
Aspects of Features on
Photographs
Characteristics of Features Helpful for Their Identification
Shape
Natural features, such as rivers, faults, coastlines, or volcanoes, tend to be irregular in shape.
Human constructed features, such as roads or buildings, tend to have regular geometric shapes.
Agricultural fields are typically rectangular. Circular crop patterns occur where center-pivot
irrigation systems are used.
Size
Some features, such as homes or highways, have approximate sizes familiar to most observers.
Where such features can be located on a photograph, they can be used as a basis to infer the
sizes of other features, such as rivers or landslides.
Pattern
Some geologic features, such as landslides, have characteristic patterns. For landslides, these
patterns commonly include a steep scarp at the top, and a lower area of displaced soil, rocks,
vegetation (trees are especially easy to see), or human structures. Refer to introductory material
in many exercises for descriptions of typical features related to geologic hazards. Unmodified
drainage patterns can reveal much about the underlying geology of an area.
Shadow
Taller objects will cast longer shadows when sun angles are low. A high sun angle, such as
directly overhead of the feature being observed, will create only a small shadow. Low-sun-angle
photography, with enhanced shadows, is often helpful in identifying geologic features such as
linear fault scarps.
Tone (or color)
Water is dark gray or black except where sunlight is reflected. Soil moisture, which is controlled
by texture and type of soil, largely determines the difference in appearance of different fields on
air photos. Wetter areas generally appear as darker grays; dryer soils as lighter grays. Vegetation
accounts for a great many differences in pattern and shades of gray. Heavily forested areas are
usually medium to dark gray. Grasslands are light gray. Cultivated fields vary greatly in color tone
depending on whether or not just plowed, or if planted, the kind of crop and stage of growth.
Texture
Roughness or smoothness of features on aerial photographs can be helpful in identifying them.
The texture of a given feature on a particular photograph depends on the scale of the
photograph. For example, a fresh lava flow may appear rough when viewed closely, but will
appear smooth when viewed from farther away. Water will often be smooth; surf zones along
coasts may, when viewed closely, appear rough.
Association
When looking at some features, other features are often also found. Landslides may cause rivers
to bend or, if the landslide is large enough, a lake may be created. Linear faults often are zones
with more water and therefore greater vegetation. Active volcanoes may have fresh lava areas
where there is no vegetation.
Site
Thinking about typical sites for features can help identify objects on photos. Large shopping
malls are not typically found in rural areas. Major airports are located near big cities. Floodplains
are a natural part of river systems.
central portion of the photo directly beneath the cam-
era. The basic scale of a vertical air photo, overlooking
variations due to distortion near the edges of the photo
and due to relief, can be computed if the focal length of
the camera (the distance between the lens and the film)
and the altitude of the camera above the ground sur-
face are known.
The scale of a photograph can also be determined
by measuring distances between objects on the ground
and comparing them with distances measured
between the same objects on the photo. Section lines,
where available, are well suited for this purpose since
they are supposed to be one mile apart and are fre-
quently visible on photos as roads or fencelines. Scale
is computed using this equation:
Sc = Dph/Dgd
where Sc = the scale of the photo,
Dph = the distance on the photograph, and
Dgd = the distance on the ground.
