Aerial Photography
Introduction
Aerial photography has two uses that are of interest within the context of this course:
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Cartographers and planners take detailed measurements from aerial photos in the
preparation of maps. |
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Trained interpreters utilize aerial photos to determine
e.g. land-use
and environmental conditions, among other things. |
Although both maps and aerial photos present a "bird's-eye" view of the earth,
aerial photographs are NOT maps. Maps are orthogonal representations of the earth's
surface, meaning that they are directionally and geometrically accurate (at least within
the limitations imposed by projecting a 3-dimensional object onto 2 dimensions). Aerial
photos, on the other hand, display a high degree of radial distortion. That is, the
topography is distorted, and until corrections are made for the distortion, measurements
made from a photograph are not accurate. Nevertheless, aerial photographs are a powerful
tool for studying the earth's environment.
Because most GISs can correct for radial distortion, aerial photographs are an excellent
data source for many types of projects, especially those that require spatial data from
the same location at periodic intervals over a length of time. Typical applications
include land-use surveys and habitat analysis.
This unit discusses benefits of aerial photography, applications, the different types of
photography, and the integration of aerial photographs into GISs.
Basic Elements of Air Photo Interpretation
Novice photo interpreters often encounter difficulties when presented with their first
aerial photograph. Aerial photographs are different from "regular" photos in at
least three important ways:
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Objects are portrayed from an overhead (and unfamiliar) position. |
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Very often, infrared wavelengths are
recorded. |
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Photos are taken at scales most people are unaccustomed to seeing. |
These "basic elements" can aid in identifying objects on aerial photographs.
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Tone (also called Hue or Color) -- Tone refers to the relative
brightness or color of elements on a photograph. It is, perhaps, the most basic of the
interpretive elements because without tonal differences none of the other elements could
be discerned. |
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Size -- The size of objects must be considered in the context of
the scale of a photograph. The scale will help you determine if an object is a stock pond
or Lake Minnetonka. |
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Shape -- refers to the general outline of objects. Regular
geometric shapes are usually indicators of human presence and use.
| Some objects can be identified almost solely on the basis of their shapes. |
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The Pentagon Building |
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(American) football fields |
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Cloverleaf highway interchanges |
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Texture -- The impression of "smoothness" or
"roughness" of image features is caused by the frequency of change of tone in
photographs. It is produced by a set of features too small to identify individually.
Grass, cement, and water generally appear "smooth", while a forest canopy may
appear "rough". |
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Pattern (spatial arrangement) -- The patterns formed by objects
in a photo can be diagnostic. Consider the difference between (1) the random pattern
formed by an unmanaged area of trees and (2) the evenly spaced rows formed by an orchard. |
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Shadow -- Shadows aid interpreters in determining the height of
objects in aerial photographs. However, they also obscure objects lying within them. |
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Site -- refers to topographic or geographic location. This
characteristic of photographs is especially important in identifying vegetation types and
land forms. For example, large circular depressions in the ground are readily identified as
sinkholes in central Florida, where the bedrock consists of limestone. This identification
would make little sense, however, if the site were underlain by granite. |
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Association -- Some objects are always found in association with
other objects. The context of an object can provide insight into what it is. For instance,
a nuclear power plant is not (generally) going to be found in the midst of single-family
housing.
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Advantages of Aerial Photography over Ground-Based Observation:
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Aerial photography offers an improved vantage point. |
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Aerial photography has the capability to stop action. |
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Aerial photography provides a permanent recording. |
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Aerial photography
has broader spectral sensitivity than the human eye. |
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Aerial photography
has better spatial resolution and geometric fidelity than many ground-based
sensing methods. |
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