Air photo




 


Aerial Photography

Introduction


Aerial photography has two uses that are of interest within the context of this course:

Cartographers and planners take detailed measurements from aerial photos in the preparation of maps.
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:

Objects are portrayed from an overhead (and unfamiliar) position.
Very often, infrared wavelengths are recorded.
Photos are taken at scales most people are unaccustomed to seeing.

These "basic elements" can aid in identifying objects on aerial photographs.

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.
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.
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.
The Pentagon Building
(American) football fields
Cloverleaf highway interchanges
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".
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.
Shadow -- Shadows aid interpreters in determining the height of objects in aerial photographs. However, they also obscure objects lying within them.
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.
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. 


Advantages of Aerial Photography over Ground-Based Observation:

Aerial photography offers an improved vantage point.
Aerial photography has the capability to stop action.
Aerial photography provides a permanent recording.
Aerial photography has broader spectral sensitivity than the human eye.
Aerial photography has better spatial resolution and geometric fidelity than many ground-based sensing methods.

 

National
Environmental
Research
Institute

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This page was updated: 19. September 1999
These pages are maintained by Michael Stjernholm, NERI

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