Aerial Photography
EB/ES/GE 351
James S. Aber

Introduction to Cameras and Film

Photography is one of the oldest and most versatile forms of remote sensing of the Earth's surface. All photographic cameras have certain basic components: lens, diaphragm, shutter, viewfinder, and image plane. Geometry of the lens and image plane determine the scene area focused onto the film or electronic detector. The diaphragm and shutter control the amount of light to expose each photograph.

Traditional film photography is based on the reaction to light of silver halide crystals, which undergo a chemical change when exposed to ultraviolet, visible or near-infrared radiation. This photochemical change can be chemically developed into a visible picture. The spectral sensitivity of conventional photography ranges from near-ultraviolet to near-infrared wavelengths (~0.3 µm to 0.9 µm). The lower limit is based on available ultraviolet energy and strong atmospheric scattering; film sensitivity determines the upper limit. Similar limits apply to digital cameras, which have largely replaced film for remote sensing.

Digital color-visible photograph. Sunken garden on the campus of Emporia State University in late summer. Note normal appearance of vegetation, flags, and other objects in the view. Photo © J.S. Aber.
Digital color-infrared photograph of same scene. Active vegetation appears red and pink. Also some synthetic fibers and dyes are highly reflective for near-infrared, as shown by the flags. Photo © J.S. Aber.

Different parts of the spectrum may be photographed by using various combinations of films and filters. Photographs are routinely taken in b/w panchromatic, b/w minus blue, b/w infrared, color-visible, color-infrared, and multiband types. For example, color-infrared images represent green, red, and near-infrared wavelengths, which are depicted as blue, green and red in the photograph (see above). This shifting of bands to visible colors is called false-color. Ultraviolet photography is also possible for special applications.

Aerial Photography

Aerial photographs may be taken in vertical, low-oblique or high-oblique positions; standard air photos are vertical views of the ground. Vertical photographs are normally acquired in overlapping pattern, so as to create a stereoscopic effect when adjacent pairs are viewed together. The overlapping views of the same ground area produce a parallax effect, which is also the basis for depth perception in human vision. This ability to perceive depth is quite useful for visual interpretation of air photos.

Airphotos of the Earth's surface are not maps. Photographs are single-point perspective views, and as such they contain geometric distortions. In vertical airphotos, for example, tall objects appear to lean away from the center. This phenomenon is called relief displacement and is particularly obvious toward the edges of the scene.

Relief displacement demonstrated by trees in a a conifer forest at Kojšovská hol'a, Slovakia (left) and cemetery tombstones at Liebenthal, Kansas (right). Note the person (*) for scale. Kite airphotos © S.W. and J.S. Aber.

photo scale = f ÷ H
f & H must be in same units

The scale of a vertical airphoto is determined by lens focal length (f) and flying height above the ground (H). Flying height varies with topography, however, so scale is not constant throughout a photograph. Vertical air photos can be used for ground measurements and map making. Photogrammetry is the art and science of accurate measurements based on photographs. Dimensions and locations of objects, height differences, and absolute elevations may be determined accurately. Modern topographic maps are usually produced on the basis of air photos.

Aerial photography is typically done from specially equiped airplanes or helicopters nowadays. However many other manned or unmanned platforms may be utilized to hold the camera above the ground, including balloons, tethered blimps, kites, radio-controlled model planes, and rockets—see Project Corona.

Low-Height Unmanned
Aerial Platforms
Great Plains kite aerial photography
UAS applications for soil degradation
Helium blimp aerial photography
Rocket aerial photography

Applications of Aerial Photography

Aerial photographs are routinely employed for all manner of mapping and evaluation of natural and cultural resources, including agriculture (crops and soils), archaeology, biology (habitat, wildlife census), forestry, geology, geomorphology, engineering, hydrology, industrial development, military (camouflage detection, espionage, terrain models), mineral and oil prospecting, pollution (air, land, water), reclamation, transportation, urban planning, wind energy, etc.

U.S. Sources for Aerial Photographs
Commmercial U.S. Government
Quantum Spatial National Aerial Photography Program—NAPP
Midwest Aerial Photography National High Altitude Photography NHAP
Col-East, Inc. Digital orthophoto quadrangles—DOQ

Small, unmanned drones have become quite popular in the last few years. In spite of the hype for a new industry, actually flying such drones routinely remains a difficult proposition that requires a skilled pilot. Many accidents have happened. Learning to fly a remotely controlled helicopter takes years of practice. Such drones are legal for hobby and personal, non-profit use. However, commercial applications remain limited in the United States by FAA regulations; a plan is under development for 2015. Nonetheless, many commercial photographers already are using drones.

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EB/ES/GE 351 © J.S. Aber (2016).