Encyclopedia of Environmental Science and Engineering, Volume I and II

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REMOTE SENSING

Remote sensing is the act of acquiring information about an

object from a distance. Environmental applications of remote

sensing typically involve the collection of photographic or

electronic images of the Earth’s surface or atmosphere from

airborne or spaceborne platforms. Visual interpretation or com-

puter processing can then be used to analyze these images.

The history of remote sensing dates back to the 19th cen-

tury, when the first aerial photographs were taken from bal-

loons and kites. The invention of the airplane provided a

better platform and aerial photography advanced rapidly

after World War I. With the dawn of the Space Age in 1957,

the field of photographic remote sensing expanded to include

pictures taken from satellites and other space platforms, as

demonstrated by pictures taken with a variety of camera

types during the Apollo missions.

At the same time, non-photographic remote sensing sys-

tems such as electronic multispectral scanners (MSS) were

developed for use on airborne platforms as well as on meteo-

rological satellites, Earth resources satellites, and other

spaceborne platforms. The 1990s have seen a further expan-

sion of the field of remote sensing, with the appearance of

new imaging radar satellites, imaging spectrometers, and

high-resolution MSS systems, and with the improvement of

methods for computer processing of remotely-sensed data.

TYPES OF REMOTE SENSING SYSTEMS

Several types of remote sensing systems can be differentiated

based on the principles employed for measuring electromag-

netic radiation. The most common types fall into four broad

categories: photographic systems, videographic systems,

multispectral scanners, and imaging radar systems. Within

these categories, particular instruments are designed to oper-

ate in specific portions of the electromagnetic spectrum. The

Earth’s atmosphere scatters and absorbs many wavelengths

of electromagnetic radiation, limiting the portion of the spec-

trum that can be used for remote sensing.

Photographic Systems

Many types of cameras have been used to acquire photo-

graphs of the Earth’s surface from airplanes and from space.

Common formats include 35-mm, 70-mm, and 9 
 9-inch

film sizes, although specialized cameras that employ other

film sizes are also used. Film types include black and white

panchromatic, black and white infrared, color, and color

infrared, covering the visible and near-infrared portions of

the electromagnetic spectrum from approximately 0.4 to 0.9

μ m. (Photography in the ultraviolet range, from 0.3 to 0.4

μ m, is also possible but is rarely done due to atmospheric

absorption and the need for quartz lenses.) Once the film has

been processed, photographs can be electronically scanned

at a variety of resolutions for use in a digital environment.

Photographic systems provide relatively high-resolution

images, with the nominal scale of a vertical aerial photo-

graph being dependent on the focal length of the camera and

the flying height of the sensor.^1

Videographic Systems

Video cameras can be used to record images in analog form

on videotape. Video systems have been designed to operate

in the visible, near-infrared, and mid-infrared portions of the

electromagnetic spectrum. The advantages of video systems

include low cost, near-real-time image availability, and the

ability to collect and store many image frames in sequence.

The primary disadvantage of video is its low spatial resolu-

tion, with approximately 240 lines per image for standard

video cameras. 2,3

Multispectral Scanners

MSS systems use electronic detectors to measure electro-

magnetic radiation in selected bands of the spectrum from

approximately 0.3 to 14 m m, including the visible and near-,

mid-, and thermal-infrared regions. These individual bands

may be fairly wide (greater than 0.2 m m in width) or quite

narrow (less than 0.01 m m in width). The designs used for

MSS systems fall into two categories. Across-track scanners

employ a rotating or oscillating mirror to scan back and forth

across the line of flight. Along-track (“push-broom”) scan-

ners use a linear array of charge-coupled devices (CCDs) to

scan in parallel along the direction of flight. Distinct subcat-

egories of MSS systems include thermal scanners, which

measure emitted radiation in the thermal infrared portion of

the spectrum, and imaging spectrometers, or “hyperspectral

scanners,” which generally collect data in over 100 continu-

ous, narrow spectral bands, producing a complete reflectance

spectrum for every pixel in the image. 1,4

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