Remote Sensing_Engineering Handbook_1996
LIMNOLOGY LIBRARY — __..
JAN 3 02003
151
Remote Sensing
151.1 Electromagnetic Energy
151.2 Atmospheric Effects
151.3 Remote Sensing Systems
Aerial Photography e Multispectral, Thermal, and Hyperspectral Scanners e Side-Looking Radar
151.4 Remote Sensing from Earth Orbit
151.5 Digital Image Processing
Ralph W. Kiefer
University of Wisconsin, Madison
Thomas M. Lillesand
University of Wisconsin, Madison
Remote sensing involves the use of airborne and space-imaging systems to inventory and monitor
earth resources. Broadly defined, remote sensing is any methodology employed to study the
characteristics of objects from a distance. Using various remote sensing devices, we remotely
collect data that can be analyzed to obtain information about the objects, areas, or phenomena of
interest. This chapter discusses sensor systems that record energy over a broad range of the
electromagnetic spectrum, from ultraviolet to microwave wavelengths.
— Remote sensing affords a practical means for frequent and accurate monitoring of the earth's
resources from a site-specific to global basis. This technology is aiding in assessing the impact of a
range of human activities on our planet's air, water, and land. Data obtained from remote sensors
have provided information necessary for making sound decisions and formulating policy in a host
of resource development and land use applications. Remote sensing techniques have also been
used in numerous special applications. Expediting petroleum and mineral exploration, locating
forest fires, providing information for hydrologic modeling, aiding in global crop production
estimates, monitoring population growth and distribution, and determining the location and extent
of oil spills and other water pollutants are but a few of the many and varied applications of remote
sensing that benefit humankind on a daily basis. It should be pointed out that these applications
almost always involve some use of ground truth or on-the-ground observation. That is, remote
sensing is typically a means of extrapolating from, not replacing, conventional field observation.
© 1998 by CRC PRESS LLC
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151.1 Electromagnetic Energy
The sun and various other sources radiate electromagnetic energy over a range of wavelengths.
Light is a particular type of electromagnetic radiation that can be seen or sensed by the human
eye. All electromagnetic energy, whether visible or invisible, travels in the form of sinusoidal
waves. Wavelength ranges of special interest in remote sensing are shown in Table 151.1.
Table 151.1 Components of the Electromagnetic Spectrum
Wavelength | Spectral Region
0.3 to 0.4 um Ultraviolet
0.4 to 0.7 um Visible:
0.4 to 0.5 um = blue
0.5 to 0.6 um = green
0.6 to 0.7 um = red
0.7 to 1.3 um | | Near infrared
1.3 ю 3.0 um — Mid-infrared
3 to 14 um Thermal infrared
Immtolm Microwave
When electromagnetic energy is incident upon an object on the earth's surface, it can interact
with the object in any or all of three distinct ways. The incident energy can be reflected,
transmitted, or absorbed. The absorbed component goes into heating the body and is subsequently
re-emitted from the object. The particular mix of these three possible interactions is dependent
upon the physical nature of objects. For example, healthy vegetation normally appears green
because the blue and red components of the incident light are absorbed by chlorophyll present in
plant leaves. In contrast, concrete surfaces strongly reflect blue, green, and red wavelengths nearly
equally and appear light gray. Remote sensors record such variations in energy interaction (both in
visible and invisible wavelengths) in order to discriminate between earth surface features and to
assist in quantifying their condition.
All objects at a temperature greater than absolute zero radiate energy according to the formula
M =oeT* (151.1)
where M is the total radiant exitance (radiated energy) from the surface of a material (W m™), o is
the Stefan-Boltzmann constant (5.6697 - 10-8 W m2 K-4), ¢ is the emissivity (efficiency of
radiation) of the material, and 7 is the absolute temperature (K) of the emitting material. The
general shape of the resulting curves of emitted energy (radiant exitance) versus wavelength is
shown in Fig. 151.1(a). The wavelength at which the amount of emitted energy is a maximum is
related to its temperature by Wien's displacement law, which states that
A
Am =
= (1512)
where A, 1s the wavelength of maximum spectral radiant exitance (¿m), A = 2898 um K, and 7 =
temperature (K). Note that the sun's energy has a peak wavelength of 0.48 um (in the visible part
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of the spectrum), whereas the earth's energy has a peak wavelength of 9.67 um (invisible to the
human eye, but able to be sensed by a thermal scanner).
Figure 151.1 Spectral characteristics of (a) energy sources, (b) atmospheric effects, and (c) sensing
systems. Note that wavelength scale is logarithmic. (Source: Lillesand, T. M. and Kiefer, R. W. 1994.
Remote Sensing and Image Interpretation, 3rd ed. John Wiley & Sons, New York. With
permission.)
Ie or
| ` - Sun's energy (at 6000” К)
© | ®
E — ——— A | ———
03um lum 10 um 100 um 1 mm 1m
Wavelength —m
=
2
#
=
E |
Wavelength ——
(6) Atmospheric transmittance
— |= Human eye
Photography Thermal scanners
Multispectral scanners Radar and passive microwave
+ rrr 1 ¥ FY TRERY ¥ 1 1 PETER NN 1 1 1 1 i
© 0.3um | um 10 um 100 um 1 mm 1m
Wavelength ——a
(с) Common remote sensing systems
Reflected energy is recorded when objects are sensed in sunlight in the ultraviolet, visible,
near-infrared or mid-infrared portions of the spectrum. Radiated energy is recorded in the thermal
infrared portion of the electromagnetic spectrum using radiometers and thermal scanners. This
allows, for example, the detection and recording of the heated effluent from a power plant as it
flows into a lake at a cooler temperature. |
Remote sensing systems can be active or passive systems. The examples cited earlier are passive
© 1998 by CRC PRESS LLC
systems in that they record reflected sunlight or emitted radiation. Radar systems are called active
systems because they supply their own energy. Pulses of microwave energy are transmitted from
radar systems toward objects on the ground, and the backscattered energy is then received by radar
antennas and used to form images of the strength of the radar return from various objects.
151.2 Atmospheric Effects
Because the atmosphere contains a wide variety of suspended particles, it offers energy interaction
capabilities just as "ground" objects do. The extent to which the atmosphere transmits
electro-magnetic energy is dependent upon wavelength, as shown in Fig. 151.1(b). The sensing
systems typically used in various wavelength ranges are shown in Fig. 151.1(c). Energy in the
ultraviolet wavelengths is scattered greatly, which limits the use of ultraviolet wavelengths from
aerial or space platforms. The atmosphere is transparent enough in the visible, near-infrared, and
mid-infrared wavelengths to permit aerial photography and multispectral sensing in these
wavelengths. In this region the blue wavelengths are scattered the most and the mid-infrared
wavelengths are scattered the least. In the thermal infrared region there are two "windows" where
the atmosphere is relatively transparent: 3—5 um and 8-14 um wavelength (most aerial thermal
scanning is done in the 8—14 um band). At microwave wavelengths the atmosphere 1s extremely
transparent, and many radar systems can be operated in virtually all weather conditions.
151.3 Remote Sensing Systems
Aerial Photography
Aerial photographs can be taken on any of several film types, from a variety of flying heights.
Mapping cameras typically use an image size of 230 by 230 mm. Smaller-format cameras (70 mm
and 35 mm) can be used where large-area coverage with great geometric fidelity is not required.
The interpretability of aerial photographs is highly dependent on the selection of film type and
image scale.
Principally because of cost considerations, the films most widely used for aerial photography are
the black and white (b/w) films. Panchromatic films are b/w films sensitive to the visible portion of
the electromagnetic spectrum (0.4 to 0.7 um). The sensitivity of black and white infrared films
includes both the visible part of the spectrum and also the wavelengths 0.7 to 0.9 um (near
infrared). It is important to note that infrared energy of these wavelengths does not represent heat
emitted from objects, but simply reflected infrared energy to which the human eye is insensitive.
Color and color infrared films are also widely used for aerial photography. Although the cost of
using these films is greater than for black and white films, they provide greater information content
due to the human eye's ability to discriminate substantially more colors than shades of gray.
Normal color films have three separate emulsion layers sensitive to blue, green, and red
wavelengths, respectively. Color infrared films have three similar emulsion layers, but they are
sensitive to green, red, and near-infrared wavelengths, respectively. Again, as in the case of b/w
infrared films, it is reflected sunlight, not emitted energy, that is photographed with color infrared
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film.
Digital cameras use a camera body and lens but record image data with charge-coupled devices
(CCDs) rather than film. The electrical signals generated by these detectors are stored digitally,
typically using media such as computer disks. Although this process is not "photography" in the
traditional sense (images are not recorded directly onto photographic film), it is often referred to as
"digital photography." Of course, hard copy photographs can also be converted into an array of
digital picture elements (pixels) using some form of image scanner.
Video cameras are sometimes used as a substitute for small-format (70 mm and 35 mm)
cameras. Video camera data are recorded on videotape, typically in HI-8 format.
The scale of photographs affects the level of useful information they contain. Small-scale
photographs (1:50 000 or smaller) are used for reconnaissance mapping, large-area resource
assessment, and large-area planning. Medium-scale photographs (1:12 000 to 1:50 000) are used
for the identification, classification, and mapping of such items as tree species, agricultural crop
types, vegetation communities, and soil types. Large-scale photographs (larger than 1:12 000) are
used for the intensive monitoring of specific items such as surveys of the damage caused by plant
diseases, insects, or tree blow-downs. Applications such as hazardous waste site assessment often
require very-large-scale photographs.
The principles of photogrammetry can be used to obtain approximate distances and ground
elevations from aerial photographs using relatively unsophisticated equipment and simple
geometric concepts, as well as to obtain extremely precise maps and measurements using
sophisticated "soft copy" instrumentation, digital images, and complex computational techniques.
Multispectral, Thermal, and Hyperspectral Scanners
Multispectral scanners are electro-optical devices that sense selectively in multiple spectral bands
using electronic detectors rather than film. They sense one small area on the ground at a time and,
through scanning, build up two-dimensional images of the terrain for a swath beneath an aircraft or
spacecraft. Through this process, they collect rows and columns of image data than can be
computer processed. As shown in Fig. 151.1, multispectral scanners can sense in a much broader
spectral range than film. Multispectral scanners are the sensing devices used in the Landsat and
SPOT satellites (discussed later).
Thermal scanners are electro-optical devices that sense in the thermal infrared portion of the
electromagnetic spectrum. They do not record the true internal temperature of objects (kinetic
temperature), but rather their apparent temperature based on the radiation from their top surfaces
(radiant temperature). Because they sense energy emitted (rather than reflected) from objects,
thermal scanning systems can operate day or night. Multiple thermal bands can be sensed
simultaneously, as in the case of NASA's Thermal Infrared Multispectral Scanner. Successful
interpretations of thermal imagery have been made in such diverse tasks as determining rock type
and structure, locating geological faults, mapping soil type and moisture conditions, determining
the thermal characteristics of volcanoes, studying evapotranspiration from vegetation, locating cold
water springs, determining the extent and characteristics of thermal plumes in lakes and rivers,
delineating the extent of active forest fires, and locating underground coal mine fires.
Hyperspectral scanners acquire multispectral images in many very narrow, contiguous spectral
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bands throughout the visible, near-infrared, and mid-infrared portions of the electromagnetic
spectrum. These systems typically collect 200 or more channels of data, which enables the
construction of an effectively continuous reflectance spectrum for every pixel in the scene (as
opposed to the 4-6 broad spectral bands used by the Landsat and SPOT satellites).
Side-Looking Radar
An increasing amount of valuable environmental and resource information is being acquired by
active radar systems that operate in the microwave portion of the spectrum. Microwaves are
capable of penetrating the atmosphere under virtually all conditions. Depending on the specific
wavelengths involved, microwave energy can penetrate clouds, fog, light rain, and smoke.
Side-looking radar uses an antenna pointed to the side of the aircraft or spacecraft. Because the
sensor is mounted on a moving platform, it is able to produce continuous strips of imagery
depicting very large ground areas located adjacent to the flight line.
Microwave reflections from earth materials bear no direct relationship to their counterparts in the
visible portion of the spectrum. For example, many surfaces that appear rough in the visible
portion of the spectrum may appear smooth as seen by microwaves (e.g., a White sand beach). The
appearance of various objects on radar images depends principally on the orientation of the terrain
relative to the aircraft or spacecraft (important because this is a side-looking sensor), the object's
surface roughness, its moisture content, and its metallic content.
Radar image interpretation has been successful in applications as varied as mapping major rock
units and surficial materials, mapping geologic structure (folds, faults, and joints), discriminating
vegetation types (natural vegetation and crops), determining sea ice type and condition, and
mapping surface drainage patterns (streams and lakes).
151.4 Remote Sensing from Earth Orbit
The use of satellites as sensor platforms has made possible the acquisition of repetitive
multispectral data of the earth's surface on a global basis. The principal earth resources satellites to
date have been the U.S. Landsat and the French SPOT systems. The application of satellite image
interpretation has already been demonstrated in many fields, such as agriculture, botany,
cartography, civil engineering, environmental modeling and monitoring, forestry, geography,
geology, geophysics, habitat assessment, land resource analysis, land use planning, oceanography,
range management, and water resources.
The Landsat satellites image each spot on the earth's surface once each 16 days, providing for
frequent, synoptic, repetitive, global coverage. The Landsat multispectral scanner currently in
operation is the Thematic Mapper, which senses in six bands of the spectrum, from the blue
through the mid-infrared, with a ground resolution cell size of 30 by 30 m. A seventh band senses
in the thermal infrared with a ground resolution cell size of 120 by 120 m.
The SPOT satellites image each spot on the earth's surface once each 26 days, but the satellite
can be aimed (using ground commands) as much as 27° from nadir to allow more frequent viewing
opportunities (e.g., at a latitude of 45° a total of 11 viewing opportunities exist during each 26-day
cycle). Stereoscopic imaging is also possible due to the off-nadir viewing capabilities. The
© 1998 by CRC PRESS LLC
multispectral scanner of the current SPOT satellite senses in three bands of the spectrum (green,
red, and near infrared) with a ground resolution cell size of 20 by 20 m. In its "panchromatic"
(black and white) mode it senses in one broad band with a ground resolution cell size of 10 by 10
m.
The Canadian Radarsat satellite is planned for launch in 1995. This satellite will provide radar
data with many possible swath widths and resolutions on a 1-3 day repeat coverage (depending on
latitude). The primary applications for which Radarsat has been designed include ice
reconnaissance, coastal surveillance, land cover mapping, and agricultural and forest monitoring.
Other earth resources satellites include those of the European Space Agency and the National
Space Agency of Japan. Likewise, several other countries operate, or are planning to launch, earth
resources satellites.
151.5 Digital Image Processing
The digital data acquired by multispectral and thermal scanners, radar systems, and digital cameras
are typically computer processed to produce images through digital image processing. Through
various image-processing techniques, digital images can be enhanced for viewing and human
image interpretation. Digital data can also be processed using computer-based image classification
techniques to prepare various thematic maps, such as land cover maps. Digital image-processing
procedures are normally integrated with the functions of geographic information systems (GIS).
Defining Terms
Electromagnetic radiation: The transmission of energy in the form of waves having both an _
electric and a magnetic component.
Electromagnetic spectrum: Electromagnetic radiation is most simply characterized by its
frequency or wavelength. When electromagnetic waves are so ordered, the resulting array is
called the electromagnetic spectrum. The spectrum is normally considered to be bounded by
cosmic rays at the short wavelength end and by microwaves at the long wavelength end.
Emitted energy: The energy radiated by an object resulting from its internal molecular motion
(heat). All objects above "absolute zero" in temperature radiate energy.
Ground truth (or reference data): Field observations or other information used to aid or verify
the interpretation of remotely sensed data.
Photogrammetry: The science, art, and technology of obtaining reliable measurements, maps,
digital elevation models, thematic GIS data, and other derived products from photographs.
Pixel: The cell representing each combination of row and column (picture element) in a digital
data set.
Reflected energy: That component of incident energy that is reflected from an object.
Remote sensing: Studying the characteristics of objects from a distance by recording and
analyzing electromagnetic energy, typically from ultraviolet to microwave wavelengths.
© 1998 by CRC PRESS LLC
References
American Society of Photogrammetry. 1980. Manual of Photogrammetry, 4th ed. ASP, Falls
Church, VA.
American Society of Photogrammetry. 1983. Manual of Remote Sensing, 2nd ed. ASP, Falls
Church, VA.
American Society for Photogrammetry and Remote Sensing. 1995. Manual of Photographic
Interpretation, 2nd ed. ASPRS, Bethesda, MD.
American Society for Photogrammetry and Remote Sensing. 1995. Manual of Remote
Sensing—FEarth Observing Platforms and Sensors, 3rd ed. CD-ROM. ASPRS, Bethesda,
MD.
Avery, T. E. and Berlin, G. L. 1992. Fundamentals of Remote Sensing and Airphoto
Interpretation, 5th ed. Macmillan, New York.
Campbell, J. B. 1987. Introduction to Remote Sensing. Guilford, New York.
Elachi, C. 1987. Introduction to the Physics and Techniques of Remote Sensing. John Wiley
& Sons, New York. |
Jensen, J. R. 1986. Introductory Digital Image Processing: A Remote Sensing
Perspective. Prentice Hall, Englewood Cliffs, NJ.
Lillesand, T. M. and Kiefer, R. W. 1994. Remote Sensing and Image Interpretation, 3rd ed. John
Wiley & Sons, New York.
Sabins, F. F., Jr. 1987. Remote Sensing: Principles and Interpretation, 2nd ed. Freeman, New
York.
Wolf, P. R. 1983. Elements of Photogrammetry, 2nd ed. McGraw-Hill, New York.
Further Information
The leading professional society dealing with remote sensing and photogrammetry is the American
Society for Photogrammetry and Remote Sensing, 5410 Grosvenor Lane, Suite 210, Bethesda, MD
20814. The society publishes the monthly journal Photogrammetric Engineering & Remote
Sensing, as well as many books and special publications.
Information on the availability of cartographic and image data throughout the U.S., including
aerial photographs and satellite images, can be obtained from the Earth Science Information Center
(ESIC), U.S. Geological Survey, 507 National Center, Reston, VA 22092.
Information on the availability of Landsat data on a worldwide basis can be obtained from
EOSAT Corporation, 4300 Forbes Boulevard, Lanham, MD 20706.
Information on the availability of SPOT data on a worldwide basis can be obtained from SPOT
Image, 16 bis, avenue Edouard-Belin, 31030 Toulouse Cedex, France. For North America contact
SPOT Image Corporation, 1897 Preston White Drive, Reston, VA 22091.
Information on the availability of Radarsat data can be obtained from Radarsat International Inc.,
Building D, Suite 200, 3851 Shell Road, Richmond, British Columbia V6X 2W2, Canada.
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