Environmental Engineering Reference
In-Depth Information
Fig. 3.5
Multispectral images
resolution, but these are classi
ed military satel-
lites or very expensive commercial systems.
A panchromatic image consists of only one
band. It is usually displayed as a greyscale
image, i.e., the displayed brightness of a partic-
ular pixel is proportional to the pixel digital
number which is related to the intensity of solar
radiation reflected by the targets in the pixel and
detected by the detector. Thus, a panchromatic
image may be similarly interpreted as a black-
and-white aerial photograph of the area, though
at a lower resolution.
Multispectral and hyperspectral images
(Fig.
3.5
) consist of several bands of data. For
visual display, each band of the image may be
displayed one band at a time as a greyscale image
or in combination of three bands at a time as a
colour composite image. Interpretation of a
multispectral colour composite image will
require the knowledge of the spectral re
Temporal Resolution
Temporal resolution is a measure of the repeat
cycle or frequency with which a sensor revisits
the same part of the Earth
'
s surface. The fre-
quency will vary from several times per day, for
a typical weather satellite, to 8
20 times a year
for a moderate ground resolution satellite, such
as Landsat TM. The frequency characteristics
will be determined by the design of the satellite
sensor and its orbit pattern.
-
Spectral Resolution
The spectral resolution of a sensor system is the
number and width of spectral bands in the sensing
device. The simplest form of spectral resolution is
a sensor with one band only, which senses visible
light. An image from this sensor would be similar
in appearance to a black-and-white photograph
from an aircraft. A sensor with three spectral
bands in the visible region of the EM spectrum
would collect similar information to that of the
human vision system. The Landsat TM sensor has
seven spectral bands located in the visible and
near- to mid-infrared parts of the spectrum.
fl
ectance
signature of the targets in the scene.
3.1.7 Geodesy, Geodetic Data
and Map Projections
Geodesy is the branch of science concerned with
the determination of the size and shape of the
Earth. Geodesy involves the processing of survey
measurements on the curved surface of the Earth,
as well as the analysis of gravity measurements.
