Page 47 - Digital Analysis of Remotely Sensed Imagery

20 Cha pte r O n e

makes it possible to differentiate subtle variations in the condition of
targets. A fine radiometric resolution is critical in studying targets
that have only a subtle variation in their reflectance, such as detection
of different kinds of minerals in the soil and varying levels of
vegetation stress caused by drought and diseases. Also, remotely
sensed data of a fine radiometric resolution are especially critical in
quantitative applications in which a ground parameter (e.g., sea
surface temperature and concentration level of suspended solids in a
water body) is retrieved from pixel values directly. Data of a higher
quantization level enable the retrieval to be achieved more accurately,
while a coarse radiometric resolution causes the pixels to look similar
to one another.

1.5.4 Temporal Resolution
Also known as revisit period, temporal resolution refers to the
temporal frequency at which the same ground area is sensed
consecutively by the same sensing system. Since remote sensing
satellites are revolving around the Earth 24 hours a day and 365 days
a year, the temporal resolution is directly related to the satellite orbital
period. A short period means more revolutions per day and is
equivalent to a high temporal resolution. Temporal resolution of the
same satellite varies with latitude of the geographic area being sensed.
At higher latitude there is more spatial overlap among images
acquired over adjoining orbits. The same ground is sensed more
frequently, or at a finer temporal resolution, than at a lower latitude.
One method of refining temporal resolution of satellite data is to tilt
the scanning mirror of the sensing system. In this way the same scene
is able to be scanned repeatedly at a close temporal interval from the
adjoining orbits either to the left or to the right of the current path.
Temporal resolution is very significant in applications in which the
object or phenomenon of study is temporally dynamic or in a state of
constant change, such as weather conditions, floods, and fires. In
general, satellite remote sensing data have a higher temporal resolution
than airborne remote sensing data.
Among the four image resolutions, temporal resolution bears no
direct relationship to the other three resolutions. Although spectral
and spatial resolutions are independent of each other, both are tied
closely to radiometric resolution. In recording images at a finer
spectral or spatial resolution, the returned energy emitted from or
reflected by the ground is sliced into numerous units either spectrally
or spatially. The successful detection of such a tiny quantity of energy
over a unit imposes a stringent demand on the radiometric sensitivity
of the detectors (Fig. 1.12). Consequently, a fine radiometric resolution
is achievable by compromising either spectral or spatial or both
resolutions in order to accumulate sufficient energy from the target
to be accurately identifiable. Conversely, a low radiometric resolution
may be adopted in order to achieve a finer spectral or spatial