An Introduction to Remotely Sensed Data Analysis - Sampling Spatial Units for Agricultural Surveys - page 67

Fig. 4.2 Electromagnetic spectrum

used for data acquisition, satellite images can be classified into: mono-spectral or
panchromatic (a single wavelength band), multispectral (several spectral bands),
superspectral (tens of spectral bands), and hyperspectral (hundreds of spectral
bands).
The spatial resolution describes the level of spatial accuracy, and is a measure of
the smallest linear or angular separation between two objects that can be detected
by the sensor. This can be considered as a measure of the smallest object that can be
distinguished as a single entity in the image. A better spatial resolution means that
the sensor will more accurately detect phenomena. In terms of the spatial resolu-
tion, the images can be classified into: low resolution (approximately 1 km or
more), medium resolution (from 100 m to 1 km), high-resolution (from 5 to
100 m), and very high-resolution (5 m or less).
The radiometric resolution defines the different sensitivities of sensors to the
radiation emitted or reflected from the Earth. The radiometric range is the maxi-
mum number of quantization levels that may be recorded by a particular sensing
system. Most sensors record data in eight bits, with values ranging from 0 to
255 (i.e., 256 gray levels).
The temporal resolution (or revisit period) concerns the frequency with which a
sensor receives images from a specific area. The ability to collect images of the
same area at different periods of time is one of the most important elements when
analyzing remotely sensed data. For instance, by analyzing data received at differ-
ent times, we can study the evolution of a phenomenon.
Another important concept is the swath of a satellite, which can be defined as the
width of the strip observed by each satellite pass. A sensor sees a certain portion of
the Earth

s surface as a satellite orbits around the Earth. Image swaths for sensors
are generally between tens and hundreds of kilometers wide. As the satellite circles
the Earth from pole to pole, its east-west position would not change if the Earth did
not rotate. However, because the Earth is rotating from west to east, the satellite
appears to shift. This allows the satellite swath to cover a new area with each
consecutive pass.

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