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2.2 Remotely Sensed Image Classification
2.2.1 Remote Sensing
Remote sensing is the science and art of obtaining information about an object, area, or phenomenon through the analysis of data acquired by a device that
have no any contact with the object, area, or phenomenon investigated Lillesand and Kiefer 1993. Earth observation by remote sensing is the interpretation and
understanding of measurements made by airborne or satellite-borne instruments of electromagnetic radiation that is reflected from or emitted by objects on the
Earth’s land, ocean, or ice surfaces or within the atmosphere, and the establishment of relationships between these measurements and the nature and
distribution of phenomena on the Earth’s surface or within the atmosphere Mather 2004.
The characteristics of imaging remote sensing instruments operating in the visible and infrared spectral region can be summarized in terms of their spatial,
spectral and radiometric resolutions. The spatial resolution of an imaging system is not an easy concept to define, because it can be measured in a number of
different ways, depending on the user’s purpose. Most sensors operating in the visible and infrared bands collect multispectral or multi-band images, which are
sets of individual images that are separately recorded in discrete spectral bands, and the term spectral resolution refers to the width of these spectral bands
measured in micrometers μm or nanometers nm. Radiometric resolution or
radiometric sensitivity refers to the number of digital quantization levels used to express the data collected by the sensor. In general, the greater the number of
quantization levels, the greater the detail in the information collected by the sensor Mather 2004.
An important principle underlying the use of remotely-sensed data is as follow different objects on the Earth’s surface and in the atmosphere reflect,
absorb, transmit or emit electromagnetic energy in different proportions, and that such differences allow these components to be identified Mather 2004. The great
advantage of having remotely-sensed data available digitally is that it can be processed by computer either for machine assisted information extraction or for
8 enhancement of its visual qualities in order to make it more interpretable by a
human analyst Richards and Jia 2006.
2.2.2 Image Pre-processing
Image pre-processing is concerned to perform correction on any errors that occurred in the remotely sensed images. There are two common types of image
pre-processing techniques, namely radiometric correction and geometric correction. Radiometric correction, aim to remove errors on remotely-sensed data,
either resulted from the presence of the atmosphere as a transmission medium through which radiation must travel from its source to the sensors, or from
instrumentation effects Richards and Jia 2006. Atmospheric correction might be a necessary pre-processing technique to compute a ratio of the values in two bands
of a multispectral, relate upwelling radiance from a surface to some property of that surface in terms of a physically based model, and compare results or ground
measurements made at one time to results achieved at a later Mather 2004. Radiometric errors within a band and between bands may due to effects of design
and operation of the sensor system which normally ignored by comparison to band errors from atmospheric effects. An ideal radiation detector should have a
transfer characteristic radiation in, signal out as shown in Figure 1.a. which should be linear, and therefore there is a proportional increase and decrease of
signal with detected radiation level Richards and Jia 2006.
a b
Figure 1. Transfer characteristic of a radiation detector: a. Ideal transfer
characteristic, b. Hypothetical mismatches in detector characteristics in the same
band Richards and Jia 2006.
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