Principles of remote sensing

The interpretation of aerial photography has for many years proved to be a valuable source of data for civil engineers. Until the early 1960s civilian 'remote sensing' was concerned primarily with the interpretation of such imagery. Since then, however, developments in orbiting satellites, sensor technology and computing have led to the creation of a discipline which now impinges on many areas in science and engineering. Although still in an embryonic state, particularly in the field of image processing, it has already proved to be a cost-effective method of investigating engineering phenomena of both large aerial extent, e.g. surface drainage, and those which are more localized, e.g. landslides, unstable land, etc.

7.5.1 The electromagnetic spectrum

Electromagnetic (EM) energy is all energy which travels in a periodic harmonic manner at the velocity of light. Electromagnetic energy is normally considered to consist of a continuum of wavelengths referred to as the EM spectrum (Figure 7.18).

It can be seen from Figure 7.18 that several regions of the EM spectrum are of particular importance in remote sensing. For example, the visible and reflected infra-red regions of the spectrum are important since they enable reflected solar radiation to be measured. In contrast, at longer wavelengths in the infra-red (8 to 14 jim waveband) the sensing of emitted thermal energy is of more significance. Measurements within the visible and infra-red (reflected and emitted) regions are considered to be passive in nature since the radiation being recorded occurs naturally. As the wavelength increases to the order of several millimetres it becomes more convenient actively to generate EM radiation of this wavelength and record the reflected radiation from the terrain. Thus, a typical active system would be side-

Wavelength |(im)

i(r7i(r6i(r5icr4io-3io-2io-1 1 101 102103 104 105106

Reflected Thermal i__ _/

Infrared

Visible Microwave

Figure 7.18 The electromagnetic spectrum looking airborne radar (SLAR). It should be noted that instruments also exist for the measurement of passive microwave emission. However, since the emitted EM energy at this wavelength is very small, microwave radiometers are much less common than SLAR instruments.

Depending on the nature of the radiation being measured, it is possible to record the reflected or emitted energy by using either a lens and photographic emulsion or by using a linescanner and crystal detector. The geometrical distinction between the two approaches is illustrated in Figure 7.19. The primary advantage of using a linescanner approach is that it is possible to record radiation of wavelengths greater than about 0.9 pm. It is also possible to measure the variations in radiation within narrow spectral regions and to record directly these variations in digital form.

Mention should also be made here of the distinction between the terms 'photograph' and 'image'. A 'photograph' is an image which has been detected by photographic techniques and recorded on photographic film. In contrast, an 'image' is a more general term used to describe any pictorial representation of

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detected radiation data. Therefore, although scanner data may be used to create a photographic product, this result is normally referred to as an 'image' since the original detection mechanism involved the use of crystal detectors creating electrical signals, rather than a lens focused on to photographic film.

7.5.2 Classification of remote sensing systems

Remote sensing systems can be classified using various criteria, such as sensor sensitivity range, mode of recording (photographic or scanning), mode of operation (active or passive) or type of sensor platform (aircraft or satellite). Table 7.5 provides a framework for the classification of data acquisition systems in remote sensing by using the latter two criteria; it also provides some common examples of each category.

Table 7.5 Classification of data acquisition systems

Aircraft Satellite

Overlap

Overlap

Passive systems

Wild RC-10 camera Spacelab Daedalus 1268 MSS metric camera Daedalus 1230 Thermal NASA large-format

Scanner camera

Barr and Stroud IR18 Landsat MSS, RBV, TM TVFS SPOT

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