Introduction to remote sensing

The field of remote sensing is very broad and has been described from many angles by numerous authors, e.g. Campbell [19], Lillesand and Kiefer [66], and Sabins [88]. In his book, Campbell tried to identify the central concepts of remote sensing and came up with the following definition:

Remote sensing is the practice of deriving information about the earth's land and water surfaces using images acquired from an overhead perspective, using electromagnetic radiation in one or more regions of the electromagnetic spectrum, reflected or emitted from the earth's surface.

Although this definition certainly does not cover all areas (e.g. meteorological or extraterrestrial remote sensing), it does serve well as a description of remote sensing in this thesis. In the remainder of this section, several of the above principles will be explained and some additional concepts necessary to understand this thesis will be introduced.

As stated above, remote sensing makes use of electromagnetic radiation. The strongest and best-known source of electromagnetic radiation is our sun, which emits radiation over the entire electromagnetic spectrum—see Table 1.1. Besides this natural source of illumination, which is used for passive remote sensing, it is also possible to use an artificial source of electromagnetic radiation, in which case we speak of active remote sensing. When the radiation reaches the surface of the earth, part of it will be reflected. Another part will be absorbed and subsequently emitted, mainly in the form of thermal (far infrared) energy. The fraction of the irradiance that is reflected (or absorbed and re-emitted) is dependent on the wavelength and differs for each material, as is illustrated in Figure 1.1. By measuring the amount of electromagnetic radiation that is reflected or emitted and comparing it to the spectral reflectance curves of known materials, information about the earth's land and water surfaces can be derived.

To measure the reflected and emitted radiation, usually an imaging scanner aboard an airplane or satellite is used. Basically, the two types of passive imaging scanners that exist are the mechanical scanner and the pushbroom scanner. The mechanical scanner

Division

Wavelengths

Gamma rays

<0.03 nm

X-rays

0.03-3.0 nm

Ultraviolet

3.0-380 nm

Visible

0.38-0.72 /m

Blue

0.4-0.5 im

Green

0.5-0.6 im

Red

0.6-0.7 /m

Infrared

0.72-1000 /m

Near infrared

j> reflected

0.72-1.30 /m

Mid infrared

1.30-3.00 /m

Far infrared

emitted

3.00-1000 /m

Microwave

0.1-30 cm

Radio

>30 cm

Table 1.1: Principal divisions of the electromagnetic spectrum (Campbell [19], corrected).

Table 1.1: Principal divisions of the electromagnetic spectrum (Campbell [19], corrected).

wavelength (m m)

Figure 1.1: Generalised spectral reflectance curves for three different materials (adopted from Schowengerdt [98]).

Figure 1.2: A multispectral mechanical scanner uses a scan mirror to direct the radiation inside the instantaneous field of view towards a spectrometer. Here, the incoming energy is dispersed into a spectrum and led to detectors that are sensitive to specific wavelength bands. Rotation of the scan mirror moves the IFOV across-track, while the along-track movement is provided by the platform motion (after Sabins [88]).

Figure 1.2: A multispectral mechanical scanner uses a scan mirror to direct the radiation inside the instantaneous field of view towards a spectrometer. Here, the incoming energy is dispersed into a spectrum and led to detectors that are sensitive to specific wavelength bands. Rotation of the scan mirror moves the IFOV across-track, while the along-track movement is provided by the platform motion (after Sabins [88]).

uses a scan mirror to direct the surface radiation onto an electronic detector, taking a measurement at regular intervals. Figure 1.2 shows an example of a multispectral mechanical scanner. The pushbroom scanner uses a linear array of detectors—usually CCDs1—to take a number of measurements simultaneously. Apart from these across-track readings, both scanners also take measurements in the along-track direction, which is defined by the platform's motion. With some effort, this two-dimensional grid of measurements can be transformed to a digital image consisting of picture elements or pixels. Not only do the corresponding ground locations of the measurements have to be corrected due to factors like the earth's curvature and irregular movements of the scan mirror and the platform (geometric corrections), but the measurements themselves must also be corrected for atmospheric and sensor effects (radiometric corrections). The resolution of the resulting image or series of images, which expresses the level of fine detail that can be distinguished, has four aspects. The spatial resolution is the ground area that is represented by a single pixel; this area is approximately equal to the geometrical projection of a single detector element at the earth's surface, which is sometimes called the instantaneous field of view (IFOV). The radiometric resolution is defined by the number brightness levels that can be distinguished and is dependent on the number of bits into which each measurement is quantified. The spectral resolution denotes the width of the wavelength interval at which the electromagnetic radiation is recorded. If a multispectral (e.g. Thematic Mapper) or hyperspectral scanner (e.g. AVIRIS) is used, which take measurements in a few up to

1 Charge-Coupled Device.

Scanner

TMa

AVHRR6

AVIRISC

Platform

Landsat-4/5

NOAAd-7/9/l 1/12/14

NASA6 ER-2

satellite

satellite

airplane

Scene coverage

185x170 km2

2399x4752 km2

11x10 km2

Image size

6167x5667 pixels

2048x4320 pixels

614x512 pixels

Resolution

• spatial

30x30 m2 f

1.1x1.1 km2

20x20 m2

• radiometric

8 bits

10 bits

12 bits

• temporal

16 days

12 hours

not applicable

• spectral

band 1

0.45-0.52

¡m

band 1

0.58-0.86 ¡m

224 bands,

band 2

0.52-0.60

¡m

band 2

0.73-1.10 ¡m

each 10 nm wide,

band 3

0.63-0.69

¡m

band 3

3.55-3.93 ¡m

over 0.38-2.45 ¡m

band 4

0.76-0.90

¡m

band 4

10.3-11.3 ¡m

band 5

1.55-1.75

¡m

band 5

11.5-12.5 ¡m

band 6

10.4-12.5

¡m

band 7

2.08-2.35

¡m

"Thematic Mapper.

6 Advanced Very High Resolution Radiometer, Local Area Coverage (LAC) mode. c Airborne Visible InfraRed Imaging Spectrometer. dNational Oceanic and Atmospheric Administration eNational Aeronautics and Space Administration ^Spatial resolution of band 6 is 120x120 m2.

"Thematic Mapper.

6 Advanced Very High Resolution Radiometer, Local Area Coverage (LAC) mode. c Airborne Visible InfraRed Imaging Spectrometer. dNational Oceanic and Atmospheric Administration eNational Aeronautics and Space Administration ^Spatial resolution of band 6 is 120x120 m2.

Table 1.2: Characteristics of a few well-known scanners. The exact specifications may differ for other models carried by different platforms.

several hundreds of spectral bands, the spectral resolution may well not be unique (c.f. TM bands 3 and 4). The temporal resolution, finally, only applies to time series of images and describes how long the interval between two successive recordings of the same scene is. In case the scanner is carried by a satellite, the temporal resolution is determined by the satellite's orbit. The characteristics of a few well-known scanners are listed in Table 1.2.

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