Another factor controlling the type of information able to be extracted from
remotely sensed images of coral reefs is the image processing algorithm used to
transform the images into maps of benthic cover types, water depth, percent
macro-algal cover, or other relevant parameters. This is the process of transforming an image from a qualitative picture into a quantitative digital map that can
be used for science and management (Table 1.2). The image processing algorithm
is an equation, or series of equations, applied to every pixel in an image to identify
habitat characteristics and/or estimate environmental parameters.
1.2.2 Spectral Characteristics
The spectral dimension of a remotely sensed image determines if it can be used to
map particular coral reef biophysical variables. As introduced above, spectral
dimension refers to the quantities of light or electromagnetic energy measured in
each image pixel. More specifically, spectral dimension refers to the location,
width and number of spectral bands measured by the sensor. Remote sensing
instruments use detectors, including light-sensitive film and light-sensitive detector
materials (e.g., silicon) to measure the strength of electromagnetic energy, or
number of photons per unit time, in selected portions of the electromagnetic
spectrum. These film and solid detector materials are sensitized to specific regions
(i.e., bands) of the electromagnetic spectrum for measurement purposes. Traditionally, remote sensing science uses a wavelength notation (as opposed to frequency) to denote the different portions of the electromagnetic spectrum.
Significant amounts of work have been completed on radiative transfer processes in gases, liquids, solids and plants; hence there is a high level of understanding about how specific structural and chemical attributes of these features
control absorption and scattering at specific wavelengths. Radiative transfer refers
to the processes of transmission, absorption and scattering of electromagnetic
energy. Based on this knowledge, remote sensing detectors, especially multispectral and hyperspectral systems, are designed to measure electromagnetic
energy in pre-defined portions of the spectrum known to be sensitive to specific
structural and chemical attributes of features or associated processes in the
environment.
The individual spectral bands used for any particular sensor cover a set range of
wavelengths. For example, the multispectral system shown in Fig. 1.3 covers the
blue, green, red and near-infrared portions of the electromagnetic spectrum using
100 nm wide spectral bands. In contrast, the hyperspectral system in Fig. 1.3
covers the same range of wavelengths using hundreds of 10 nm wide spectral
bands. Multispectral systems provide broadly applicable spectral reflectance signatures suitable for mapping coral reef benthic features at a coarse level (e.g.,
geomorphic zones; Table 1.1 and Chaps. 2 and 3). Hyperspectral systems provide
highly detailed spectral reflectance signatures enabling better discrimination of
coral reef benthic features, and improved quantitative estimation of biophysical,
1 Visible and Infrared Overview
13
remotely sensed images of coral reefs is the image processing algorithm used to
transform the images into maps of benthic cover types, water depth, percent
macro-algal cover, or other relevant parameters. This is the process of transforming an image from a qualitative picture into a quantitative digital map that can
be used for science and management (Table 1.2). The image processing algorithm
is an equation, or series of equations, applied to every pixel in an image to identify
habitat characteristics and/or estimate environmental parameters.
1.2.2 Spectral Characteristics
The spectral dimension of a remotely sensed image determines if it can be used to
map particular coral reef biophysical variables. As introduced above, spectral
dimension refers to the quantities of light or electromagnetic energy measured in
each image pixel. More specifically, spectral dimension refers to the location,
width and number of spectral bands measured by the sensor. Remote sensing
instruments use detectors, including light-sensitive film and light-sensitive detector
materials (e.g., silicon) to measure the strength of electromagnetic energy, or
number of photons per unit time, in selected portions of the electromagnetic
spectrum. These film and solid detector materials are sensitized to specific regions
(i.e., bands) of the electromagnetic spectrum for measurement purposes. Traditionally, remote sensing science uses a wavelength notation (as opposed to frequency) to denote the different portions of the electromagnetic spectrum.
Significant amounts of work have been completed on radiative transfer processes in gases, liquids, solids and plants; hence there is a high level of understanding about how specific structural and chemical attributes of these features
control absorption and scattering at specific wavelengths. Radiative transfer refers
to the processes of transmission, absorption and scattering of electromagnetic
energy. Based on this knowledge, remote sensing detectors, especially multispectral and hyperspectral systems, are designed to measure electromagnetic
energy in pre-defined portions of the spectrum known to be sensitive to specific
structural and chemical attributes of features or associated processes in the
environment.
The individual spectral bands used for any particular sensor cover a set range of
wavelengths. For example, the multispectral system shown in Fig. 1.3 covers the
blue, green, red and near-infrared portions of the electromagnetic spectrum using
100 nm wide spectral bands. In contrast, the hyperspectral system in Fig. 1.3
covers the same range of wavelengths using hundreds of 10 nm wide spectral
bands. Multispectral systems provide broadly applicable spectral reflectance signatures suitable for mapping coral reef benthic features at a coarse level (e.g.,
geomorphic zones; Table 1.1 and Chaps. 2 and 3). Hyperspectral systems provide
highly detailed spectral reflectance signatures enabling better discrimination of
coral reef benthic features, and improved quantitative estimation of biophysical,
1 Visible and Infrared Overview
13