significant spatial distortions due to the geometry of the photograph acquisition
process. As a result, the scale, or relation of ground distance to the same distance
in a photograph, may vary. Uncorrected photographs can therefore not be used to
produce spatially accurate maps for comparison over time or for integration with
other spatial data until they are first ortho-corrected. The ortho-correction process
transforms photographs to a digital format with consistent spatial scale, allowing
them to be more effectively used for comparative mapping purposes.
1.2.4 Multispectral Imaging Systems
Multispectral systems on airborne and satellite platforms, including the current
generation of large-format digital mapping cameras, typically have 3–10 spectral
bands per pixel, resulting in a simplified spectral reflectance signature (Figs. 1.3 and
1.5). For thematic mapping of coral reef features, image pixel size and spectral band
placement will control the type and amount of information able to be discriminated.
Several published papers, including the images in Fig. 1.4, show that multispectral
data with moderate pixel sizes (20–30 m) can be used to map 5–6 coral reef benthic
classes at accuracy levels of 80 %, while multispectral data with smaller pixel sizes
(\5.0 m) can map 10–12 classes of coral reef benthic cover features at comparable
accuracy (Andréfouët et al. 2003; Roelfsema and Phinn 2010).
Due to the broad spectral bands used in multispectral systems, their utility for
mapping quantitative biophysical properties (e.g., pigment concentration) is limited
since the narrow width of absorption features associated with photosynthetic and
non-photosynthetic pigments cannot be resolved. Multispectral data do contain
Fig. 1.5 Example spectral signatures from the same patch of live coral. The progression of
reflectance signature graphs, from left to right is: in-situ reflectance from field spectrometry,
modelled at-surface reflectance with 1.0 m of water, at-surface reflectance from airborne
hyperspectral (CASI 2); and at-surface reflectance (x 10,000) from satellite multispectral
(QuickBird 2) image (provided by Ian Leiper)
16
S. R. Phinn et al.
process. As a result, the scale, or relation of ground distance to the same distance
in a photograph, may vary. Uncorrected photographs can therefore not be used to
produce spatially accurate maps for comparison over time or for integration with
other spatial data until they are first ortho-corrected. The ortho-correction process
transforms photographs to a digital format with consistent spatial scale, allowing
them to be more effectively used for comparative mapping purposes.
1.2.4 Multispectral Imaging Systems
Multispectral systems on airborne and satellite platforms, including the current
generation of large-format digital mapping cameras, typically have 3–10 spectral
bands per pixel, resulting in a simplified spectral reflectance signature (Figs. 1.3 and
1.5). For thematic mapping of coral reef features, image pixel size and spectral band
placement will control the type and amount of information able to be discriminated.
Several published papers, including the images in Fig. 1.4, show that multispectral
data with moderate pixel sizes (20–30 m) can be used to map 5–6 coral reef benthic
classes at accuracy levels of 80 %, while multispectral data with smaller pixel sizes
(\5.0 m) can map 10–12 classes of coral reef benthic cover features at comparable
accuracy (Andréfouët et al. 2003; Roelfsema and Phinn 2010).
Due to the broad spectral bands used in multispectral systems, their utility for
mapping quantitative biophysical properties (e.g., pigment concentration) is limited
since the narrow width of absorption features associated with photosynthetic and
non-photosynthetic pigments cannot be resolved. Multispectral data do contain
Fig. 1.5 Example spectral signatures from the same patch of live coral. The progression of
reflectance signature graphs, from left to right is: in-situ reflectance from field spectrometry,
modelled at-surface reflectance with 1.0 m of water, at-surface reflectance from airborne
hyperspectral (CASI 2); and at-surface reflectance (x 10,000) from satellite multispectral
(QuickBird 2) image (provided by Ian Leiper)
16
S. R. Phinn et al.