to a known map projection (Robinson et al. 2000). A complete background on the
use of astronaut-acquired photography and subsequent digital data for remote
sensing purposes is explored fully in Robinson et al. (2002).
After preprocessing, georeferenced digital images may be analyzed individually, or mosaicked together to cover an entire coral reef region (Chavez et al.
2000). Occasionally, the differences in cross-scene illumination due to bidirectional sun angles, common on low-altitude aerial photography as an aircraft flies
back and forth across a study area, are apparent when creating a mosaic of individual aerial images. In such cases, additional radiometric corrections will be
required to normalize the pixels across the individual images prior to mosaicing
(Lillesand and Kieffer 1994; Beisl and Woodhouse 2004; Beisl et al. 2006).
Finally, if desired, a land mask can be applied in order to focus subsequent
computer analysis on the in-water features only.
Analysis of aerial photography for coral reef studies can range from a basic
visual interpretation to more complex computer-aided classifications. At its simplest and most effective form, aerial photography can be used for mapping coral
reefs by hand-drawing polygons on the hard photographic copies themselves, with
or without transparent overlays (e.g., Manoa Mapworks 1984), or digitized on a
visual display in either image processing or geographic information software, a
technique known as ‘heads up’ digitizing (e.g., Coyne et al. 2003; Scopélitis et al.
2009).
Supervised or unsupervised computer classification may be performed using the
spectral information in digitized aerial photographs. Color film records light
reflected in three wavelengths—red, green, and blue (RGB). Color-infrared (CIR)
film, also known as false-color photography, records light reflected in the nearinfrared, red, and green wavelengths. CIR imaging is often used in land-mapping
applications, as healthy vegetation reflects more radiation in the near-infrared
(NIR) wavelengths than green wavelengths. However, NIR wavelengths are
absorbed within the first few cm of water, and the use of false-color photography
for coral reef applications must take this into consideration. Hopley and Catt
(1988) describe the use of NIR photography to monitor the ecological response of
sea-level rise on several sites in the Great Barrier Reef, noting that the acquisitions
were timed to coincide with negative daytime tides. So while CIR images are
useful for mapping mangroves or other emergent vegetation, as well as some
shallow (less than *3 m) reef flat habitats (Fig. 2.2), CIR photography is typically
not used in coral reef management applications where it is necessary to image
reefs in greater than *3 m water depth (Hopley 1978).
Because lack of light penetration in deeper water is a limitation when using
aerial photographs to study coral reefs (as well as multispectral and hyperspectral
remote sensing), they are limited in their use as the sole source of information for
inferring coral abundance, species types, and other benthic studies across the entire
reef. For these types of investigations, it is helpful to combine aerial photography
with other remotely sensed or field-collected data. For example, Fig. 2.3 shows (a)
aerial photography of a coral reef ecosystem off the west coast of the island of
Hawai‘i, and (b) the same imagery merged with high-resolution LiDAR
34
S. A. Cochran
use of astronaut-acquired photography and subsequent digital data for remote
sensing purposes is explored fully in Robinson et al. (2002).
After preprocessing, georeferenced digital images may be analyzed individually, or mosaicked together to cover an entire coral reef region (Chavez et al.
2000). Occasionally, the differences in cross-scene illumination due to bidirectional sun angles, common on low-altitude aerial photography as an aircraft flies
back and forth across a study area, are apparent when creating a mosaic of individual aerial images. In such cases, additional radiometric corrections will be
required to normalize the pixels across the individual images prior to mosaicing
(Lillesand and Kieffer 1994; Beisl and Woodhouse 2004; Beisl et al. 2006).
Finally, if desired, a land mask can be applied in order to focus subsequent
computer analysis on the in-water features only.
Analysis of aerial photography for coral reef studies can range from a basic
visual interpretation to more complex computer-aided classifications. At its simplest and most effective form, aerial photography can be used for mapping coral
reefs by hand-drawing polygons on the hard photographic copies themselves, with
or without transparent overlays (e.g., Manoa Mapworks 1984), or digitized on a
visual display in either image processing or geographic information software, a
technique known as ‘heads up’ digitizing (e.g., Coyne et al. 2003; Scopélitis et al.
2009).
Supervised or unsupervised computer classification may be performed using the
spectral information in digitized aerial photographs. Color film records light
reflected in three wavelengths—red, green, and blue (RGB). Color-infrared (CIR)
film, also known as false-color photography, records light reflected in the nearinfrared, red, and green wavelengths. CIR imaging is often used in land-mapping
applications, as healthy vegetation reflects more radiation in the near-infrared
(NIR) wavelengths than green wavelengths. However, NIR wavelengths are
absorbed within the first few cm of water, and the use of false-color photography
for coral reef applications must take this into consideration. Hopley and Catt
(1988) describe the use of NIR photography to monitor the ecological response of
sea-level rise on several sites in the Great Barrier Reef, noting that the acquisitions
were timed to coincide with negative daytime tides. So while CIR images are
useful for mapping mangroves or other emergent vegetation, as well as some
shallow (less than *3 m) reef flat habitats (Fig. 2.2), CIR photography is typically
not used in coral reef management applications where it is necessary to image
reefs in greater than *3 m water depth (Hopley 1978).
Because lack of light penetration in deeper water is a limitation when using
aerial photographs to study coral reefs (as well as multispectral and hyperspectral
remote sensing), they are limited in their use as the sole source of information for
inferring coral abundance, species types, and other benthic studies across the entire
reef. For these types of investigations, it is helpful to combine aerial photography
with other remotely sensed or field-collected data. For example, Fig. 2.3 shows (a)
aerial photography of a coral reef ecosystem off the west coast of the island of
Hawai‘i, and (b) the same imagery merged with high-resolution LiDAR
34
S. A. Cochran