underwater objects (Tewinkel 1963) and estimate underwater topography (Murase
et al. 2008), which could be potentially helpful in mapping the geomorphology of
reef systems. Andréfouët et al. (2002) show that using high-resolution spatial
information from aerial photographs, rather than optical data from other airborne
or satellite sensors, results in more accurate detection and mapping of the spatial
patterns found during coral bleaching events. Fletcher et al. (2003) merged aerial
orthophoto mosaics with NOAA topographic surveys (T-sheets) and hydrographic
surveys (H-sheets) to map historical shoreline position and calculate coastal erosion rates on the Hawaiian island of Maui.
Since aerial photography has the longest history in coral reef remote sensing, it
offers unparalleled usefulness to those studies documenting change through time.
Armstrong (1981) used vertical panchromatic aerial photographs to calculate over
40 years of change, including the effects from two hurricanes, on a coral reef off
Puerto Rico. To compute the exact scale of each photograph, which is necessary
for accurate calculations of habitat areas and measuring change, known man-made
structures were measured in the field and compared to their image on the photographs. In addition to the vertical photographs, oblique photographs were used to
assist in defining different reef features. Yamano et al. (2000) used aerial photography to track a 21 year change in coral zonation. Lewis (2002) used aerial
photographs to track the loss of geomorphologic reef structure over a 40 year
period, while Hernandez-Cruz et al. (2006) used aerial photographs to document
seagrass extents over a 63 year period.
Most recently, commercial and government aerial photography has transitioned
away from film-based systems to digital cameras. This transition has significantly
improved the extent of coverage, consistency of photo quality, ability to conduct
automated processes, and ease of integration of aerial photographs with other
spatial information. For example, Palandro et al. (2003) combined aerial photographs with IKONOS imagery from an orbital satellite to track changes on a coral
reef over a 19 year period. In order to merge film-based prints with modern digital
data, it is first necessary to scan the prints. Often, different sets of aerial photographs are acquired at different altitudes using different cameras and lenses,
resulting in different spatial resolutions and scales. Scanning the photographs at a
high resolution (typically 300 dpi or higher) and then employing resampling and
geographic correction provides direct comparison on a pixel-to-pixel basis
between different sets of imagery. Advanced computer techniques simplify the
merging process of different datasets.
2.3 Photography Analysis and Classification Techniques
Low-altitude aerial photography is an excellent resource for studies of coral reef
habitats due to its high resolution. As with other remote sensing technologies,
aerial photography also enables a synoptic view of large areas of study, something
that is not typically possible when conducting on-the-ground or in-the-water
32
S. A. Cochran
et al. 2008), which could be potentially helpful in mapping the geomorphology of
reef systems. Andréfouët et al. (2002) show that using high-resolution spatial
information from aerial photographs, rather than optical data from other airborne
or satellite sensors, results in more accurate detection and mapping of the spatial
patterns found during coral bleaching events. Fletcher et al. (2003) merged aerial
orthophoto mosaics with NOAA topographic surveys (T-sheets) and hydrographic
surveys (H-sheets) to map historical shoreline position and calculate coastal erosion rates on the Hawaiian island of Maui.
Since aerial photography has the longest history in coral reef remote sensing, it
offers unparalleled usefulness to those studies documenting change through time.
Armstrong (1981) used vertical panchromatic aerial photographs to calculate over
40 years of change, including the effects from two hurricanes, on a coral reef off
Puerto Rico. To compute the exact scale of each photograph, which is necessary
for accurate calculations of habitat areas and measuring change, known man-made
structures were measured in the field and compared to their image on the photographs. In addition to the vertical photographs, oblique photographs were used to
assist in defining different reef features. Yamano et al. (2000) used aerial photography to track a 21 year change in coral zonation. Lewis (2002) used aerial
photographs to track the loss of geomorphologic reef structure over a 40 year
period, while Hernandez-Cruz et al. (2006) used aerial photographs to document
seagrass extents over a 63 year period.
Most recently, commercial and government aerial photography has transitioned
away from film-based systems to digital cameras. This transition has significantly
improved the extent of coverage, consistency of photo quality, ability to conduct
automated processes, and ease of integration of aerial photographs with other
spatial information. For example, Palandro et al. (2003) combined aerial photographs with IKONOS imagery from an orbital satellite to track changes on a coral
reef over a 19 year period. In order to merge film-based prints with modern digital
data, it is first necessary to scan the prints. Often, different sets of aerial photographs are acquired at different altitudes using different cameras and lenses,
resulting in different spatial resolutions and scales. Scanning the photographs at a
high resolution (typically 300 dpi or higher) and then employing resampling and
geographic correction provides direct comparison on a pixel-to-pixel basis
between different sets of imagery. Advanced computer techniques simplify the
merging process of different datasets.
2.3 Photography Analysis and Classification Techniques
Low-altitude aerial photography is an excellent resource for studies of coral reef
habitats due to its high resolution. As with other remote sensing technologies,
aerial photography also enables a synoptic view of large areas of study, something
that is not typically possible when conducting on-the-ground or in-the-water
32
S. A. Cochran