HDTV video) was accomplished using the satellite image as a reference by means
of easily identifiable control points found on the atoll rims.
The near infrared (NIR) bands from the satellite imagery and the red bands
from the Space Shuttle imagery were stretched and thresholded to create binary
images with the lagoon water as one endmember (i.e., 0 = black) and with clouds
and sub-surface pinnacles as the other (i.e., 255 = white). The binary images were
compared and any pixels that were not the same across images were considered to
be clouds. A tolerance factor was included to compensate for any errors in geometric rectification, and newly confirmed pinnacles were used as additional control
points to refine the geometric rectification of the Space Shuttle imagery.
Andréfouët and Robinson (2003) noted that this method of evaluation required
the images to be relatively cloud-free, as clouds that appear on the same pixel
across multiple images could be misinterpreted as pinnacles. Additional temporally different imagery can help further refine the interpretation. The authors also
found that deeper sub-surface pinnacles were sometimes difficult to distinguish
using the processed red bands due to their weaker spectral signatures. Stretching
and thresholding the green bands in these instances helped with the interpretation.
Astronaut-acquired space photography provides a low-cost alternative to
commercial satellite imagery, which can be important for developing countries and
for other budget-restricted studies requiring multi-temporal sets of imagery.
Although limited in its usefulness as a primary data source for coral reef mapping
studies due to challenges with spatial rectification, astronaut-acquired photography
is well suited as a secondary data source to complement other forms of remotely
sensed data.
2.4.4 Suspended Sediment Studies
Historical changes to tropical island watersheds often result in increased landbased pollution, including sediment, nutrients, and other pollutants that threaten
many coral reefs worldwide. Terrestrial sediment is introduced to the nearshore
environment through erosion and storm runoff, and while a natural background
amount is expected in a healthy ecosystem, excessive amounts can lead to degradation of reefs (Field et al. 2008). Sediment that settles on coral can inhibit
feeding, reduce recruitment, and decrease calcification rates. Fine sediment suspended in the water column can block light and thereby decrease sunlight needed
for photosynthesis (Fabricius 2005).
Historical land-use changes on the Hawaiian island of Moloka‘i have resulted
in large amounts of sediment delivered from the uplands to the coast and deposited
on the inner reef flat. This sediment is then resuspended in a daily cycle of tides,
wind, and waves. In 2005, the U.S. Geological Survey used digital aerial photography, along with LiDAR bathymetry data and in situ water sampling, to
quantitatively map suspended sediment concentrations in the top 0.5 m of the
water column on the south shore of Moloka‘i (Cochran et al. 2008).
2 Photography Application
43
of easily identifiable control points found on the atoll rims.
The near infrared (NIR) bands from the satellite imagery and the red bands
from the Space Shuttle imagery were stretched and thresholded to create binary
images with the lagoon water as one endmember (i.e., 0 = black) and with clouds
and sub-surface pinnacles as the other (i.e., 255 = white). The binary images were
compared and any pixels that were not the same across images were considered to
be clouds. A tolerance factor was included to compensate for any errors in geometric rectification, and newly confirmed pinnacles were used as additional control
points to refine the geometric rectification of the Space Shuttle imagery.
Andréfouët and Robinson (2003) noted that this method of evaluation required
the images to be relatively cloud-free, as clouds that appear on the same pixel
across multiple images could be misinterpreted as pinnacles. Additional temporally different imagery can help further refine the interpretation. The authors also
found that deeper sub-surface pinnacles were sometimes difficult to distinguish
using the processed red bands due to their weaker spectral signatures. Stretching
and thresholding the green bands in these instances helped with the interpretation.
Astronaut-acquired space photography provides a low-cost alternative to
commercial satellite imagery, which can be important for developing countries and
for other budget-restricted studies requiring multi-temporal sets of imagery.
Although limited in its usefulness as a primary data source for coral reef mapping
studies due to challenges with spatial rectification, astronaut-acquired photography
is well suited as a secondary data source to complement other forms of remotely
sensed data.
2.4.4 Suspended Sediment Studies
Historical changes to tropical island watersheds often result in increased landbased pollution, including sediment, nutrients, and other pollutants that threaten
many coral reefs worldwide. Terrestrial sediment is introduced to the nearshore
environment through erosion and storm runoff, and while a natural background
amount is expected in a healthy ecosystem, excessive amounts can lead to degradation of reefs (Field et al. 2008). Sediment that settles on coral can inhibit
feeding, reduce recruitment, and decrease calcification rates. Fine sediment suspended in the water column can block light and thereby decrease sunlight needed
for photosynthesis (Fabricius 2005).
Historical land-use changes on the Hawaiian island of Moloka‘i have resulted
in large amounts of sediment delivered from the uplands to the coast and deposited
on the inner reef flat. This sediment is then resuspended in a daily cycle of tides,
wind, and waves. In 2005, the U.S. Geological Survey used digital aerial photography, along with LiDAR bathymetry data and in situ water sampling, to
quantitatively map suspended sediment concentrations in the top 0.5 m of the
water column on the south shore of Moloka‘i (Cochran et al. 2008).
2 Photography Application
43