3.2 PAST AND PRESENT SENSORS
Over the past 20 years, sensors have evolved in their capability to capture spatial
and temporal characteristics of coastal resources inhabited by coral reef ecosystems
(Table 4). The evolutionary route of mapping coastal environments began with
measurements of ocean temperatures, chlorophyll, and sediment concentrations
(Robinson, 1985). Features with a large spatial extent, consistent behavior over time,
and strong and distinct spectral response, were readily observed using passive remote
sensing techniques (Robinson, 1985). These techniques offered the advantage of
quickly retrieving environmental information for a region that normally could be
examined on the ground only within a timeframe of weeks or months. Areal coverage,
timely data acquisition, digital representation of marine features, and sensitivity to
temporal changes in environmental attributes are still prominent advantages of airborne
or spaceborne imaging.
In recent years, there has been an explosion of interest in hyperspectral and high
spatial resolution satellite imagery (Table 5) (Green et al., 2000). The appeal is in the
enormous versatility of these sensors, as operators mix and match spatial and spectral
resolutions (Klemas, 2001) to create an ideal suite of specifications for mapping reef
features. The finer spatial and spectral detail is useful for improving discrimination
between similar reef features, while the temporal flexibility allows the operator to avoid
unfavorable environmental conditions (Green et al., 1996). In 1999, the Ikonos satellite
was launched and has since collected radiometric detail in 1-m panchromatic and 4-m
multi-spectral detail. This increased spatial resolution has proven to be beneficial in the
classification of reef structure (Mumby et al., 2004).
A disadvantage of some of these newer satellites with commercial mandates is a
minimum coverage order requirement for non-US regions that is typically large and
costly; hence satellite use is primarily suited to well-funded organizations (Green et al.,
2000). In addition, computer storage space and processing requirements increase
exponentially with higher spatial and spectral resolution data (Aplin et al., 1997). A
further disadvantage to some airborne systems is that they are often restricted to their
country of origin (Green et al., 2000, Whitehouse and Hutt, this book). Transporting
sensors is difficult, and even when a system is permitted to enter a country, necessary
modifications to the host aircraft may not be permitted (Monaco, Personal
Communication, February 1999).
3.3 ADVANCEMENTS IN MAPPING TECHNIQUES
The differences between remote sensing efforts for mapping reefs just a few years
ago and today is revealed in widely available literature (Table 6) and can be categorized
into four separate areas: (1) clearly defining a reef environment within a biological
context, but modified to match resolution capabilities of a pre-defined sensor;
(2) identifying the most appropriate techniques for discriminating the spectral
wavelengths of coral reef classes; (3) designing the most appropriate field survey
techniques to match spatial and spectral characteristics of selected sensors; and (4)
capitalizing on the increasingly available multi-temporal image data sets to detect
changes in reef features. The focus of most recent studies has been to improve imagefeature delineation, thereby identifying feature type, extent, and change with heightened
precision and accuracy.
257
Mapping and Management of Coral
Over the past 20 years, sensors have evolved in their capability to capture spatial
and temporal characteristics of coastal resources inhabited by coral reef ecosystems
(Table 4). The evolutionary route of mapping coastal environments began with
measurements of ocean temperatures, chlorophyll, and sediment concentrations
(Robinson, 1985). Features with a large spatial extent, consistent behavior over time,
and strong and distinct spectral response, were readily observed using passive remote
sensing techniques (Robinson, 1985). These techniques offered the advantage of
quickly retrieving environmental information for a region that normally could be
examined on the ground only within a timeframe of weeks or months. Areal coverage,
timely data acquisition, digital representation of marine features, and sensitivity to
temporal changes in environmental attributes are still prominent advantages of airborne
or spaceborne imaging.
In recent years, there has been an explosion of interest in hyperspectral and high
spatial resolution satellite imagery (Table 5) (Green et al., 2000). The appeal is in the
enormous versatility of these sensors, as operators mix and match spatial and spectral
resolutions (Klemas, 2001) to create an ideal suite of specifications for mapping reef
features. The finer spatial and spectral detail is useful for improving discrimination
between similar reef features, while the temporal flexibility allows the operator to avoid
unfavorable environmental conditions (Green et al., 1996). In 1999, the Ikonos satellite
was launched and has since collected radiometric detail in 1-m panchromatic and 4-m
multi-spectral detail. This increased spatial resolution has proven to be beneficial in the
classification of reef structure (Mumby et al., 2004).
A disadvantage of some of these newer satellites with commercial mandates is a
minimum coverage order requirement for non-US regions that is typically large and
costly; hence satellite use is primarily suited to well-funded organizations (Green et al.,
2000). In addition, computer storage space and processing requirements increase
exponentially with higher spatial and spectral resolution data (Aplin et al., 1997). A
further disadvantage to some airborne systems is that they are often restricted to their
country of origin (Green et al., 2000, Whitehouse and Hutt, this book). Transporting
sensors is difficult, and even when a system is permitted to enter a country, necessary
modifications to the host aircraft may not be permitted (Monaco, Personal
Communication, February 1999).
3.3 ADVANCEMENTS IN MAPPING TECHNIQUES
The differences between remote sensing efforts for mapping reefs just a few years
ago and today is revealed in widely available literature (Table 6) and can be categorized
into four separate areas: (1) clearly defining a reef environment within a biological
context, but modified to match resolution capabilities of a pre-defined sensor;
(2) identifying the most appropriate techniques for discriminating the spectral
wavelengths of coral reef classes; (3) designing the most appropriate field survey
techniques to match spatial and spectral characteristics of selected sensors; and (4)
capitalizing on the increasingly available multi-temporal image data sets to detect
changes in reef features. The focus of most recent studies has been to improve imagefeature delineation, thereby identifying feature type, extent, and change with heightened
precision and accuracy.
257
Mapping and Management of Coral