Satellite imagery is a valuable tool for natural resource managers and researchers since they provide an excellent record of the location and extent of habitats. Benthic habitat maps of the main 8 Hawaiian Islands were created through visual interpretation of multispectral IKONOS™ and Quickbird™ imagery using the NOAA Habitat Digitizer extension. Habitat boundaries were delineated around signatures (e.g., areas with specific color and texture patterns) in the orthorectified imagery mosaic corresponding to habitat types in the Classification Scheme. The custom Habitat Digitizer extension was used, digitizing at a scale of 1:6,000 with a 1-acre minimum mapping unit. Generally, feature detection of seafloor habitats was possible from the shoreline to water depths of approximately 30 meters, depending on water clarity. In order to optimize the satellite imagery for visual interpretation, a number of processing steps were implemented to enhance the geopositioning and clarity of the imagery. These steps include: orthorectification to remove spatial distortions in the imagery due to relief displacement; pansharpening; deglinting; and generating normalized reflectance values.
Multispectral IKONOS™ and Quickbird™ satellite imagery, from Space Imaging Inc. and Digital Globe respectively, was used for creating all maps. IKONOS imagery was 11-bitprecision and included both pan-chromatic and multispectral four-band data. The IKONO Ssatellite orbits the Earth every 98 minutes at an altitude of approximately 680 kilometers or 423 miles. IKONOS is in a sun-synchronous orbit, passing a given longitude at about the same local time (10:30 A.M.) daily and can produce 1-meter imagery of the same geography every 3 days. The Satellite sensor elevation angle, the angle from horizon to sensor as seen from the area of interest (AOI), is typically > 60°. Swath size for a single scene is 11 km x 11 km. Information on IKONOS was taken from the Space Imaging website ( http://www.geoeye.com).
Quickbird imagery had similar characteristics to IKONOS, 11-bit precision pan-chromatic band and four-band multispectral, but has slightly higher ground resolution and increased blue signal in the panchromatic band. The Quickbird satellite orbits every 93.4 minutes at an altitude of 450 km with a 98 degree, sun-synchronous inclination. Views are revisited with a frequency of 3-7 days depending on latitude at 60-cm resolution and viewing angle can be changed for in-track and cross-track pointing. Swath size for a single scene is 16.5 km x 16.5 km. Information about Quickbird data from the Digital Globe website: http://www.digitalglobe.com/downloads.
Band centers, ground resolution and calibration coefficients for IKONOS and Quickbird are shown below in Table 1 and Table 2 respectively. Radiometric calibration was done by multiplying raw imagery in digital units (DN) by the calibration factor and then dividing by the spectral bandwidth. Imagery received from Space Imaging was evaluated for quality before any processing commenced; any raw data containing undesirable environmental features, such as excessive glint, cloud cover, or other factors that obscured bottom features, were rejected.
| Band | λ center (nm) | λ range (nm) | Resolution (m) | Radiometric Cal Factor DN*cm 2 *sr/mW |
|---|---|---|---|---|
| Pan | 727.1 | 525.8 - 928.5 | 1 | Not used |
| Blue | 480.3 | 444.7 - 516 | 4 | 728 |
| Green | 550.7 | 506.4 - 595 | 4 | 727 |
| Red | 664.8 | 631.9 - 697.7 | 4 | 949 |
| NIR | 805 | 757.3 - 852.7 | 4 | 843 |
| Band | λ center (nm) | λ range (nm) | Resolution (m) | Radiometric Cal Factor DN*cm 2 *sr/mW |
|---|---|---|---|---|
| Pan | 675 | 450 - 900 | 0.61 | Not used |
| Blue | 485 | 450 - 520 | 2.44 | 623 |
| Green | 560 | 520 - 600 | 2.44 | 695 |
| Red | 660 | 630 - 690 | 2.44 | 789 |
| NIR | 830 | 760 - 900 | 2.44 | 648 |
The initial imagery evaluation centered around one main criterion; "Is the bottom visible?" After the satisfaction of the technical and quantifiable parameters were verified, each image was deemed either acceptable or not based mainly on whether bottom features were visible between the shoreline and the 30 meter isobath. The first group of quantified parameters could be confirmed by the imagery metadata, those parameters were:
Each image was required to meet certain spatial parameters as well:
Furthermore, each acceptable image was required to meet certain spectral quality criteria:
After an image had met the more quantifiable criteria, it could then be qualitatively evaluated based bottom visibility. Overall, the main obstruction of the bottom was suspended sediment but white caps, white wash, foam, and breaking waves also caused visibility issues. Images that were not pristine but deemed marginally acceptable were evaluated to estimate the percentage of usable imagery. The area of obstructed bottom (be it from sediment, waves, clouds, etc.) was converted to a shape file and subtracted from the total mapping area of that image. If the unusable area was 10% or less of the total, the image was acceptable. Another consideration made regarding marginal imagery was the likelihood of getting better data over the same area at a later time due to the calm nature of winter waves and weather patterns.
