Creating and Interpreting Digital Orthophotographs

Introduction

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 American Samoa, Guam, and the Commonwealth of the Northern Mariana Islands were created through visual interpretation of multispectral IKONOS 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.

Satellite Imagery

The IKONOS satellite, owned and operated by Space Imaging Incorporated, LLC, provides commercially available panchromatic (black and white) and multispectral (blue/green/red/near-infrared) imagery. The panchromatic imagery has a 1 m pixel dimension (meaning features larger then 1 m² can be detected in the imagery). The multispectral imagery has a 4 m pixel dimension (meaning features larger then 16 m² can be seen in the imagery). The IKONOS imagery is purchased in 11 km wide swaths that are mosaicked together to produce complete images of locales. IKONOS satellite imagery provides precise and robust data with spectral and spatial resolution suitable for shallow water benthic mapping. Furthermore, satellite imagery provides efficient and effective global coverage for repeated imaging of remote islands that are often obscured by cloud cover.

All of the IKONOS imagery was purchased in National Imagery Transmission Format (NITF) with the associated Rational Polynomial Coefficients (RPCs or satellite ephemeris data). When using image analysis software capable of reading NITF files with associated RPCs, the initial horizontal positioning error never exceeded 15 m (for locations where there is little or no vertical relief to affect image pixel displacement).

Orthorectification

During orthorectification, digital imagery is subjected to algorithms that eliminate each source of spatial distortion. The result is a georeferenced digital mosaic of several imagery scenes with uniform scale throughout the mosaic. After an orthorectified mosaic is created, visual interpreters can accurately and reliably delineate the boundaries of features in the imagery as they appear on the computer monitor using a software interface such as the Habitat Digitizer. Through this process, natural resources managers and researchers are provided with spatially accurate maps of habitats and other features visible in the imagery.