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Digital image mosaics of the nearshore coastal waters of selected areas on the Hawaiian Islands of Hawai'i, Maui, Moloka'i, and O'ahu using aerial photographs and SHOALS airborne lidar bathymetry data

December 21, 2016

The lack of geographic and thematic maps of coral reefs limits our understanding of reefs and our ability to assess change. The U.S. Geological Survey (USGS) has the capability to compile digital image mosaics that are useful for creating detailed map products. Image maps covering the shallow near-shore coastal waters have been produced for several of the main Hawaiian Islands, including Hawaii, Maui, Molokai, and Oahu and are presented in JPEG2000 (.jp2) format.

The digital-image mosaics were generated by first scanning historical aerial photographs. At the time, available satellite image resolutions were not acceptable and the aerial photographs used were the best option. The individually scanned digital images were tone-and color-matched and then digitally mosaicked together using spatial matching. Separately, black and white digital orthophoto quads (DOQs) or digital raster graphics (DRGs) of the same areas were merged with shaded-relief images generated from lidar bathymetry data. The resulting black and white images covering both near-shore coastal waters and on-land areas became the geometric masters for the mosaics generated from the aerial photographs. The aerial-photograph mosaics were geometrically corrected to overlay the master data set by using hundreds of image-to-image geometric control points and slaving the mosaic onto the master.

The USGS has been investigating the use of remotely sensed image and spatial data to help map and study coral reef environments. Interpretation of these data is corroborated by extensive field mapping and correlation with field measured distribution and density of coral cover and other coral reef cover types. An immediate result of this effort was the generation of very-high spatial resolution georeferenced image maps of critical coral reef habitat areas being studied by the USGS and others.

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