Data

Creating and Interpreting Digital Orthophotographs

Creating the Digital Mosaic

Aerial photographs were acquired for the Main Eight Hawaiian Islands Benthic Mapping Project in 2000 by NOAA Aircraft Operation Centers aircraft and National Geodetic Survey cameras and personnel. Approximately 1,500, color, 9 by 9 inch photos were taken of the coastal waters of the Main Eight Hawaiian Islands at 1:24,000 scale (see Table 1). Specific sun angle and maximum percent cloud cover restrictions were adhered to when possible during photography missions to ensure collection of high quality imagery for the purpose of benthic mapping. In addition, consecutive photos were taken at 60 percent overlap on individual flight lines and 30 percent overlap on adjacent flight lines to allow for orthorectification and elimination of sun glint.

Table 1: Mosaic Specifications for each Island. Error values are in meters.

Location	UTM Zone	Photo Scale	Pixel Width (m)	Number of Photos	RMSE X (m)	RMSE Y (m)	RMSE Z (m)
Niihau	4	1:24000	1.0	15	0.054	1.120	0.300
Kauai	4	1:24000	1.0	66	2.582	2.884	1.453
Oahu	4	1:24000	1.0	25	1.437	1.382	1.139
Lanai	4	1:24000	1.0	11	1.687	1.873	1.116
Molokai	4	1:24000	1.0	31	.887	1.024	0.027
Maui	4	1:24000	1.0	37	1.417	1.223	1.502
Hawaii	5	1:24000	1.0	21	1.169	1.093	.566

Prints and diapositives

Prints and diapositives (color transparencies) were created from the original negatives. Diapositives were then scanned at a resolution of 500 dots per inch (DPI) using a metric scanner, yielding 1.0 by 1.0 meter pixels for the 1:24,000 scale photography. All scans were saved in tagged image file format (TIFF) for the purposes of orthorectification and photointerpretation. Original TIFFs were also converted to *.jpg format to reduce file size and facilitate web-based image distribution, and are currently available on the NOAA Biogeography Program's Web Site at 72, 150, and 500 DPI resolution.

Georeferencing/mosaicing of the TIFFs

Georeferencing/Mosaicing of the TIFFs was performed using a variety of softcopy photogrammetric software including Socet Set Version 4.4.0, Autometric Softplotter, PCI OrthoEngine, and Erdas OrthoBase. First, lens correction parameters were applied to each frame to eliminate image distortion. Airborne kinematic GPS (location of the aircraft at the time of each exposure) was then used when available to provide a first order geolocation. When this information was not available, measurements were made between flightline strips for input into aerial triangulation software to provide preliminary co-registration.

Image to Image Tie-Points

Image to image tie-points (distinct features visible in overlap areas of each frame such as street intersections, piers, coral heads, reef edges, and bridges) were then used to further co-register the imagery, especially for photos taken over open water where ground control points were not available (see below). Softcopy photogrammetry software has limited ability to automatically find such features common to overlapping photographs but this automated function performs poorly for submerged features.

Ground Control Points (GCPs) for Georeferencing

Fixed ground features visible in the scanned photos were selected for ground control points (GCPs) which were then used to georeference the imagery (i.e. link the image pixels to a real world coordinate system such as latitude/longitude). GCPs were measured and then differentially corrected to the closest Continuously Operating Reference System (CORS). We obtained points with a wide distribution throughout the imagery, especially on peninsulas and outer islands whenever possible since this results in the most accurate registration throughout each image. Only ground control points for terrestrial features were collected due to the difficulty of obtaining precise positions for submerged features (see Appendix 1: Ground Control Points).

Digital Elevation Models (DEM)

Pre-existing U.S. Geological Survey (USGS) 10 meter digital elevation models used to correct for relief displacement (http://biogeo.nos.noaa.gov/products/mapping/dems/). Once draft orthorectified mosaic was produced, a set of independent ground control points (i.e. check points) were used to measure the quality of each mosaic's rectification and ensure that it required horizontal and vertical spatial accuracy limits. If the spatial accuracy was not acceptable based on this comparison, additional modifications were made to the GCPs, tie-points, etc., until a satisfactory mosaic was created for each island. In general, mosaics were georeferenced such that pixels are positioned within one pixel width of their correct location.

Finalizing the Process

Spatial accuracy of the mosaics is reported in Table 1. Values reported are an average for the entire mosaic, therefore, accuracy of features near land (near GCPs) are generally better than the values reported while the accuracy of features away from land is generally not as good. Where no land is in the original photographic frame only kinematic GPS and tie points were used to georeference the images.

Once all the photos were orthorectified, the best segments of each photo were selected for creation of the final mosaic. Segments of each photo were selected to minimize sun glint, cloud interference, turbidity, etc. in the final mosaic. Where possible, parts of images obscured by sun glint or clouds were replaced with cloud/glint free parts of overlapping images. As a result, most mosaics have few or no clouds or sun glint obscuring bottom features. However, in some cases, clouds, sun glint, or turbid areas could not be replaced with overlapping imagery. In these areas, such obstructions were minimized but could not be eliminated completely.

Segments from 500 out of the approximately 1,500 original aerial photos were selected to create the final mosaic (Table 1). Final mosaics were created in "geoTIF" file format (georeferenced image file) with the following projection parameters:

• North American Datum 83
• Universal Transverse Mercator (UTM)
• UTM Zone 4 for Niihau, Kauai, Oahu, Molokai, Lanai, and Maui, and UTM Zone 5 for Hawaii

These files are available on the Benthic Habitats of theMain Eight Hawaiian Islands CD-ROM and at the NOAA Biogeography Program's Web Site in Mr. SID format. No color balancing was attempted since this alters color and textural signatures in the original imagery and interferes with the photointerpreter's ability to delineate habitats. As a result, mosaics have visible seams between adjacent photos. This provides the photointerpreter with "true color" imagery for maximum ability to identify and delineate benthic features.