These distortions of scale within an image can be removed through orthorectification. During orthorectification, digital scans of aerial photos are subjected to algorithms that eliminate each source of spatial distortion. The result is a georeferenced digital mosaic of several photographs with uniform scale throughout the mosaic. Features near land are generally georeferenced with greater accuracy 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 locations and image tie points were used to georeference the images. After the orthorectified mosaics were created, photointerpreters were able to accurately and reliably delineate boundaries of features in the imagery as they appear on the computer monitor.
Department of Commerce (DOC), National Oceanic and Atmospheric Administration (NOAA), National Environmental Satellite Data, and Information Service (NESDIS), National Geophysical Data Center (NGDC), 20010501, Benthic Habitats of Puerto Rico and the U.S. Virgin Islands; Photomosaic of U.S. Virgin Islands (St. Thomas), 1999: NOAA's National Geophysical Data Center (NGDC), Boulder, CO.This is part of the following larger work.
Department of Commerce (DOC), National Oceanic and Atmospheric Administration (NOAA), National Ocean Service (NOS), National Centers for Coastal Ocean Science (NCCOS), Center for Coastal Monitoring and Assessment (CCMA), Biogeography Program, 20011201, Benthic Habitat Maps of Puerto Rico and the U.S. Virgin Islands Prepared by Visual Interpretation from Remote Sensing Imagery Collected by NOAA Year 1999: NOAA's Ocean Service, National Centers for Coastal Ocean Science (NCCOS), Silver Spring, MD.
This is a Raster data set.
The map projection used is Transverse Mercator.
Planar coordinates are encoded using Coordinate Pair
Abscissae (x-coordinates) are specified to the nearest 2.4
Ordinates (y-coordinates) are specified to the nearest 2.4
Planar coordinates are specified in meters
The horizontal datum used is North American Datum of 1983.
The ellipsoid used is Geodetic Reference System 80.
The semi-major axis of the ellipsoid used is 6378137.
The flattening of the ellipsoid used is 1/298.257.
National Geophysical Data Center
The National Ocean Service is conducting research to digitally map biotic resources and coordinate a long-term monitoring program that can detect and predict change in U.S. coral reefs, and their associated habitats and biological communities.
National Oceanic and Atmospheric Administration (NOAA), National Ocean Service (NOS), National Geodetic Survey (NGS), 1999, National Geodetic Survey Aerial Photography of Puerto Rico and the U.S. Virgin Islands, 1999: NOAA's Ocean Service, National Geodetic Survey (NGS), Silver Spring, MD.
Print and diapositives were created from the original negatives. Diapositives were then scanned at a resolution of 500 dots per inch (DPI) using a metric scanner, yielding 2.4 by 2.4 meter pixels for the 1:48000 scale photography. All scans were saved in TIFF format for the purposes of orthorectification and photointerpretation.
Georeferencing/mosaicing of the TIFF's was performed using Socet Set Version 4.2.1. Lens correction parameters were applied to each frame to eliminate image distortion. Airborne kinematic GPS 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 Socet Set to provide preliminary co-registration.
Image to image tie-points were then used to further co-register the imagery, especially for photos taken over open water where ground control points were not available. Fixed ground features visible in the scanned photos were selected for ground control points (GCP's) which were then used to georeference the imagery. GCP's were measured using real-time DGPS (differential Global Positioning System). Points were obtained 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 difficulty of obtaining precise positions for submerged features.
A custom digital terrain model (DTM) was then created using the Socet Set software to correct for feature displacement due to terrain effects. To accomplish this, water features and the shoreline were set to an elevation of zero. Preliminary experimentation revealed that the effects of refraction on the position of submerged features in the imagery were not significant enough to make a correction for underwater displacement according to Snell's law. Selected land elevation points were then inserted from USGS 1:24000 Digital Elevation Models or other elevation data sets where clouds or other sources of interference prevented the Socet Set software from automatically making an accurate DTM.
Once the terrain models were complete and a draft orthorectified mosaic was produced, a set of independent ground control points was used to measure the quality of each mosaic's rectification and ensure that it met acceptable limits of horizontal spatial accuracy. If spatial accuracy was not acceptable based on this comparison, additional modifications were made, 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.
Person who carried out this activity:
Horizontal accuracy was determined by solution of Socet Set generated model (RMS less than 1) and by comparison to independent ground control data. x=4.3 +/-5.2, y=2.1 +/-8.4 values are in meters +/- standard deviation
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.
Once all of the photographs were orthorectified, the best segments of each photograph were selected for creation of the final mosaic. Segments of each photograph were selected to minimize sun glint, cloud interference, and turbidity 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.
Are there legal restrictions on access or use of the data?
- Access_Constraints: none
- Use_Constraints: none
Data are not to be used for navigation. Disclaimer- While every effort has been made to ensure that these data are accurate and reliable within the limits of the current state of the art, NOAA cannot assume liability for any damages caused by any errors or omissions in the data, nor as a result of the failure of the data to function on a particular system. NOAA makes no warranty, expressed or implied, nor does the fact of distribution constitute such a warranty.