Data Management Plan

Habitat maps of Puerto Rico and the U.S. Virgin Islands were created by visual interpretation of aerial photographs using the Habitat Digitizer Extension. Aerial photographs are valuable tools for natural resource managers and researchers since they provide an excellent record of the location and extent of habitats. However,spatial distortions in aerial photographs due to such factors as camera angle, lens characteristics, and relief displacement must be accounted for during analysis to prevent incorrect measurements of area, distance, and other spatial parameters. 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.

Process Steps:

• 2000-01-01 00:00:00 - Aerial photographs were acquired for Puerto Rico and the U.S. Virgin Islands Benthic Mapping Project in 1999 by NOAA Aircraft OperationCenters aircraft and National Geodetic Survey cameras and personnel.Approximately 600, color, 9 by 9 inch photos were taken of thecoastal waters of Puerto Rico and the U.S. Virgin Islands at 1:48000scale. Specific sun angle and maximum percent cloud coverrestrictions were adhered to when possible during the photographymission to ensure collection of high quality imagery for the purposeof benthic mapping.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 forthe 1:48000 scale photography. All scans were saved in TIFF formatfor the purposes of orthorectification and photointerpretation.Georeferencing/mosaicing of the TIFF's was performed using Socet SetVersion 4.2.1. Lens correction parameters were applied to each frameto eliminate image distortion. Airborne kinematic GPS was then usedwhen available to provide a first order geolocation. When thisinformation 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 theimagery, especially for photos taken over open water where groundcontrol points were not available. Fixed ground features visible inthe scanned photos were selected for ground control points (GCP's)which were then used to georeference the imagery. GCP's were measuredusing real-time DGPS (differential Global Positioning System). Pointswere obtained with a wide distribution throughout the imagery,especially on peninsulas and outer islands whenever possible sincethis results in the most accurate registration throughout each image.Only ground control points for terrestrial features were collecteddue to difficulty of obtaining precise positions for submergedfeatures.A custom digital terrain model (DTM) was then created using the SocetSet software to correct for feature displacement due to terraineffects. To accomplish this, water features and the shoreline wereset to an elevation of zero. Preliminary experimentation revealedthat the effects of refraction on the position of submerged featuresin the imagery were not significant enough to make a correction forunderwater displacement according to Snell's law. Selected landelevation points were then inserted from USGS 1:24000 DigitalElevation Models or other elevation data sets where clouds or othersources of interference prevented the Socet Set software fromautomatically making an accurate DTM.Once the terrain models were complete and a draft orthorectifiedmosaic was produced, a set of independent ground control points wasused to measure the quality of each mosaic's rectification and ensurethat it met acceptable limits of horizontal spatial accuracy. Ifspatial accuracy was not acceptable based on this comparison,additional modifications were made, until a satisfactory mosaic wascreated for each island. In general, mosaics were georeferenced suchthat pixels are positioned within one pixel width of their correctlocation.