Project Methods
Creating and Interpreting Digital Orthophotographs
Creating the Digital Mosaic
Aerial photographs were acquired for the Puerto Rico and U.S. Virgin Islands Benthic Mapping Project in 1999 by NOAA Aircraft Operation Centers aircraft and National Geodetic Survey cameras and personnel. Approximately 600, color, 9 by 9 inch photos were taken of the coastal waters of Puerto Rico and the U.S. Virgin Islands at 1:48000 scale (photography scale varied for some specific islands, see Table 3.1). 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.
| Location | UTM Zone | Photo Scale | Pixel Width (m) | Number of Photos | Avg. Spatial Accuracy X | Avg. Spatial Accuracy Y |
|---|---|---|---|---|---|---|
| St. John | 20 | 1:48000 | 2.4 | 14 | 4.31 ± 5.2 | 2.14 ± 8.4 |
| St.Thomas | 20 | 1:48000 | 2.4 | 20 | 1.48 ± 1.3 | 1.05 ± 3.4 |
| St.Croix | 20 | 1:48000 | 2.4 | 27 | 1.21 ± 3.0 | 0.69 ± 3.4 |
| Culebra | 19 | 1:48000 | 2.4 | 14 | 5.51 ± 20.1 | 7.04 ± 18.2 |
| Mona | 19 | 1:28000 | 1.5 | 14 | 2.76 ± 9.1 | 4.06 ± 4.5 |
| Desecheo | 19 | 1:20000 | 1.0 | 3 | 4.26 ± 30.0 | 9.47 ± 36.4 |
| Puerto Rico: South | 19 | 1:48000 | 2.4 | 72 | 0.06 ± 3.0 | 0.89 ± 4.4 |
| Puerto Rico: East | 19 | 1:48000 | 2.4 | 55 | 0.85 ± 9.5 | 2.59 ± 7.8 |
| Puerto Rico: West | 19 | 1:48000 | 2.4 | 34 | 1.65 ± 5.1 | 1.04 ± 6.7 |
| Puerto Rico: North | 19 | 1:48000 | 2.4 | 51 | 4.88 ± 9.6 | 4.06 ± 5.3 |
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 2.4 by 2.4 meter pixels for the 1:48000 scale photography (pixel size varied for some specific islands due to the scale of the original photography, see Table 3.1). All scans were saved in tagged image file (TIF) format for the purposes of orthorectification and photointerpretation. Original TIF's 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 TIF’s
Georeferencing/Mosaicing of the TIF’s was performed using Socet Set Version 4.2.1. 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 flight line strips for input into Socet Set 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). Socet Set has limited ability to automatically find such features common to overlapping photographs but this automated function performs poorly for submerged features.
Ground Control Points (GCP’s) for Georeferencing
Fixed ground features visible in the scanned photos were selected for ground control points (GCP's) which were then used to georeference the imagery (i.e. link the image pixels to a real world coordinate system such as latitude/longitude). GCP's were measured using real-time DGPS (differential Global Positioning System). 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).
Creating a Custom Digital Terrain Model (DTM)
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 (less than one pixel) 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.
Finalizing the Process
Spatial accuracy of the mosaics is reported in Table 3.1. Values reported are an average for the entire mosaic, therefore, accuracy of features near land (near GCP’s) 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. Also, spatial accuracy may be especially poor near clouds over land since this interferes with creation of an accurate DTM.
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 309 out of the approximately 600 original aerial photos were selected to create the final mosaic (Table 3.1). Final mosaics were created in "geoTIF" file format (georeferenced image file) with the following projection parameters:
- North American Datum 83
- Universal Transverse Mercator (UTM) Zone 19 for Puerto Rico
- UTM Zone 20 for the U.S. Virgin Islands
These files are available on the Benthic Habitats of Puerto Rico and the U.S. Virgin Islands CD-ROM and at the NOAA's Biogeography Program 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.