This tile represents a portion of a ten county area located in south-central Pennsylvania. The following ten counties are covered within the project area: Fulton County Franklin County Adams County York county Huntingdon County Cumberland County Dauphin County Mifflin County Juniata County Perry County
The ground control and airborne GPS data was integrated into a rigid network through the completion of a fully analytical aerotriangulation adjustment. The original aerial film was scanned at a resolution of 21 microns. The scans were produced using Z/I Imaging PhotoScan flatbed metric scanners. The scanned images were then used as input, along with ground control, airborne GPS data and camera calibration data into a Z/I Imaging softcopy system for final adjustment.
The digital orthophotography was produced in natural color at an ortho scale of 1:2400 with a 2- foot pixel resolution. A step-by-step breakdown of the digital orthophoto production process follows. 1. A representative number of raster image files were visually checked for image quality on the workstation. 2. The digital image files were oriented on the digital orthophoto production workstation. The following information was then loaded onto the workstation. - The camera calibration parameters and flight line direction. - ground control and pass point locations. - The exterior orientation parameters from the aerotriangulation process. - ASCII file containing the corner coordinates of the orthophotos. - The digital elevation model in a DGN format. - Project-specific requirements such as final sheet size and resolution. - Orientation parameters developed from the aerotriangulation solution. 3. A coordinate transformation based on the camera calibration fiducial coordinates was then undertaken. This transformation allowed the conversion of every measured element of the plates to a sample/line location. Each pixel in an image was then referenced by sample and line (its horizontal and vertical position) and matched to project control. The newly rectified image was visually checked for pixel drop-out and/or other artifacts that may degrade the final orthophoto image. 4. DTM data was imported and written to the correct subdirectory on disk. The final digital orthophotos were referenced to the Pennsylvania State Plane coordinates, South Zone, NAD83, Survey Foot. 5. The DTM file was reinspected for missing or erroneous data points. 6. A complete differential rectification was carried out using an exponential algorithm that removed image displacement due to topographic relief, tip and tilt of the aircraft at the moment of exposure, and radial distortion within the camera. Each final orthophoto was produced at a scale of 1/200 with a 1-foot pixel resolution. 7. Each digital orthophoto image was visually checked for accuracy on the workstation screen. The digital orthophotos were then edge-matched using a module of the Ortho Pro software package. 8. Once the orthophotos were inspected and approved for accuracy, the files were copied to the network and downloaded by the orthophoto finishing department. This production unit was responsible for radiometrically correcting the orthophotos prior to completing the mosaicking and clipping of the final tiles. 9. The processed images were mosaicked using the Ortho Pro software. File names were assigned. 10. The finishing department performed final visual checks for orthophoto image quality. The images were inspected using Adobe Photoshop, which enabled the technician to remove dust and lint from the image files interactively. 11. The final orthophoto images were written out into compressed MrSID format, using 10:1 compression, untiled, Arc/Info readable, GeoTIFF format.