ortho_hf

                                The accuracy and quality of the CITIPIX ODIs in ORTHOS_HF meet National Map Accuracy  Standards 1:1,200 scale, plus/minus 1 meter or 3.33 feet, and is accurate for mapping applications at 1" = 100'.
                              

                                ORTHO_HF contains high resolution 24-bit color CITIPIX Ortho-rectified Digital Images (ODIs). Flown in April 2001, from Portland to Auburn ME.
                              

                                The ArcSDE in Oracle version of ORTHO_HF is a seamless mosaic of CITIPIX (GlobeXplorer trademark) Ortho-rectified Digital Images (ODIs). CITIPIX ODIs are derived from color photographs, taken from an altitude of approximately 10,800 feet (3,300 meters) above mean terrain, using a 12 inch (305-mm) focal length Zeiss TOP30 aerial camera. The aircraft is equipped with a high accuracy dual frequency GPS2 receiver operating in kinematic relative mode and camera orientation is measured by inertial3 sensor technology.  The 12-inch (305 mm) focal length lens, when compared to the usual six-inch, minimizes the perspective distortion in high buildings down town contexts. The nominal scale of the photography is approximately 1:10,800. Since ODIs are mosaics and considering flight overlaps, only the central portion of each aerial photograph is used in the final assembly, representing approximately 28% of each photo's coverage.


Zeiss-Intergraph PhotoScan TD high-resolution photogrammetric scanners are used to scan 10-inch wide color negative film rolls in a controlled environment room (temperature, humidity and dust). Scanner aperture (pixel size) is 1,814 dots per inch or 14 *m4  which, when used on a 1:10800 photo-scale image, yields a ground pixel size of approximately 6 inches (15 cm), and which produces a color image of 16,700 by 16,700 24-bit pixels. This level of scanning resolution closely matches the film grain, which is between 10 and 12 *m, thus very close to 100% of the information present on the film itself is recorded in the digital file. Accuracy of these scanners is better than 2 *m, and lens distortion falls within the same order of magnitude. This insures that each measured pixel is right where it should be.


Ortho-rectification was performed by Kodak Global Imaging, a former Eastman Kodak Company no longer in operation, using it's own technology involving high accuracy digital terrain models (DTMS) (created from CITIPIX stereo-models), geometric corrections and mosaicking processes. Each of these operations was performed by highly trained personnel, operating a tailor-made computerized processing chain.


Once assembled in a continuous image covering sometimes thousands of photographs, base image data is separated in 2 km x 2km tiles, cut along Universal Transverse Mercator (UTM) even-numbered coordinate grid lines. A typical tile overlaps the UTM grid by an added 15 meters in all directions, the result being 2030 meters x 2030 meters tiles, overlapping each other by 30 meters: this is the default format of all ODI tiles produced and archived... 

Georeference for each tile is computed from accurate ortho-rectification and mosaicking processes. Each tile is given a pixel/ground transformation matrix (with zero-rotation), included in the file georeferencing parameters. UTM or State Plane Coordinate System (SPCS7), North American Datum (NAD83) or NAD27, X and Y coordinates are computed for each tile...Due to the high level of accuracy in the ortho-rectification and georeferencing processes, seam lines are often hardly visible to untrained eyes, thanks to operator-controlled, computer assisted seam line positioning on the final mosaic.
                              

                                The MrSid versions of the ORTHO_HF images were cropped and compressed using LizardTech's GeoExpress Command Line 3.1 software (www.lizardtech.com).  The original 2km x 2km .tif images from Globexplorer were provided to MEGIS as multi-band TIFF images with no compression.  The images were cropped to 1km x 1km tiles using 1000-meter UTM boundaries, plus a 2-pixel (.3m) buffer.  Images were compressed at a ration of 12:1 using 5 zoom levels and were stored as MrSID Generation 2 files.  The compression and cropping were all done in a single operation using command-line tools in the software.  The images were done in batches of roughly 100GB and processed using an ArcInfo Arc Macro Language (AML) script which determined the UTM coordinates and passed the appropriate arguments to the GeoExpress software.