An orthophoto is remotely sensed image data in which displacement of features in the image caused by terrain relief and sensor orientation have been mathematically removed. Orthophotography combines the image characteristics of a photograph with the geometric qualities of a map. The source imagery was obtained from November 2006 through March 2007 and used to produce orthophotos with a one foot ground sample distance (GSD). Imagery was acquired at 0.9 foot GSD resolution. Flight height maintained during mission was 8,650 feet AGL. The imagery was captured at 12-bit radiometric resolution and converted to 8-bit radiometric resolution during post processing. The imagery was captured with 30% sidelap between all adjacent flight lines. The PRVI project area was divided into 20 flight blocks due to neccessary base station and flight line length requirements. The imagery was obtained and processed by all digital means beginning with data acquisition using an ADS40 digital sensor. The orthophotos are also available in GeoTIFF form from the distributor.
The triangulated strips were rectified with a recent DEM of the area provided by 3001, Inc. The vertical accuracy of the DEM varies based on the elevation postings and is generally estimated at 30-45cm. The red, green and blue bands were combined to generate a final natural color orthophoto, and the near - infrared, red and green bands were combined to generate a final false color infrared orthophoto.
The ADS40 sensor collects twelve bit image data which requires radiometric adjustment for output in standard eight bit image channels. Converting to eight bit results in a reduction of the color range from 4096 to 256 - thus loss of radiometric detail is inevitable. The extra dynamic range of the sensor permits greater object differentiation in shadows and in bright areas. The sixteen bit dynamic range permitted the imagery to be more effectively color balanced than is possible with eight bit imagery. This was accomplished by performing tonal enhancements immediately prior to the reduction from sixteen bit to eight bit data. In addition to color banancing these eight bit images were adjusted to create seamless imagery to the highest extent practically achievable.
The imagery was mosaicked using a combination of automated and manual seamlines generation. Project pecified tiles were extracted from the mosaic. Final image tiles were reviewed for artifacts and anomalies and adjusted as part of quality control procedures. When necessary, local corrections to the imagery were performed to minimize such effects.
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