Process_Description:
The digital orthophotography was produced in 24-bit
natural color at a ratio of 1 in 4,800 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 MGE format
- Project-specific requirements such as final tile size
and resolution.
-Orientation parameters developed from the
aerotriangulation solution.
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.
3. DTM data was imported and written to the correct
subdirectory on disk. The final digital orthophotos were
referenced to Pennsylvania State Plane, South Zone,
NAD83, NAVD88, in US Survey Feet.
4. The DTM file was re-inspected for missing or erroneous
data points.
5. A complete differential rectification was carried out
using a cubic convolution 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 natural scale of 1 in 4800 with a 2 foot pixel
resolution. The digital orthophotos were partially imagery
filled with a black background outside a 1 mile buffer on
each side of the river banks.
6. Each digital orthophoto image was visually checked
for accuracy on the workstation screen. Selected control
points (control panels or photoidentifiable points) that are
visible on the original film were visited on the screen, and
the X and Y coordinates of the location of the panel or
photoidentifiable point were measured. This information
was cross-referenced with the X and Y information
provided by the original ground survey. The digital
orthophotos were then edge-matched using proprietary
software that runs in Z/I Imaging OrthoPro software
package. Adjoining images were displayed in alternating
colors of red and cyan. In areas of exact overlap, the
image appears in gray-scale rendition. Offsets were
colored red or cyan, depending on the angle of
displacement. The operator panned down each overlap
line at a map scale to inspect the overlap area.
7. Once the orthos were inspected and approved for
accuracy, the files were copied to the network and
downloaded by the ortho finishing department. This
production unit was charged with radiometrically correcting
the orthophotos prior to completing the mosaicking and
clipping of the final tiles. The image processing technician
performed a histogram analysis of several images that
contained different land forms (urban, agricultural, forested,
etc.) and established a histogram that best preserves detail
in highlight and shadow areas. EarthData Intrernational
has developed a proprietary piece of software called
"Image Dodging." This radiometric correction algorithm
was utilized in batch and interactive modes. Used in this
fashion, this routine eliminated density changes due to sun
angle and changes in flight direction. A block of images
were processed through image dodging, in batch mode
and displayed using Z/I Imaging OrthoPro software. At this
point the images have been balanced internally, but there
are global differences in color and brightness that were
adjusted interactively. The technician assigned correction
values for each orthophoto then displayed the corrected
files to assess the effectiveness of the adjustment. This
process was repeated until the match was considered near
seamless. The files then were returned to digital
orthophoto production to mosaic the images.
8. The processed images were mosaicked using the Z/I
Imaging software. The mosaic lines were set up
interactively by the technician and were placed in areas
that avoided buildings, bridges, elevated roadways, or
other features that would highlight the mosaic lines. File
names were assigned.
9. The finishing department performed final visual checks
for orthophoto image quality. The images were inspected
using Adobe Photoshop, which enabled the technician to
remove some imperfections. However, the scope of this
emergency response limited the amount of intensive visual
checks in photoshop.
10. The final orthophoto images were written out into
TIFF format with the corresponding georeference files for
ESRI platforms. Three MrSID files were also created for
the project. The MrSID files were mosaiced from groups of
individual TIFFs.