Process_Description:
Aerial photography was acquired from June 01, 2009 until September 30, 2009.
A Digital Mapping Camera was utilized for direct digital image acquisition
and no scanning conversion from film to digital was needed. The Z/I DMC -
Frame Digital Mapping Camera - was utilized for the entire State. The
DMC is a well-known digital aerial camera system manufactured by
Z/I Imaging, Intergraph. There was 7 DMC cameras assigned and used for the
acquisition of aerial photography for the entire State of Virginia:
DMC serial numbers 001, 012, 039, 044, 046, 107 and 109.
The DMC contains four panchromatic and four multispectral camera heads.
The panchromatic heads are looking downwards in a slightly tilted position
to ensure maximum ground coverage. The mounting of the panchromatic camera
heads is modeled by four sets of six exterior orientation parameters, three
positions xyz of projections centers and three orientation angles ??? for
the tilting of the heads inside the DMC cone. The positions xyz of the
panchromatic camera projection centers within the DMC cone are precisely known.
The determination of the mounting angles is essential for the performance
of the DMC and is called ?platform calibration?. Geometric and radiometric
calibration for each one of the 8 camera heads is determined in the
manufacturing laboratory. This step is also known as ?geometric and
radiometric calibration of a single camera head? and done once when the
DMC is being manufactured. The 4 multispectral heads collect 4 separated bands:
Red, Green, Blue and Near Infrared wavelength. The DMC collect 12bit data and
its CCDs are 12 microns pixel size with frame (footprint) of 7,680 x 13,824
pixels. GSD distance is a 1:10,000 relationship to the flying height.
For this project Twin Engine Navajo, Turbo Commander and Conquest aircrafts
were used, flying at average flight height of 6,700 to 8,600 meters above
ground, allowing image acquisition of 0.67 to 0.86 meters Ground Sample
Distance.
An INS - Inertial Navigation System, with AGPS (Airborne GPS) and IMU
(Inertial Measurement Unit) was utilized for all aerial missions. The INS
allows for accurate photo-center registration and image orientation.
For inspection and verification of accuracy of the exterior orientation
computed with the Inertial Navigation System, stereo compilation was
performed for every flight mission and its result compared against higher
accuracy source data. Using the Direct Georeferencing method, each flight
mission was loaded in Softplotters and photo identifiable features were
collected. Edge/intersection of roads, driveways and parking lots, were
collected and Easting and Northing position saved in MicroStation Design
Files (DGN). These features were then checked against orthophotos with
30cm (0.30m) pixel size (1: 2400 scale) to validate the direct georeferencing
accuracy. This task was performed using Z/I IRAS-C software and Global Mapper
software. If a test of positional accuracy failed (> 5 meters) a full
analytical triangulation is processed for that mission, assuring accurate
horizontal positioning.
The aerial photographs were separated per day of flight and per camera used.
Each digital image was dodged for even distribution of lightness and darkness
of the aerial photo. The dodging of each rectified image alleviates the effects
of hotspots, side-to-side shading within an exposure.
Each set was loaded and color setting applied to match the histogram requirements
of NAIP program. Apertune, the best digital color balancing software, manufactured
by Agfa specifically for aerial films and aerial images was used with every photo.
Tonal balance setting were applied between flight lines to adjust dissimilar
conditions at the time of capture.
When analytical aerotriangulation was needed, we made use of ISAT (Intergraph)
with automated measurement of passpoints and tie-points between adjacent photos
and flight lines.
USGS seamless 2009 NED (National Elevation Data) was used as surface for
ortho-rectification. Sanborn utilized commercial software called OrthoPRO,
form Z/I Intergraph, to ortho-rectify the digital aerial images.
The ortho rectification was performed using results of the aerotriangulation
for each individual digital image, and projecting it over the Digital Elevation
Model - NED - and by differential ortho rectification creating a true scaled
imagery for each aerial photo. Cubic convolution resampling method was utilized
during the ortho-rectification.
Automated seamlines were processed and create between each ortho-rectified image.
Seams between exposures were determined by the software to use the best exposure based
on proximity to nadir and ground angle to the camera. Seams were then manually
modified only to reduce apparent differences in contrast and brightness betweem two
adjacent images, or to improve feature visibility. Tile/Grids
or so called "CHIPS" were then extracted using OrthoPRO's NDOPDOQ database, appropriately
named to meet the DOQQ naming convention, and formatted as defined in the task order.
Quality control was performed on each extracted tile to ensure no artifacts or
anomalies were present in the imagery. A histogram check tool was developed by Sanborn
and used in every DOQQ to test the image quality in accord to USDA requirements of
clipping, contrast, brightness, color balance, band-to-band registration
and "blemish.
Horizontal accuracy was once again tested in each DOQQ, for 6-meter absolute accuracy.
If failed, then entire flight mission would be revised, a new aerotriangulation
performed and new set of doqqs produced.
Once all the DOQQs that cover the County were finalized and approved by the QC Department,
the CCM, Compressed County Mosaic, was created using GeoExpress creating LizardTech's MrSid
files, in Generation 3 format, block size 64, transparency and background RGB 0,0,0
"maximum zoom level and 15:1 compression ratio.
CCMs where delivered to USDA in DVD, being one County per DVD.