PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
August 2017
529
•
Orthorectification
•
Mosaicking
•
Radiometric balancing
•
Cutting to tiles for delivery and web hosting
Only imagery pre-processing, aerotriangulation, and initial
orthorectification were accomplished using the Leica XPro
software suite that exploits ADS100 imagery and data.
All mosaicking, imagery processing, and production tiles
were generated using an extensive suite of custom software
developed by Surdex over the last decade. All processing is
performed in a “4x12” environment (4 bands x 12 bits/pixel
depth) until the final orthoimagery tiles are generated. This
provides the maximum amount of latitude in radiometric
balancing from both a global and localized perspective. It
also supports generation of color (red, green, blue) and color
infrared (near infrared, red, green) products in addition to
standard 4-band data sets useful in automated image classi-
fication.
Surdex’s software creates a final mosaic in a “master tile”
layout of contiguous tiles covering a project area. Custom soft-
ware can then be used for final product deliverables such as:
•
Produce 4-band, color, or color infrared data renditions.
•
Output in various map projections and differing linear
units (US Survey Foot, International Foot, Meter, etc.).
•
Generation to desired tile layouts, such as the US
Public Land Survey System (PLSS), Digital Orthophoto
Quarter-Quadrangles, etc.
•
Down-sampling of the data to differing resolutions, such
as 1’ to 1-meter.
•
Differing compressions such as JPEG, JPEG200,
MrSID, etc.
A “relative aerotriangulation” is performed with absolute
position solely relying upon ABGPS and IMU data. This
approach reduces strip-to-strip shear during mosaicking,
and the relatively short task in the overall timeline ensures
a good quality product for even expedited projects. For de-
tailed analysis at a later date, control points can be intro-
duced into the solution and orthorectification, mosaicking,
and balancing repeated to achieve more traditional produc-
tion quality standards.
Owing to the necessity of rapid delivery during a disaster,
only existing elevation data (LiDAR, USGS NED, local data,
etc.) is used for orthorectification with no effort at updat-
ing the data. For the Meramec River project, Surdex used
the USGS National Elevation Dataset (NED). This means
elevated structures such as bridges and overpasses may
exhibit layover and/or smearing since localized elevation
models are not employed as on a typical project. However,
the pushbroom aspect of the ADS100 ensures the maximum
amount of near-nadir aspect during acquisition, minimizing
the distortion of elevated features.
For expedited projects, generally only minor editing of
automated seamlines by technicians is performed in order to
reduce the timeline. Surdex uses a custom automated seam-
line generation tool to generate initial seamlines. However,
in the case of cloud and cloud shadow cover for the Meramec
situation, Surdex technicians had to selectively use imagery
from each of the two acquisition missions to maximize clear
imagery. This was guided by a delineation of cloud cover
during imagery inspection.
Surdex has developed a vast array of tools to address global
balancing of the final orthoimagery. Using what-you-see-
is-what-you-get (WYSIWYG) prototyping in the company’s
specialized tools, the distributed processing environment
ensures high quality results – even for expedited projects.
Radiometric balancing and “colorimetry” (tone, brightness,
contrast, etc.) were based on image metrics employed on all
projects and the expertise of highly skilled image processing
technicians.
Illustration of orthoimages from the first flight (left) and second flight (right) used to mosaic the orthoimagery to a final product.
