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A Case Study in Executing Large Photogrammetric
Projects
Large photogrammetric projects with challenging schedules require
creative approaches. This article uses a case study to illustrate
how a team led by Surdex was able to successfully execute a
very large digital orthophoto production project. The case study
involves the National Agriculture Imagery Program (NAIP) administered
by the United States Department of Agriculture Aerial Photography
Field Office. This project required the production of 1- or
2-meter resolution Digital Orthophoto Quarter Quadrangles for
the states of Missouri, Oklahoma, and Kansas over a three-month
time period in the summer of 2003. More than 31,000 frames of
photography covering 220,000 square miles were acquired and
processed into nearly 16,000 digital orthophotos.
Craig Molander
and Tim Bohn
Most photogrammetric projects experience challenges
of some type during execution, but few have the combined elements
of schedule, volume, and weather. A team headed by Surdex Corporation
of Chesterfield, Missouri was awarded a large digital orthophotography
contract in the spring of 2003 that challenged the standard
tools, processes, and management generally employed.
This project required approximately 220,000 square miles of
color or color-infrared photography to be acquired in less than
10 weeks and more than 15,000 digital orthophotos produced within
30 days of the close of the photography windows.
2003 NAIP
The Surdex team was awarded a digital orthophotography project
by the United States Department of Agriculture (USDA), Farm
Service Agency, Aerial Photography Field Office in Salt Lake
City, Utah. The National Agriculture Imagery Program (NAIP)
administered by the USDA is unique from the standpoint that
photography must be acquired during the peak crop growing
periods. Thus, “leaf-on” color or color infrared photography
must be captured in windows of roughly 60-80 days ranging from
early summer to early fall and specific to each state. The photography
is acquired at a scale of 1:40,000 to support the generation
of 1- or 2-meter Digital Orthophoto Quarter-Quandrangles (DOQQ).
The entire 2003 NAIP effort involved approximately 800,000 square
miles of coverage and more than 64,000 digital orthophotos produced
by nine contracting teams.
Each DOQQ is 3.75 arc-minutes square in longitude and latitude.
The absolute accuracy requirement for the 2-meter DOQQs is only
20 meters, whereas the one-meter DOQQ product must be accurate
to within 3 meters relative to existing “reference” DOQQs. All
photography is pre-planned for exposures at each quarter-quadrangle
center and edge along north-south flight lines, resulting in
acceptable stereoscopic coverage. Although DOQQs are constructed
only from the quarter-quadrangle-centered exposures (mosaicking
is not required), aerotriangulation requires all exposures
to be used.
Surdex Corporation was awarded the states of Missouri, Oklahoma,
and Kansas. The following table summarizes the three state project
areas.
The NAIP deliverables and timeframes expressed with respect
to the closure of photography windows are:
-- Delivery of county mosaics, compressed in MrSID format, within
30 days.
-- Delivery of uncompressed DOQQs within 90 days.
-- Delivery of titled film within 90 days.
Program Challenges
The NAIP posed several serious challenges:
-- The acquisition of aerial photography during difficult Midwestern
U.S. summer flying conditions.
-- Scanning imagery after film processing, inspection, and titling.
-- Processing large volumes of data through aerotriangulation
and product processing in very short timeframes.
The acquisition of photography within the allowable time windows,
and with adequate time to allow data processing, was the most
critical factor in the program. Midwestern summers are notorious
for relatively high temperatures coupled with extreme haze,
and frequented by large thunderstorm lines running at a frequency
of 3-5 days. Flight planning required around-the-clock monitoring
of weather and close coordination of flight crews. Very “aggressive”
flying was required to take advantage of the average 1-5 hour
flight windows and GPS ground stations had to be maintained
over large areas to support the multiple flight crews. Close
coordination with the Federal Aviation Administration was required
since the aircraft operated at altitudes in the Positive Controlled
Airspace (PCA) realm. In addition, a number of Military Operations
Areas (MOA) also dotted the three-state area, requiring advance
arrangements for airspace clearance— largely resolved by flying
on holidays.
Scanning of the photography became the second limiting
factor in the project. High speed image scanners (4 Leica DSW500/600
systems) were driven to operate on an average 20-hour day, with
operators monitoring progress in an around-the-clock fashion.
The aggressive flying produced countless rolls of film
with discordant strips of photography, especially in the latter
stages of the program as hard-to-get areas remained. It was
necessary to constantly assess the status of image scanning
to establish priorities ensuring the processing of data into
final county deliveries.
Aerotriangulation had to be accomplished in very short periods
of time, with a focus on the selection of adequately sized areas
based on product delivery areas. Though the accuracy of the
end product was not in itself a challenge, the fact that 1-meter
resolution DOQQs had to agree with prior versions meant that
absolute and comparative accuracies needed to be monitored and
achieved.
Orthorectification involved the use of the United States Geological
Survey (USGS) National Elevation Dataset (NED) surface model.
This data was purchased by Surdex for the entire United States,
with subsets extracted and re-projected for each project area
and distributed to teammates requiring its use.
Approach
The Surdex approach to this program was based on the use of
airborne GPS (ABGPS) and full softcopy aerotriangulation to
meet accuracy requirements. Visually identifiable points were
extracted from the USDA-supplied reference DOQQs and used
as check and quality control points in the aerotriangulation
process.
In the investigation of the upcoming NAIP during the fall and
winter of 2002-2003, it became clear that a number of steps
would have to be taken to execute successfully:
-- Extremely high automation must be utilized given the volume
of work to be accomplished in a very short time frame.
-- Each functional area (photography, image scanning, etc.)
had to be approached using conservative capacity estimates
to ensure success—essentially arranging for excess capacity.
-- Each teammate must utilize, or be equipped with, common software
operating at the highest levels of performance.
Various vendor offerings in a number of functional areas were
reviewed for possible assistance, but it quickly became obvious
that a high level of customization was required. The specific
requirements of NAIP required even more customized software
to solve logistics issues and to lessen the scripting efforts
generally required of the production staff. Some investigation
time was also spent in focusing on software that literally “looked
for something to do” in an attempt to diminish reliance upon
operator initiation of tasks that completed at any hour of the
day. A number of custom database systems and applications were
developed to support project logistics and reporting requirements
of the USDA. These focused on providing information from a variety
of viewpoints and were developed to operate within the context
of an existing Surdex-developed Internet/intranet project management
solution. The Collaborative Project Management System (CPMS)
has been used within Surdex for more than three years and provides
comprehensive status and reporting for Surdex, the client, and
teammates via a Web interface.
The use of common software amongst the team members was mandatory
to achieve consistent results within program specifications
and allow progress monitoring and planning. This included:
-- The Z/I Imaging ImageStation Automatic Triangulation (ISAT)
software for softcopy aerotriangulation environment, already
extensively used by Surdex.
-- Digital image dodging, reduced resolution data set (RRDS)
generation, automated digital orthophoto accuracy assessment,
and orthorectification software were standardized based on software
developed by Surdex over the past few years. These tools were
conveyed to all teammates requiring its use.
The coordination of flight crews, daily reporting of photography
status, prioritizing and tracking image scanning progress, and
overall progress reporting became the immediate focus of the
Surdex research and development staff shortly after program
award. A series of databases, data entry forms, and reporting
applications were developed using SQL Server and made available
to the production staff in an intranet environment. The most
important data contained within the databases included:
-- Acquired photography and ABGPS data, using both USDA exposure
nomenclature and aliases required for mission planning
and mission support systems.
-- Tracking reflights for specific frames.
-- Tracking of film rolls and mapping of individual frames to
rolls.
-- Image scanning progress including mapping image names to
rolls and frames.
-- Aerotriangulation information, including tracking
exposures and county coverages.
-- Digital orthophoto production progress.
-- Delivery and quality control progress.
The tracking of progress and prioritization of the aerial photography
portion proved to be the one of the most critical components
of the project. Since photography crews took an aggressive approach
and worked around weather patterns on a daily basis, a number
of logistical problems were created. The collection of large,
but often isolated, blocks of photography were common early
in the project, leaving holes in the coverage. In addition,
occasional excessive cloud cover or cloud shadows resulted in
a number of rejections leaving more holes. Thus, the final 5-10%
of the aerial photography collection resulted in inefficiency
as the isolated holes and reflights were addressed.
Each field crew was responsible for submitting electronic or
hardcopy (facsimile) reports at the end of each day of activity.
Surdex project management staff used this information to update
the internal databases via an intranet form. Electronic versions
of progress were transmitted to the USDA each day, as per contractual
requirements, and field crews updated as to the current situation.
All aircrews were also responsible for performing initial checks
on ABGPS data after each mission, although final data reduction
was performed by Surdex prior to aerotriangulation.
The scanning of photography early in the project was focused
simply on scanning each roll as it completed inspection and
titling. However, the filling of holes and reflights beginning
midway through the project resulted in rolls with coverage ranging
over large areas. Priorities were then set on a daily basis
to ensure that most of the data could be obtained from the backlog
of film rolls. Aerotriangulation and scanning technicians constantly
reviewed pending work and priorities to ensure production
could continue efficiently.
A “NAIP Database” was created for distribution to all teammates
involved in processing and was based on information distributed
by the USDA. A Relational Data Base Management System (RDBMS)
interface provided the following for various application programs:
-- Lists of all counties involved in each state and the Universal
Transverse Mercator (UTM) zone required as a reference frame
(the three states involved four UTM zones).
-- Lists of DOQQs required for each county delivery.
-- Federal Information Processing System (FIPS) state and county
codes required in deliverable metadata files.
-- Extracted and re-projected elevation model surfaces for each
UTM zone area derived from the NED.
Photography
The photography was successfully accomplished, partially benefitted
by a 7-10 day span of good weather but mainly due to aggressive
flying and coordination. All told, seven aircraft were used
involving five companies (three Surdex aircraft were employed).
Though a 2-week extension was granted late in the period for
the states of Missouri and Oklahoma due to weather preventing
reflights, this stage was assessed as being a total success.
The NAIP Database proved to be crucial in the prioritization.
Figure 4 shows the early stages of acquisition for Missouri
and was gleaned from the NAIP Database. Figure 5 shows the end-result
of the flying, color-coded by aircraft. Note that large areas
were accomplished by systems such as the Cessna Conquest operated
by Keystone Aerial Surveys of Philadelphia, Pennsylvania (light
blue), while twin-piston and turbo-charged aircraft provided
large area coverage using multiple flights per day. A time-lapse
view of the photography acquisition was recently created from
the database and clearly shows the staged opening windows for
each state and the effects of weather fronts.
Image Scanning
Image scanning was accomplished with virtually no hardware downtime.
The NAIP Database successfully guided priorities, aided by constant
dialogue between the image scanning and aerotriangulation personnel.
Surdex-developed software for digital image dodging and RRDS
generation provided quality results at very high throughput—roughly
five minutes total per image.
Using custom “photo mosaicking” software, it was possible to
review the quality of the image scanning and dodging as early
as during image scanning. Using ABGPS data or aerotriangulation
results (initial or final), all images were rectified or orthorectified
(using the NED) and “collaged” into an overview for each aerotriangulation
block or arbitrary area. Problems identified in image tone and
contrast quality were either further digitally dodged or re-scanned.
Figure 6 shows a collage for the entire state of Missouri.
Aerotriangulation
Aerotriangulation (AT) exceeded expectations due to very
good performance from the Z/I Imaging ISAT product and around-the-clock
attention from the staff. The automated tie/ pass point collection
provided excellent results with little cleanup required. All
told, more than 75 blocks of photography, averaging roughly
400 frames per block, were processed by a staff of four people.
To ensure accuracy and quality were met during this critical
step, a number of Surdex-developed software modules were used:
-- A final analysis of each block was performed in a semi-automated
fashion by comparing photography footprints to actual tie and
pass point connections. Missing or suspect ties between models
and strips were highlighted for the aerotriangulation staff
to investigate.
-- The elevations of all aerotriangulation points were compared
with the NED by interpolation, thus highlighting both suspicious
points and possible problems with the NED itself.
-- All points involved in the AT were automatically digitally
matched with the reference DOQQs to attempt to isolate accuracy
issues, particularly with regards to the 1-meter resolution
product. Though color or color-infrared was compared to older
panchromatic imagery, the matching was overall quite successful—finding
approximately 30% of the points.
-- In-house procedures for AT review were followed despite schedule
pressures, thus ensuring a quality product.
Digital Orthophoto Processing
The generation of digital orthophotos was accomplished
using several high-speed processor workstations (dual 2-3.5
GHz) and a Surdex-developed orthorectification software module
tailored to the characteristics of the NAIP. The NAIP Database
was used to generate scripts immediately after aerotriangulation
was complete for a block. The database supported the determination
of which frames were to be processed—very critical when realizing
that counties “shared” DOQQs to a significant extent. At one
point, more than 8,000 DOQQs were orthorectified in a 48-hour
period using two workstations.
All AT points were automatically digitally matched with the
resulting DOQQs to assess overall agreement of the combined
effects of orthorectification and the NED. In the case of 1-meter
resolution DOQs, areas showing more than three meters of error
were highlighted and subjected to more detailed accuracy
analysis by the orthophoto department.
Though there was not a program requirement to do so, each county
mosaic (or groups of mosaics) were radiometrically balanced
to assure a consistent product to the FSA offices. This was
especially critical in the state of Missouri, where color infrared
is notoriously difficult to work with.
Summary
Successfully executing large programs such as the 2003 NAIP
required extensive software and database customization, the
use of premium vendor software, and aggressive project
management.
About the Authors
Craig Molander is a Senior Vice President of Business Development
at Surdex Corporation.
Tim Bohn is a senior project manager at Surdex Corporation.
He served as the project manager for the 2003 NAIP effort.
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