By Dan Paulsen, Nathan Kempf and Twyla Caudill,
Pacific Western Technologies, Ltd. (www.pwt.com),
Albuquerque, N.M.
Pueblo de Cochiti is one of New Mexico’s 19 sovereign Native American
Pueblos. Continuously occupied for more than 500 years, the Pueblo is
situated along the Rio Grande and encompasses the Cochiti Dam and
Reservoir. Spanning 81 square miles, Pueblo de Cochiti hosts a variety
of land and vegetation types—from mesas and canyons to riverside
floodplain, juniper-studded grasslands to riparian cottonwood forests.
Residential housing and irrigated agricultural fields are small but
vital land uses within this spectacular setting.
The Pueblo de Cochiti people are proud of their heritage and protective
of their beautiful homeland. When they decided to build a comprehensive
geographic information system (GIS), they selected the Geospatial
Solutions Group of Pacific Western Technologies Ltd. (PWT) to provide
GIS consultation and color digital orthophoto support for GIS base
mapping. In addition, PWT was able to compile historical orthophoto and
topographic data of Cochiti Dam and Reservoir from aerial photography
acquired while the dam was under construction and before the reservoir
was inundated.
Digital Photogrammetry Solutions
PWT uses Intergraph Z/I Imaging digital photogrammetric systems for its
geospatial data collection and processing, which automates many
processes and permits integration of various geospatial data sets. “We
have completed the transition to digital photogrammetric operations
throughout our production process,” says Robert Gray, PWT’s senior vice
president. “This allows for more automated data throughput and
eliminates redundant operations. Quality and accuracy are maintained,
and in some cases augmented, while reducing the time required to
complete a project.”
Representatives from Pueblo de Cochiti and PWT held several meetings
to discuss how GIS would be used to help manage the diverse cultural
and natural resources within the Pueblo. Producing accurate,
high-resolution, color digital orthophotos of the entire Pueblo
constituted the project’s first phase.
PWT hired a subcontractor to acquire the aerial photography, using
an Intergraph Z/I Imaging RMK Top aerial camera. Color aerial
photography was acquired during the early spring, before the
riparian vegetation began to leaf out. Airborne, Global Positioning
System (GPS)-controlled aerial photography was well suited for the
project given the size and rugged terrain of the Pueblo. In addition
to aerial photography of the Pueblo lands, large-scale aerial
photography was acquired over residential and agricultural areas to
produce high-resolution orthophotos of the sites. The boundaries of
these areas were clearly defined by Pueblo staff during their
initial discussions with PWT.
Professional surveyors from Jeff Mortensen & Associates Inc.
established the ground control required to supplement the
airborne-GPS work. Pueblo de Cochiti tribal members provided
invaluable assistance to the surveyors in gaining access to some of
the Pueblo’s remote areas.
Once the primary data were collected, PWT took advantage of its
digital photogrammetric systems, beginning with project setup. This
important first step establishes the basic geospatial and
operational parameters that define the project through its
completion. Items such as control datum and projection, image scale,
sensor calibration data, flight lines and images are input, and
automated inner orientation and image registration are completed.
Then analytical aerotriangulation is enhanced with automated
processes. The use of digital image correlation techniques allowed
PWT to accurately measure pass and tie points, which are especially
critical when using airborne-GPS procedures that limit ground
control. The end result is a rigorous aerotriangulation solution
achieved in a fraction of the time compared with conventional
methods. Image orientations don’t need to be repeated once they’ve
been established, which is a distinct advantage of using digital
systems from one supplier throughout the production process.
One of PWT’s most significant automated procedures is the use of
stereo auto-correlation to create the digital elevation model (DEM)
required for orthophoto rectification. Auto-correlation is an
automated process that places 3-D mass points in a regular grid
throughout the stereo model. PWT’s technical staff then performs a
quality-assurance/quality-control check of the auto-correlated data
in stereo, removing any points that aren’t on the ground, and
supplementing the DEM as needed. In addition, the data can be
filtered to remove points that are on buildings and trees, similar
to the “bare earth” filtering techniques employed for light
detection and ranging data. Such filtering software streamlines
auto-correlation and significantly reduces the time required for DEM
creation.
PWT uses Intergraph Z/I Imaging ImageStation OrthoPro software to
produce digital orthophotos. Once again, the use of compatible
digital systems allows for the ready import of setup, orientations
and DEMs from the previous steps. There are no redundant operations,
nor do any data need to be reformatted. Greater efficiency is
achieved from start to finish.
Historical Aerial Photography/Surface Comparison
PWT maintains an extensive collection of historical aerial photography
dating back to the 1950s. One of the most interesting aspects of the
Pueblo project involved using historical aerial photography to create
digital orthophotos and a digital terrain model of the surface of the
reservoir and dam. The black-and-white aerial photography was acquired
in 1972 for the U.S. Army Corps of Engineers. Because it was taken while
the dam was under construction and before the reservoir was inundated,
the imagery provided a unique window onto a surface that has been under
water for more that 30 years. Both the Pueblo and the Corps of Engineers
were interested in learning how much sediment deposition has occurred in
Cochiti Reservoir during that time.
One way to estimate the amount of sediment that has accumulated in the
reservoir is to create a georeferenced surface model from the historical
aerial photography, then compare it with a current georeferenced surface
model of the reservoir bottom. This is similar to the volumetric
calculations used by photogrammetrists for earthworks, coal piles, etc.
The current surface model of the reservoir bottom was developed from
bathymetric measurements conducted in 2004 by Parametrix (www.parametrix.com).
The company incorporated the ground control located on Cochiti Dam into
its control network, and used the same datum and projection that PWT
used for the current color ortho project (NAD83/NAVD88, NM State Plane).
Parametrix provided PWT with the bathymetric data, and PWT created a
surface model of the reservoir bottom to compare with the historical
data developed from the 1972 aerial photography.
The 1972 aerial photography was scanned to a digital format, and
camera-calibration data were acquired from the archives at the U.S.
Geological Survey Optical Sciences Lab. PWT photogrammetrists selected
photo-identifiable ground control from the current aerial photography,
and transferred the photo ID points to the historical aerial
photography. Most of the terrain and some of the roads surrounding the
area hadn’t changed during the years, making the photo ID transfer and
aerotriangulation possible. Thus, the bathymetric data and the current
and historical aerial photography were georeferenced to the same control
network.
PWT then compiled a DEM of the dam area (under construction) and the
future reservoir (not yet inundated), from the historical aerial
photography. The DEM provided the historical surface data for comparison
with the current bathymetric data. A quantitative analysis of the two
datasets was performed to yield a model of the deposition and erosion on
the reservoir bottom.
PWT was able to import the georeferenced bathymetric data into the
oriented stereo models from the historic aerial photography, allowing
planners to view the bathymetric data overlaid on the visual historical
surface of the reservoir. This approach provided a unique 3-D view of
how the bottom surface of the reservoir has changed during the last 30
years. Areas of sediment deposition and channel erosion were readily
apparent, as was the historic river channel. The relative ease in setup
and display afforded by these digital systems will allow for specialists
from either Pueblo de Cochiti or the Corps of Engineers to view the 3-D
perspective for themselves. PWT will only need to retrieve the
information from its archives and prepare it for viewing. This
qualitative view complements the quantitative information derived from
the surface comparisons.
Enhanced Applications
The integration, management and display of these multiple datasets are
greatly enhanced by the use of Z/I Imaging’s digital photogrammetric
systems. Employing these technologies allows PWT to offer its clients a
variety of options to meet their geospatial needs. In many cases,
traditional methods of data capture, such as planimetric and contour
mapping, will remain the best option. These systems perform those
functions admirably. However, the continuing evolution of geospatial
technology, services and applications will further drive the application
of these systems. Concludes Gray, “These systems will contribute to the
creativity of users such as PWT and Pueblo de Cochiti in providing
innovative solutions to their geospatial needs.”
