Richard J Szmajter
Lance W Frisby
In 2001, the City of Durango in southwestern Colorado determined it needed to expand its existing GIS database from a coverage area of 28.5 square miles to approximately 104 square miles, to better reflect the expanding urbanization that was occurring in the mountain community. Durango contracted Bohannan Huston, Inc. (BHI) of Albuquerque, New Mexico to develop an aerial photography/digital mapping project to enhance the City's spatial data resources. To make the project economically feasible on a limited budget, the City recommended that aerial photography be flown at a 1:7200 scale in developable areas and at 1:14,400 scale in surrounding public lands. To further offset costs, it was also recommended that surface mapping use a mixture of 2., 5. and 10. contouring intervals for urban, rural, and public lands, respectively. BHI.s Spatial Data Group (SDG) proposed using a hybrid of conventional mapping methodologies and newer photogrammetric technologies to fulfill Durango.s mapping needs while still remaining affordable and very cost effective. Using aerial Light Detection and Ranging (LIDAR) surveys in conjunction with conventional stereo photogrammetric DTM supplementation, a detailed LIDAR digital surface model was created based upon the City of Durango.s specifications. This mixed-technology approach also yielded, as a collateral product, a "cloud" of raw LIDAR reflective data that contains valuable information regarding tree canopy, biomass, building elevations, and other valuable information that could be used to support planning and community development decisions.
In January of 2001, the City of Durango in La Plata County Colorado embarked on a new aerial photography and digital mapping program with Bohannan Huston, Inc. of Albuquerque, New Mexico. The photogrammetric project was scheduled to coincide with the release of 2000 U.S. Bureau of Census data and was designed to evaluate growth and development trends within the mountain community of 14,000 and the surrounding unincorporated county. The primary objective of the project was to enhance and expand the City.s existent GIS base map that was most recently updated in 1994. The secondary goal was to compile a tiled set of full-color digital orthophotography for the city and surrounding countryside.
Several criteria were evaluated while assessing the need to supplement Durango's GIS database.
1) Recent large-scale residential and commercial developments within the city limits were not captured and mapped in the previous mapping cycle. Transportation, utility, structure, and parcel coverages needed to be updated to reflect the expansion of the city.s infrastructure into these new areas.
2) Future annexations were proposed for previously developed areas outside the boundaries of the existing GIS coverage areas. Planning and community development concerns were no longer fully supplemented by the 1994 mapped layers.
2) A shortage of vacant property within Durango, coupled with the unique topographic constraints of the region was forcing land developers to seek commercial and residential property outside of the city limits. This has resulted in the emergence of rural subdivisions and proposed commercial development atop the mesas to southeast and along Animas River corridor to the north of the city center. Expanded contour interval coverage was needed outside of the city limits to assure structures were not being built in areas of greater than 30 percent slope and within potential flood zones.
4) Evolving emergency preparedness needs accompanied the recent growth and required enhancement of the city coverage area to service the needs of the City of Durango Police Department, La Plata County Sheriff.s Department, the 911 Dispatch Center, and the newly-created Durango Fire and Rescue Authority.
Urban expansion was not the only factor that motivated the project. The need to address several quality of life criteria was also factored in the project development stages. These included the local community.s desire for expanded parks and recreational facilities, hiking and biking trails, protection of historical Victorian structures and neighborhoods, affordable housing, and an efficient transit system servicing both the town and the nearby Durango Mountain Ski Resort. Efforts were also made to provide spatial data that would prove useful in the preservation of the aesthetic quality of the nearby Animas River Valley and the surrounding hillsides. Digital Terrain Models (DTM's), hydrologic and vegetation data, and contour coverage in public lands were needed to develop hillside development standards, viewshed and watershed protection strategies, ridgeline preservation and setback standards, and open space designation.
Mapping objectives were defined prior to the implementation of the photogrammetric mission.These included:
1) Broadening the City.s digital map base from 28.5 square miles to 104 square miles.
2) Developing a digital terrain model to generate contour intervals in town and in peripheral areas undergoing growth pressures.
3) Update existing planimetric feature layers in the current GIS database.
4) Generate 2., 5. and 10. contouring intervals for urban, rural, and public lands, respectively
5) generate an array of tiled digital orthorectified color images in Colorado State Plane South Zone coordinates.
Once these objectives were defined, it became imperative to develop a strategy to fulfill the City.s needs and objectives while staying within a relatively modest budget. It was also important to city officials to receive the data in a timely manner.
To save City funds and compress the project schedule, the Spatial Data Group of Bohannan Huston, Inc (BHI) proposed a hybrid of conventional photogrammetric base mapping and newer technology and mapping methodologies to provide aerial photography and digital mapping. BHI created a detailed LIDAR digital surface model using aerial LIDAR surveys in conjunction with conventional stereo photogrammetric DTM supplementation. The combined process enhanced breakline information quickly, efficiently and with increased accuracy. This approach also yielded a reflective LIDAR surface model that contains valuable information regarding biomass, building elevations, and other valuable information as a collateral product
Aerial photography took place in early May to minimize snow cover and take advantage of "leaf-off" conditions. To minimize ground photo control surveys, Airborne GPS was used to record location (x, y, and z coordinates) for all aerial photography and was fixed to 25 ground targets to assure accuracy. BHI established and monitored a GPS ground station at an existing 1st order control point during the aerial photography acquisition. An on-board inertial measurement unit (IMU) navigation was used to control the camera/scanner orientation and record the pitch, roll, and yaw of the aircraft. Flight lines were flown with 30% sidelap and 60% overlap between exposures. To further minimize costs, only populated and developable areas were flown at scale 1:7200. Public lands were flown at 1:14,400.
The aerial LIDAR survey mission was scheduled to coincide as close as possible with the aerial photography mission. The post-processing of the airborne GPS and Inertial Measurement Unit (IMU) information from the LIDAR mission was used to accurately georeference the LIDAR pulse data with respect to the earth. The 25 ground control points provided a crosscheck to the horizontal and vertical accuracy of the LIDAR data.
Once LIDAR data was acquired, flight lines were butt-matched and preliminarily filters were used to remove most vegetation and man-made features. Then the entire LIDAR data set was loaded into a seamless spatial database. This enabled the BHI.s staff to extract LIDAR data from the database in sizes and shapes efficient for use in several mapping workflows. The LIDAR data was then thinned by the removal of redundant points not needed to accurately describe the ground. The "thinning" algorithm did a comparison of the thousands of LIDAR points collected in a flat field, with the points compared to each other and the DTM. Redundant points were then intelligently removed, leaving only those necessary to accurately define the field. This was done to further reduce the large LIDAR data set without affecting its integrity and achieved an efficient working size for engineering applications.
LIDAR data was extracted from the spatial database by photogrammetric model boundaries as DTM points and placed into MicroStation design files. After extraction, the compilation staff graphically edited each model in softcopy stereo and introduced breakline data into the model where LIDAR point data had not clearly defined the project surface. The staff also verified that the LIDAR was truly on the ground and at this time removed additional vegetation, structures and other unnecessary data not removed by the preliminary filters. The compilation staff then referenced in adjacent LIDAR models and generated contours at the required contour interval. This step allows the data compiler an overall look at LIDAR model and how it interacts with its neighboring LIDAR models. It further validates the integrity of the LIDAR and the respective DTM.
Edited LIDAR models were than loaded into a seamless spatial database and extracted in the tiling format chosen by the client. The extracted LIDAR boundaries with breaklines were developed into DTM(s) and 3D contour CADD files. Contour check plots and shaded relief maps were studied as additional quality assurance steps.
All of BHI.S photogrammetry compilers are trained on Zeiss P3/P33 analytical stereo plotters and move on to ZI Imaging ImageStations for training in digital softcopy photogrammetric collection when proficient at analytical collection. For the Durango project, BHI choose to collect all 3-dimensional planimetric and engineering quality Digital Terrain Model (DTM) data on analytical plotters. This was done to generate the most accurate measurement provided by the project photography. Previous experience has shown a C factor of 1800-2500 for analytical plotters and a C factor of 1300-1800 for digital plotters. Before any compilation proceeds, the AAT solution is checked in both digital and analytical plotters. Sample data collected in the analytical plotter were reviewed in the digital plotter to ensure correct implementation of the AAT solution in the respective compilation packages. The stereo compilation manager reviewed all resulting data compilation for completeness and consistence across the project staff. He logged compilation progress graphically with GIS tools.
After a final rigorous review by the project manager, the entire dataset was loaded into an Oracle spatial database. BHI performed the final file format translations using the Feature Manipulation Engine (FME) to deliver Esri export and 3D shape files. This translation ensures topologically "clean" products, void of pseudo nodes. Features were classified as points, lines, or polygons where appropriate and included a "type" attribute describing the feature.
The hybrid approach fulfilled the City of Durango's mapping objectives by combining the added efficiency of innovative emerging technologies and mapping methodologies with time-tested conventional base mapping techniques. The result was spatial data enhancement with a high degree of precision and accuracy, quick turnaround time for data delivery, rapid distribution of planimetric data to interested parties and entities, and a methodology that has proven to be both cost effectiveness and affordable for a small community like Durango.s.
J. Szmajter, GIS Analyst, City of Durango
Bart Matthews, Project Development Manager, Bohannan Huston, Inc.
Lance W. Frisby, GIS Coordinator, City of Durango