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  • Report on the Regional Digital Orthophoto Project

    November 3, 2000


    In April 1998, the Pima Association of Governments (PAG) and the local jurisdictions contracted for a high quality orthophoto coverage of 578 square miles in the Tucson urbanized area (includes Marana, Oro Valley, Sahuarita and Green Valley). Combined with on-the-ground verification, an aerial flight at about 3600 feet captured the images and related data necessary to generate an ortho- rectified coverage for the region. As a general description, ortho-rectified images provide the detail of a high resolution aerial photograph in the spatial accuracy of a map, all in digital form.

    Two primary contractors were involved: Science Applications International Corporation (SAIC) for data collection and processing, and DTM, Inc.for quality assurance/quality control. The total cost of the project was about $2.3 million. About $132,000 was added to the original contract to enhance or add coverage for Oro Valley, Tucson Airport Authority and the City of Tucson. Data products were delivered through November 1999 and distributed to the jurisdictions, and those products are now being used in project applications.

    The project has generated considerable interest for several reasons:

    • The high resolution of the data,
    • The vertical and horizontal accuracy,
    • The extensive topographic coverage,
    • The ability to merge the data products with other existing Geographic Information System (GIS) data layers.
    The potential uses of the data span numerous disciplines (transportation, air quality, hydrology, land use planning, etc).

    Data Products

    For each section in the project area, the data products include

    • orthophoto imagery,
    • topographic data files,
    • geodetic control points.

    The imagery is provided both as grayscale (black and white) image files and as multispectral (red, green, blue, and near-infrared bands) images. The grayscale imagery is provided in 1, 2, 4, 8, and 16 foot resolutions. One foot resolution generally means that an image measuring at least one square foot on the ground may be visible in that orthophoto image. The multispectral imagery is provided in 1 meter resolution.

    The topographic data files are provided in either Digital Terrain Models (DTMs) or Digital Elevation Models (DEMs) for each section. DTMs are represented as contour lines at 2' intervals, and were created for about 338 sections, primarily in the City of Tucson and Town of Oro Valley. DEMs are represented as points (points are spaced 30 feet apart, with 48 points per acre), and they cover the remaining 240 square miles.

    Planimetric control is provided by Global Positioning System (GPS) points established on the ground plus aerotriangulation processing by SAIC. For horizontal accuracy, 95% of all planimetric features are accurate to within 2 feet of their true coordinate position. The remaining 5% are displaced by no more than 4 feet. For vertical accuracy, 90% of all elevations are within 1 foot of their true elevation. The other 10% of the elevations have no more than a 2 foot error.

    Projects and Uses of the data

    Users are just beginning to identify the numerous applications of the data.

    • The City of Tucson has been using the high resolution ortho imagery for an inventory of transportation-related facilities (including street lights), and the multispectral imagery for vegetation analysis and in determining impervious vs. pervious areas.
    • Pima County used the DEM topographic coverage in ArcInfo format for a site planning project in the Tucson Mountains, and in a slope analysis for determining compliance with their hillside development zone ordinance.
    • Oro Valley is using the DTM coverage for analyzing open space set asides in proposed master planned developments.
    • The Town of Marana has identified possible applications for analyzing illegal grading and subdivision activity, and to assist in analyzing impacts of proposed rezonings. Also, new developments and additions can be matched with building permits and compared with tax assessor records.


    Other uses being considered by local agencies include:

    • right of way and road alignment
    • transportation network inventory and analysis
    • preliminary design studies
    • gridded emissions of air pollutants
    • change detection (if subsequent updates to the coverage are provided)
    • flood control and hydrology studies
    • habitat mapping
    • vegetation classification
    • land use identification

    Other GIS data layers available

    Orthophotos are one of the most important base layers of data for GIS systems. Many other data layers can be overlaid with the orthophoto to visually check true location. For instance, one of the major GIS projects undertaken by Pima County is a rectification of the parcel land base to match their position on the ground. Once the parcel base is adjusted to fit the orthophotos, other GIS coverages will follow. They include the transportation network, jurisdictional boundaries, floodplains, zoning, land use, demographic-related coverages and numerous others. The result of this will be a comprehensive, spatially-corrected set of coverages for use in numerous applications throughout the region.

    Regional Data Center

    In July 2000, PAG launched the web-based Regional Data Center (RDC), an interactive mapping site. The RDC is housed within PAG's existing web site, and serves as the clearinghouse for the orthophoto data and other regional GIS coverages (including the parcel base, street network, jurisdictional boundaries, etc.). It allows users to view the orthophotos and other GIS data, perform on-line analysis, create customized maps, and order the digital orthophoto files and the topographic data files. As such, the RDC serves as a primary point-of-contact for the general public and the private sector for acquiring the orthophoto products. PAG contracted with Veridian/ERIM International for the design and development of the data center. Additional GIS coverages, data and applications will be available in 2001, particularly for Census 2000 data.

    Data integrity

    Despite its accuracy and resolution, the data does have some potential misuses and limitations. For instance, the ortho images and derived topography were gathered in April 1998. Grading that has occurred since then will obviously limit the use of DEM or DTM coverage for that area, unless updated digital topographic data is captured for the disturbed area. Also, the DTM files are provided in 2 foot contours. Re-interpreting those data files to generate smaller contour intervals (i.e. 1 foot) should be avoided. Also, overlaying GIS layers, with different map projections, onto the ortho images will result in misaligned coverages.

    Training sessions

    Outreach and education will be the next critical steps in the project. Public and private sector users alike need to be informed of all the products available, as well as data formats, software and hardware requirements, and potential uses. It will also be critical to advise users of the limitations and potential misuses of the data.

    Several training sessions to address these and other topics were held in April, and will continue on a regular basis. The Southern Arizona GIS consortium (SAGIS) will be used as a vehicle to inform potential users of the availability of the data.

    New orthophoto coverage

    Another orthophoto flight is underway for the region. PAG, in conjunction with the member jurisdictions and the Arizona Department of Transportation, has contracted with Merrick and Company of Aurora, Colorado for a flight covering 670 square miles, including a re-flight of the towns of Marana and Oro Valley. Grayscale imagery will be generated in ½ foot resolution, with DTM's and two foot contours created for the majority of the coverage area. The combined 1998 and 2000 coverages result in ortho products for nearly 1,200 square miles in the Tucson region.

    The technical advisory committee identified numerous reasons for updating and expanding the coverage, including the following applications:


    • regional conservation planning projects
    • hillside development zone compliance
    • coordination with Census 2000 for a complete description of the region
    • change detection and analysis (for land use and vegetation changes)


    Jack Avis Pima County
    Andy Gunning, PAG (792-1093)
    Jeff Hildebrand PAG (792-1093)
    Laura Pinnas Town of Oro Valley (797-9797)
    Ron Platt, City of Tucson (791-5279)
    Manny Rosas,
    PAG (792-1093)
    Kevin Sweeney Town of Marana (297-2920)
    Steve Whitney,
    Pima County (740-6670)


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