San Francisco Hills Challenge Orthorectification Process
By Kevin P. Corbley

Creating digital orthophotographs for an urban area in hilly terrain poses significant photogrammetric challenges. In an ongoing mapping project, the city and county of San Francisco pushed orthorectification technology to its limits by requesting an orthophoto basemap with 3-inch resolution and 2.5-foot accuracy.
     In 1992, as part of a regional strategic plan, San Francisco decided to develop a centralized basemap that all city and county agencies could use to create virtually any digitized map they might need for planning, engineering and construction. Included in the project will be parcel maps for tax assessment, street centerlines for emergency routing, and major feature and building footprints for general planning.
     "To create the basemap, coordinate geometry was considered but rejected because of the expense involved," said George White, program director for the Department of Public Works, Office of Capital Resource Management. "Scanning existing paper maps was affordable but wouldn't have provided the necessary accuracy."
      Digital orthophotography was chosen for its combination of affordability and its high accuracy and resolution. A committee of municipal departments debated the resolution and accuracy question at length before finally coming to a consensus on acceptable parameters.
      The committee determined a pixel resolution of 3 inches was required so the basemap could be used to inventory municipal property, including objects as small as manhole covers and street lights. One future beautification project will even use the basemap to locate and maintain individual trees along roadways.
      The 2.5-foot accuracy was chosen primarily to create an extremely precise street centerline map that will be used in a vehicle navigation and routing project now under development for the city's police, fire and rescue squads. The centerline information must be accurate enough to update and correct commercially purchased digital transportation maps.
      "We figured that any application requiring better accuracy than 2.5 feet would involve a ground survey anyway, so there was no need for the basemap to be more accurate," said White.

Collecting Aerial Photos
In 1993, Public Works awarded the aerial survey contract to Hammon, Jensen, Wallen & Associates (HJW), an Oakland, Calif., photogrammetric engineering firm that specializes in digital imagery and GIS services. HJW later won the contract to create the digital orthophotos from the aerial photos.
      HJW's Pacific Aerial Surveys division acquired 275 aerial photos covering the 49-square mile San Francisco area. The firm also provided ground control and aerotriangulation necessary for digital terrain model extraction.
      "Achieving a quarter-foot pixel resolution starts with the aerial survey," said Mark Safran, HJW's director of marketing. "The altitude must be low enough that details in the photos are not lost during scanning."
      The photos were taken at an altitude of 3,300-feet above ground level, to provide a map scale of 1 inch equals 550 feet. Over the downtown area, which has numerous skyscrapers, HJW switched from a 6-inch to 12-inch focal length camera and increased altitude to 6,600 feet. This minimized the effect of building lean in the resulting high resolution photos.
      HJW scanned the photos on a Vexcel VX3000 scanner at a resolution of 2,200 dots per inch to achieve the 3-inch pixel resolution.

Terrain Challenges
A major challenge to collecting DTMs in hilly, urban terrain like San Francisco's is the smear that can be created around man-made features due to abrupt changes in topographic relief. The classic example, of which many exist in San Francisco, is that of a building erected on a hillside. One side of the building may rise 50 feet above the ground, while the back side may rise 100 feet off the ground if it is on a steep slope.
      Any well-defined feature such as a building, road or retaining wall located along a sharp slope can create what looks like a smear on the orthophoto if the differences in ground elevation on two sides of the feature are not taken into account during DTM collection, explained Bill Zeman, HJW's project manager for the San Francisco program.
      "When the technicians picked DTM points in the stereoplotter, they were careful to pick additional points along streets and on all sides of buildings, especially if there were breaklines on one side of the building," said Zeman. "This improved the accuracy of the DTM in the hilly San Francisco terrain."
      The Bay Area's famous hills presented HJW technicians with numerous opportunities to deal with another challenge to orthorectification - distortion of highway overpasses and bridges. If the sudden dips in elevation beneath bridges and overpasses are not considered during DTM extraction, the orthographic positions of these features will be incorrect in the final orthophoto.
      "The overpass appears to dip down because it will follow the terrain below it unless there is compensation," said Safran. "This induces significant errors in accuracy to any street or transportation maps digitized from the orthophoto. The overpass will appear to zigzag when it's actually straight."
      HJW performs orthorectification with an in-house Unix-based software package called OrthoView. When HJW developed this orthorectification software, it built in a capability to eliminate the distortions related to overpasses and bridges.
      As the stereoplotter operator collects DTMs, a polygon is also collected around any bridge or overpass. During orthorectification, the software takes into account the change in elevation and relative height of the overpass and places it in its correct orthographic position. OrthoView then automatically fills in the relic smear on the orthophoto where the feature had been incorrectly located.
      The project area contained more than 100 overpasses or bridges that required the automated technique. About 30 percent of the 250 orthophotos contained overpasses.

The Next Phase
HJW created a second set of resampled orthophotos with 1-foot pixels because the file sizes of 3-inch orthophotos are too large for many San Francisco agencies to handle. A single 3000 x 2000-foot orthoimage at 3-inch resolution consumes 100 megabytes of disk space. Prior to delivery of hard and softcopy orthophotos, HJW also formatted and annotated each orthophoto using both NAD 27 and NAD 83 coordinate systems, both of which are used by city agencies.
      The Department of Public Works has taken delivery of the digital orthophotos and has contracted to have several thematic maps digitized for immediate use. The first maps created will be grade maps of rights of way and curbs, parcel maps of blocks and lots, feature maps of major city features and buildings, and the street centerline map. Digitizing is slated for completion in June 1996.
      Public Works oversees the project and plans to create a centralized computer library where the orthophoto basemap and digitized maps can be accessed by all city and county agencies. Because so many agencies are involved in the project and each has its own applications with specialized software, Public Works is formatting all map files in ARC/INFO, MapInfo and AutoCAD.
      "We'll have a centralized server to distribute the maps among agencies," said George White, "and we will encourage them to create map updates after they perform work and input that information back into the system so all departments are aware of changes."
      By summer of 1996, San Francisco expects to have the centralized server up and running with the orthophotos and the digitized thematic maps. Public Works estimates the aerial survey, orthophoto basemap development, and digitization of the four thematic map layers will cost only $1 million.

About the Author:
Kevin Corbley is a freelance writer specializing in remote sensing, GIS and GPS. He is located in Denver, Colo., and may be reached at 303-987-3979.

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