Airborne/Digital Ortho: Urban Planning with Orthophotos
City managers in Fort Collins, Colo. integrate their GIS with digital orthophotos for positive results.
By Mary Jo Wagner

"How is the data in the city of Fort Collins' GIS network used?" Claudia Benedict is asked. She pauses. After a brief moment, she laughs and asks, "How much time do you have?" It's a valid question indeed as Benedict, the GIS manager, begins to quickly name just a few of the applications being carried out throughout the Colorado city.
     The Department of Transportation utilizes the road network information to determine correlations between the frequency of accidents on a particular road and the road's condition. The Poudre Fire Authority develops emergency management plans using aerial photos and city maps. The Advanced Planning Division uses the GIS to monitor developments and analyze regions slated for potential future development. The Natural Resources Division is building an inventory of natural habitat areas within the city. In short, the city's geographic information management system is an integral part of many city departments.
     And that keeps the Fort Collins GIS Department busy. A service-oriented department, the GIS Division is in constant demand, its staff in constant motion. Some of the activity revolves around producing and delivering particular datasets to the appropriate departments. But a majority of the department's employees' time is spent carrying out projects for their own division.
     Visiting the city's GIS department today, one may notice a particular buzz, an excitement level higher than normal. A recent addition to the GIS may be the cause of that excited activity: digital orthophotos.

Visual Demands
One staple of the Fort Collins' GIS has been aerial photography. Although flight missions have been flown sporadically for 12 years to collect aerial photos, the datasets have been used regularly by several departments. Various paper maps have been manually created through the years using the aerial photos as a base layer on which vector data has been overlaid.
     As the system has grown and more departments have come on-line with particular mapping needs, Benedict says the GIS department decided to update its aerial photography. In December 1995 the city commissioned the Fort Collins-based company, i-cubed (i3) - information integration & imaging, LLC - to produce a black and white photomosaic of the city and the surrounding area.
     A flight was flown over the area - approximately 125 square miles - and 82 black and white photos were collected at a scale of 1 inch =1500 feet. The photos were then rubbersheeted, meaning the photos were simply registered using ground control points, and then mosaicked together.
     Although the photomosaic gave the Fort Collins GIS Department an overview of the city, it was not an accurate representation because the photos were not corrected for the distortions in the terrain or the viewing geometry of the camera. That limitation in accuracy made it difficult for the department to carry out many applications.

Enter the Orthophotos
Benedict says the department began to turn its attention to orthophotography which would provide users with higher accuracy. The GIS Department staff met with colleagues to discuss the potential of orthophotos and within a few months 10 other city departments, including Natural Resources, Advanced Planning, Wastewater and Engineering, offered their support to acquire an orthophoto dataset.
     One year later the city developed a proposal to obtain orthophotos of the same area. Once again, the city turned to i3 to carry out the project.

Going Digital
The timing turned out to be perfect. An aerial photography mission of the northern Front Range encompassing over 900 square miles of Colorado had just been flown in June 1996. As a result, Fort Collins was afforded the latest aerial photography available.
     To produce digital orthophotos, i3 needed four data sets: scanned aerial photos; a camera calibration report; ground control points; and, elevation data.
     Twelve color photos were required to cover the area of interest. They were initially captured at a scale of 1 inch = 3250 feet and given to i3 to orthorectify and mosaic. i3 had to first scan the negatives to create digital image files for each of the 12 photos.
     "The ortho process corrects for distortions caused by the terrain, the orientation of the airplane and the camera lens," said Russ Cowart, president of i3. "Because orthorectification accounts for the viewing geometry and relief displacement, it results in a more accurate planimetric image than traditional warping, or rubber-sheeting techniques. The preciseness of an orthorectified image allows users to accurately measure distances and areas."
     i3 uses OrthoEngine by PCI Enterprises, in Richmond Hill, Ontario, to create orthorectified photos. "OrthoEngine is very easy to use and allows us to create accurate digital orthophotos quickly and efficiently," said Jill Mamini, an imagery analyst at i3.
     i3's first step was to enter information about the camera including the focal length and other calibration data into the software. General project parameters such as the map projection, photo scale and resolution were input as well. To establish the geometric orientation of each photo, Mamini interactively selected four corner fiducial marks for each of the 12 photos. Using the Fort Collins GIS Department's street centerline vectors, ground control points were collected by matching intersections from the street vectors with their corresponding intersections on the imagery.
     In addition to matching the vector dataset with the images, the images themselves are referenced to one another through the collection of tie points. Mamini created the series of tie points by selecting common features between overlapping photos.
     Once the orientation information was gathered for all the photos, it was time for Mamini to specify a Digital Elevation Model and begin the orthorectification process.
     Thirty-meter DEMs were acquired from the United States Geological Survey (USGS) to correct the terrain. DEMs allow users to account for the terrain displacement created by local topography. Once the appropriate DEM and output file were specified, the orthorectification process could begin. With a simple click of the mouse OrthoEngine's batch manager processed all of the photos in less than 24 hours.
     Finally, cut-lines - where adjacent photos are joined - were selected along natural breaks in the terrain. Based on the cut-lines, OrthoEngine combined the photos to produce a seamless, black and white orthomosaic with a 1-meter resolution. The data were integrated with the city's GIS network last December.
     "We were very happy with the orthophotos," says Benedict. "Adding this additional visual layer into the GIS opens up many application opportunities."

Projects, Projects, Projects
The core of the city's GIS is a parcel basemap. A cadastral layer, the parcel basemap shows the boundaries of every individual property in the city and is the base layer of choice for about 80-90 percent of the applications carried out.
     The orthophotos now provide a picture to coincide with the parcel map's lines and polygons. Having the orthophotos as a background allows users to monitor developments in particular areas or to help identify certain issues they might be confronted with for planned developments in the future.
     "When people are presented with a series of vectors on a screen, it's very hard to visualize what those lines mean," says Mamini. "But with a photo underneath, the concept of using vectors to represent features isn't so foreign. It's easier for people to understand."
     Benedict agrees. "Orthophotos are a great communication tool. Lines on a map don't have a lot of relevance to people who are not GIS users. With those lines overlayed on a picture people can see features they can relate to. It makes it easier to communicate, to plan and to resolve issues."
     Since receiving the orthophotos the GIS Department's printers have been busy spewing out hard copies of the digital photos of various regions of the city to meet project managers' demands. "Every day we re-discover the potential of the orthophotos as new applications arise," says Benedict.
     As the orthophotos are directly georeferenced to the city's entire street network, users can easily produce maps at any scale without sacrificing the clarity and accuracy of feature locations. The imagery is also an asset for those areas where previously collected street vectors are inadequate.
     One of the earlier projects the GIS Department completed after receiving the digital photography was to produce coverage maps of two major bike and pedestrian recreational trails. The maps are used by the Park and Recreation Department to monitor the trails' condition and develop needed maintenance projects. Citizens interested in finding a trail for a bike ride or walk can use the maps. And the police and fire departments find the maps very useful as the trails are lined with call boxes. The trail coverage maps show dispatchers where the boxes are located. In the event that someone should need help, dispatchers will have the exact location of the person to aid the first responders.
     The Poudre Fire Authority frequently obtains maps generated from the orthophotos to locate fire hydrants and emergency crews and equipment. Recently, the detailed maps allowed them to strategically locate emergency vehicles during a festival and to create "attack" plans should problems arise.
     Orthophotos are substantially more accurate and detailed than uncorrected aerial photography. In Fort Collins' case, that higher level of information equates to better use of resources and less time-consuming projects.
     Creating a natural habitat inventory is a case in point. Ten years ago the Natural Resources Division created an inventory of all the natural habitat areas within the city. Field crews scoured every inch of the city and recorded each area's surroundings. It took one year and a half to develop the inventory.
     The division is presently updating the natural habitat inventory using orthophotos maps and the increased efficiency has already been noted.
     "The majority of this project can be done from the natural resource planner's desk," explains Benedict. "Using printouts of orthophotos with the parcel coverage on top, she can compare the old and new imagery to verify the boundaries of the habitat areas. Instead of having to dispatch field crews to conduct ground surveys of the entire city, she can decide where ground surveys need to be done from the orthophotos on her desk." Any needed updates are written directly onto the photo and given to the department responsible for inputting the changes.
      Roughly 640 acres, or one square mile, were verified using digital photos in about two days. Traditional methods could warrant at least one week to confirm the same area, says Benedict.
      In between delivering needed maps to colleagues, the department staff is busy updating the landuse coverage in the system. The orthophotos help fill in any data "gaps" and indicate any discrepancies between the two datasets.

Looking Ahead
Clearly, the operative word for the GIS department's future is "inventories." Benedict says a whole slew of inventories are slated for development. She begins yet another list: a parking inventory, a sidewalk inventory, a bus stop inventory and the city forester would like to build a street tree inventory - a data base of all the trees lining city streets.
     "Some of these projects could just be handled by drive-bys but some of them would be a lot easier with good digital photography," Benedict says.
     Although it is still unclear whether the GIS Department will acquire higher resolution orthophotography to carry out these future inventories, the addition of digital photos has served several city departments well so far. If the GIS Division continues on its more "visual" path, Benedict may find herself taking a much longer pause the next time she's asked how the system's data is being used.

About the Author:
Mary Jo Wagner is a freelance writer, specializing in the GeoTechnologies. She can be reached by phone or fax at: 715-235-9839; or e-mail: [email protected]

Back