Picture perfect GIS
Cities and counties have grown reliant on aerial and satellite photography to improve the accuracy of their geographic information systems (GIS). The photographs, when orthorectified (digitally corrected to remove distortions), provide a coordinate system for all other data layers in the GIS.
Using that coordinate system, cities and counties can trace items in the photos to create spatially accurate data layers, saving time and money on processes that traditionally would involve many hours of fieldwork. “If you have good photography and you want to create a [map of roads] for transportation uses, you can sit at your computer and digitize the roads on the photograph,” says Chad Rupert, local government team manager for Information Technology Outreach Services (ITOS) at the University of Georgia. “Before orthorectified photography became common, you would put a GPS unit on a car and drive all the roads in a county.”
For local governments with existing data layers, the imagery can serve as a backdrop for basic line maps. “Many rural counties don’t have a building permit system, and the only way a tax assessor has of catching improvements is from aerial photography,” says Jimmy Nolan, project coordinator for ITOS. “Swimming pools, additions to houses [and other improvements] can be seen from aerial photographs.”
Before investing in either aerial or satellite imagery, local governments consider how the imagery will be used, what resolution they need and how much money they can spend. By clarifying those issues, local governments can determine what type of imagery will fit best in their GIS.
Erie County, Ohio
In spring 2001, Erie County, Ohio, started building its GIS from the ground up. Although the county had a digital tax map system in the Auditor’s Office, it wanted to create a countywide system that all departments could use to store and manage land information, such as parcel maps and flood insurance maps.
To begin the project, the county contracted with Chesterfield, Mo.-based Image America to collect digital aerial images. “Digital imagery is a fairly new technology,” says Mark Wroblewski, GIS coordinator for the Auditor’s Office. “[Companies that use] the conventional [aerial] process still use film. They scan the film, and then they have technicians adjust it. Digital imagery is basically right out of the aircraft and processed quickly into a format that the end user can implement automatically. The advantages to the digital imagery are the cost and the time to complete the work to its usable format.”
The county wanted high-resolution (six-inch pixels), digital orthorectified photography to provide enough detail for the basemap. “To [build the GIS] the right way, we had to go through the process of having one base coordinate system, which is what the imagery was able to provide us,” Wroblewski says.
Within two months, the county had digital orthorectified photography, which it plugged into GIS software and made available to all departments. Using the imagery as a guide, departments created their own data layers. “The office edition application provides a platform for all the decision makers as well as the technicians to look at a lot of the same data sets and make decisions based on the same information,” Wroblewski says.
Last month, the county added the aerial photography to its GIS Web site so residents, municipalities and private companies could access the land information along with the pictures. “When people have concerns or questions, we’ve got all that [geographic and photographic] information now at our fingertips,” Wroblewski says. “We can explain in laymen’s terms and graphically why a decision was made or where that information came from.”
Cook County, Ill.
In 1998, Cook County, Ill., started rebuilding its GIS using aerial photography. In the late 1980s the county had used maps from the Reston, Va.-based U.S. Geological Survey to create its GIS, but by the late 1990s, those maps were out of date and did not provide the accuracy the county needed for additional data layers. “We wanted to have a standardized base for all of the new data that we’re going to be creating,” says Alan Hobscheid, GIS coordinator for Cook County. “The county had never had a fully rectified digital product before its aerial work.”
Cook County worked with Chicago to design specifications for the project so they could both use the photos. Chicago wanted to be able to see all the sidewalks downtown, which required high-resolution photography and extra flights over tall buildings. “We have a wide range of departments with all kinds of applications, and our approach was to hit the broadest swath we could,” Hobscheid says. “As you try to achieve more accuracy, your costs go up exponentially. You’re always weighing that balance. It’s not worth short-changing accuracy because of costs if your needs aren’t going to be met.”
Striking a balance, the county contracted with Aurora, Colo.-based Merrick to take aerial photographs of the county’s 960 square miles in spring 1998. From those photos, the company created a digital elevation model and digital orthorectified photography in six-inch resolution. Subsequently, the county contracted with Redlands, Calif.-based ESRI to use the orthophotos to create a GIS. By fall 1999, Cook County had a completely rebuilt, spatially accurate GIS that could be used by departments who were already familiar with GIS, such as the Highway Department, the Assessor and the County Clerk.
By the end of the first quarter this year, Cook County plans to have launched a GIS Web site for public access to the data, including the aerial imagery. “Almost every department will benefit from having access to the imagery because, even though we’ve had imagery in paper form, typically it’s divided into one square mile per sheet,” Hobscheid says. “If you wanted to [look at] very large areas, it [could] be cumbersome. In a digital environment, it is much easier to navigate the data to accomplish whatever you’re doing.”
The county also plans to incorporate the aerial imagery into its 911 dispatching maps. “Even though they’ve had mapping to support their dispatching, they’ve never had photography underneath it,” Hobscheid says. “It may provide some additional information to the dispatcher.”
Cook County plans to update its imagery this spring to have a visual record of the changes that have occurred since 1998 and to improve the accuracy of the GIS. “That original flight was certainly supportive enough to create our GIS database, but to take it to the next level for topographic mapping, we need to get a little more accurate photography,” Hobscheid says.
Jefferson County, Colo.
Unlike Erie and Cook counties, Jefferson County, Colo., already had a fairly accurate GIS when it considered adding imagery to it. However, in the late 1990s, the county was experiencing unprecedented population growth, and the Information Technology Department was having trouble keeping the GIS current. The department was devoting too much staff time to creating custom maps for county agencies, says Dave Gallaher, director of information technology development for Jefferson County. “I [had] guys with master’s degrees sitting around trying to figure out what color to make the parks on the map,” he says “We [had] to get it so our customers [could] make their own maps and pull up their own [imagery].”
When deciding what type of imagery to purchase, the county needed to balance the technical needs of the GIS with the desires of the agencies that would be using the imagery. “[The county does not] maintain or deal with infrastructure in terms of manholes, sewer systems and streetlights,” Gallaher says. “We need to be able to see the roads, but not so much the rest of it. The six-inch pixels are nice, but it’s probably overkill when you have 800 square miles to deal with. File sizes get to be kind of large.”
In 2000, the county contracted with Thornton, Colo.-based Space Imaging to take satellite photographs and to create digital orthorectified photography at one-meter resolution for the GIS. Once the imagery was in the GIS, technicians began adjusting the data layers to line up with the photography. “We had roads that were built in the county years ago that were as much as 1,000 feet out of the designated right-of-way,” Gallaher says. “We had to shift lines because of what we’ve seen. We shift roads all the time based on what we see in those images.”
The county built a Web-based GIS that residents could use to create their own maps. The site contains satellite imagery as a background layer. “[The site] is a huge value to the county just in terms of what people can do with that information,” Gallaher says. “We’ve got probably 40 data layers they can click on and add to the information [on their screens].”
Currently, the county is using the satellite imagery to create a GIS layer of building footprints. “Instead of having somebody manually go through and draw little boxes around where each building footprint is, we’re going to do it off the satellite [imagery],” Gallaher says. “We can write an algorithm, tell the computer to look for [a] signature and [identify it as] a building. Not only do we get all the building footprints, but we get them as-built, rather than as-designed.”
The county also used satellite images to help fight the Hayman Fire, which burned 137,000 acres in Colorado last summer. When helicopter pilots were having trouble identifying the exact boundaries of the fire because of heavy smoke, the county called Space Imaging to shoot pictures of the area from the firm’s satellite, which has infrared sensors.
Within three hours, the county had a photo showing exactly where the fire was burning. “This was not an orthorectified image, but one of the advantages of a satellite is the image is almost orthorectified out of the box because of the intense height of the satellite,” Gallaher says. “During the course of that fire, they took an image every chance they could get. I think we had four or five spectacular images of this thing.”
Worth a thousand words
Several factors can influence the type of imagery local governments choose to add to their GIS. If cost is a primary concern, local governments may want to follow Cook County’s lead and start with using free or low-cost digital orthophotos from the U.S. Geological Survey. “Even though it’s not extremely high resolution, it’s relatively accurate photography spatially,” Nolan says.
If cities and counties need higher-resolution images or infrared data, satellite photography like Jefferson County’s may fit the bill. For even higher-resolution images, local governments can contract for aerial photography, as Erie County did.
Regardless of the type of imagery local governments use, they find that it adds information to the GIS that would have been impossible to create otherwise. “It is critical to have imagery,” Gallaher says. “You can have all the lovely vectors you like, but [imagery] brings it alive. You could never afford the cost of digitizing every tree. Why not just simply show the image?”
