Orthorectification Alternatives Save Time and Money
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Keep it Simple part 1 of 2
Fully orthorectified aerial photographs can be an excellent source of extremely accurate mapping information in a GIS. But for some applications, orthorectification is an expensive and often unnecessary process that can be replaced by standard registration and rectification techniques carried out in a commercial image processing system.
In fact, many GIS mapping projects, e3specially land use and land cover delineation, can be conducted more quickly and at a fraction of orthorectification cost by using scanned air photos that have been registered and rectified to an existing GIS coordinate system. Although the accuracy won’t equal that of orthorectification, the savings in time and money are worth the trade-off for projects requiring less precision.
A Case in Point
For some GIS users, the thought of integrating air-photos into their digital systems and then performing registration and rectification is a daunting prospect. Despite significant effort by image processing software developers to make their packages easy to use and ready to integrate with the GIS environment, digitally processing photos and images is still considered too difficult and often avoided by many GIS users.
The City of Greeley, CO., located about 60 miles north-east of Denver, recently overcame these issues. First, it weighed its need for accuracy against the $50,000 minimum price tag for orthorectification and concluded orthoquality accuracy wasn’t necessary. Second, it purchased a commercial image processing package and dove headfirst into performing the registration and rectification work itself despite no formal image processing training.
The results were outstanding, Greeley produced rectified photographic maps with registered GIS overlays that satisfied their original purpose and subsequently were used in many other projects. The maps’ one-to three-meter accuracy was more than adequate for many applications
A Request for Zoning Boundaries
The Greeley Water and Sewer Department began building its GIS in 1987. System management shifted in 1993 to a newly formed GIS group within Public Works, which transformed the GIS into a citywide system that contains 500 local and 200 regional layers, including transportation, hydrology, building footprints, parcels, zoning, parks, open space, water, and sewer.
The city runs Arc/Info GIS software from Redlands, CA.-based ESRI Inc. on Mountain View, CA.-based Sun Microsystems Inc. SparcStations and Microsoft Windows computers. The city has three departments located in separate buildings that access the system remotely through a Local Area Network.
Aerial photographs have been purchased routinely for a variety of projects in Greeley. Public Works has the city flown every three to four years at a scale of 1:420, and the Water Department acquires 1:1,000-scale color photos nearly every two years to monitor crops, irrigation systems and reservoirs in and around the city.
The GIS group came up with the idea to merge the photos with vector data after it received a request for mapping assistance from the Community Development Department. Working with the Wildlife Committee, a citizen’s advisory board, the department was writing new regulations that would govern future building development in the city. A major part of the regulations was a new zoning overlay that delineated boundaries of wildlife habitats, open spaces and other environmentally sensitive areas, which needed to be protected and preserved in the face of development
“We wanted to draw a well-defined line around these wildlife and natural areas so developers would be aware that development would be more closely regulated there,” says Joe Lohnes, city forester in the Forestry Division of Greeley’s Parks and Recreation Department, a major participant in the zoning project.
Existing air photos were considered the ideal choice to help draw the new zoning lines, because vegetation, drainages and other key indicators of wildlife habitat are visible. Equally important was that anyone, including citizens on the committee with no map training, could look at a photograph, understand it and identify land cover features. The same couldn’t be said for vector line maps in the GIS.