Imagery, especially digital photographs derived from planes or satellites and registered to real-world coordinates, is fundamental to the effective use of GIS. All aerial images are valuable, but images that have been adjusted by removing distortions resulting from terrain relief and camera or sensor orientation are especially valuable. These orthophotographs have the geometric characteristics of maps and the image quality of photographs. The MSDI plan for imagery concerns all forms of photographic sources, but primarily focuses upon orthoimagery.
Up-to-date orthoimagery can support many GIS-supported business functions within Minnesota, but orthoimages with complementary specifications are required to meet all needs. Scale, resolution, color, and time of year are especially important considerations for some applications. Orthoimagery also serves as a basis for developing other geospatial data, including data related to transportation, hydrography, critical infrastructure, and land use/land cover. New imagery must be acquired periodically, especially where development, construction, natural processes, and other factors change the physical landscape.
The strategic plan for addressing this state data need is being developed by an imagery data workgroup with assistance from the Strategic Plan Committee of the Governor's Council on Geographic Information.
Representative applications of orthoimagery and the specifications they require are:
Statewide orthophotography for Minnesota has been acquired only twice, during the 1989-1992 period and in 2003. The first-generation products, contracted through the U.S. Geological Survey and costing about $5 million, were black and white images and flown under leaf-off conditions, which is especially useful for revealing infrastructure. The 2003 images, contracted through the U.S. Department of Agriculture Farm Services Agency, were produced in color, which is especially useful for agricultural and natural resources applications. The total cost for the second-generation images was under $1 million. All data are available through the Minnesota Geographic Data Clearinghouse as well as other sources.
Higher resolution imagery is produced on a periodic basis for specific purposes, especially by local units of government. For example, Minneapolis has an active program to acquire 6-inch resolution color imagery for the Minneapolis area every 3 years. The Metropolitan Council acquires 2-foot imagery on a three to five year cycle. Some sharing of available imagery occurs through word of mouth and organizations such as MetroGIS, LMIC and DNR. Limited coordination of planned imagery projects occurs primarily through informal communication networks.
A draft MSDI data plan for imagery was prepared in May 2003 and updated in September 2003. The plan will be completed by the summer of 2005.
Costs for imagery acquisition vary, depending on product criteria. Recently acquired six-inch resolution color orthoimagery for urbanized areas such as Scott County and the City of Minneapolis cost $700 to $1,000/square mile. Recently produced one-meter color imagery for the entire state cost about $11/square mile. Satellite imagery is becoming competitive for some applications and is available from commercial sources costs at about $11/square mile for 2.5-meter black and white images. Some agencies that use imagery depend upon a periodic budget for image acquisition.
To learn more about the strategic plan for meeting imagery data needs within Minnesota, contact one of the workgroup chairs.