- Basics of Orthophotography
- Uses of Orthophotos in Wisconsin
- Government standards and procedures
- Additional information and demonstration programs
Basics of Orthophotography
Aerial photographs contain distortion, because they do not maintain a constant scale across the image. Orthophotographs are more readily used for measurement and spatial analyses because they do maintain a constant scale across the image. Orthophotographs are digital images that are produced by making geometric corrections to scanned aerial photographs. The distortion in aerial photographs is removed by unwarping the effects of terrain, removing the perspective projection of the camera, and by fitting the image to a particular map projection to create an "image map" that has a uniform scale and a known accuracy. Hence, orthophotographs can be used as a map whereas aerial photographs cannot. In computer systems, they can be integrated with other geographic information providing a rich visual context.
Uses of Orthophotos in Wisconsin
Orthophotos have a variety of uses in Wisconsin, both at the state and local levels. Once in digital format, they can be viewed and printed at various scales, and are extremely valuable in the development of land information systems and land use planning issues such as zoning, transportation, and agriculture.
Once the NAPP coverage of Wisconsin was completed in 1992, interest began to grow in obtaining orthophotography for various areas. The Natural Resource Conservation Service (NRCS) ordered the first sets of orthophotography for several west-central counties as part of their effort to revise the soil survey for compatibility with other maps.
At this time, all 72 Wisconsin counties are covered by existing or in-production orthophotos. These coverages were requested by several agencies including the USDA Forest Service, NRCS, USGS, individual counties, regional planning commisions and the Wisconsin DNR. These coverages consist of both digital orthophoto quarter-quads and individual county project coverages all of which use photographs no older than 1992. At this time, there are orthophotos available for 51 of the 72 Wisconsin counties and the remaining 21 conuties have orthophotos currently in the production process. Interest continues to grow and the air photo coverage of the state of Wisconsin will be enhanced based on the Wisconsin 1998 and 1999 NAPP flights .
The American Geographical Society's Library offers information on how the Southeastern Wisconsin Regional Planning Commision (SEWRPC) uses orthophotos in their work. Visit this link to view an orthophoto image over the Golda Meir Library on the UW-Milwaukee campus.
Statewide orthophotography coverage is a goal of the Wisconsin Land Information Board, and significant progress towards that goal is being made. Beccause orthophotos are needed for the statewide digital soil mapping project, that began in 1999, orthophoto coverage around the state is increasing.
Government standards for producing orthophotos
The U.S. Geological Survey (USGS) and the Department of Agriculture (USDA) have established standards for the production of digital orthophotoquads (DOQs).
The following is a list of the major standards:
- The primary source of aerial photography to be used is the National Aerial Photography Program (NAPP) and National High Altitude Photography (NHAP) imagery.
- NAPP at 1:40,000-scale, is used for quarter-quadrangle images, while the older NHAP at 1:80,000-scale is used for quadrangle (7.5-minute topographic sheet) coverage.
- Photo digitization must produce 1-meter pixel resolution for NAPP imagery and 2-meter resolution for NHAP.
- The images are rectified to a Universal Transverse Mercator projection, and National Map Accuracy Standards must be met at the 1:12,000-scale for quarter-quadrangles, and at the 1:24,000-scale for quadrangles.
- Vertical accuracy of the Digital Elevation Model (DEM) used to correct the photo image must not exceed 7 meters RMSE.
For more information on standards for DOQ production, visit the USGS website. When ordering or using orthophotos keep in mind that several states have developed their own orthophotography programs with standards that may differ from those of the federal government. Wisconsin is not one of these states.
Additional information and demonstration programs
Most orthophotos are created from NAPP photographs. See their site for additional information regarding standards and procedures for the production of orthophotographs.
The Massachusetts Institute of Technology (MIT) also provides an informative page
on orthophotos.
Learn about Orthophotos
Explore these educational modules that cover the basics of digital orthophotography, potential uses, local funding, and maintenance of these data, and how to select from and obtain available imagery. Also included is a quick-reference FAQ and glossary.