This page contains an overview of orthophotography. To find out more on the latest status and availability of Digital Orthophotography, see our Status of Orthos page.
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.
If you are still not sure what an orthophoto is, visit our extended Orthophotography 101 - The Basics page.
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.
For more information on the uses of DOPs, visit our extended page on some potential uses of orthophotos and consider the most important characteristics for a particular application.
Also, from federal DOPs, to county-wide series, to historical orthophotos, here is information on the sources of a variety of orthophoto products and formats in the catalog.
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.
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