AN ORTHOPHOTOGRAPHY

An orthophoto or orthophotograph is an aerial photograph geometrically corrected ("orthorectified") such that the scale is uniform: the photo has the same lack of distortion as a map. Unlike an uncorrected aerial photograph, an orthophotograph can be used to measure true distances, because it is an accurate representation of the earth's surface, having been adjusted for topographic relief, lens distortion, and camera tilt.

Orthophotographs are commonly used in the creation of a Geographic Information System (GIS). Software can display the orthophoto and allow an operator to digitize or place linework, text annotations or geographic symbols (such as hospitals, schools, and fire stations). Some software can process the orthophoto and produce the linework automatically

An orthophoto is an aerial photograph planimetrically corrected. Orthophotographs have the positive attributes of a photograph such as detail and timely coverage, and the positive attributes of a map including uniform scale and true geometry. This enables orthophotographs to be used in their primary role as a backdrop on which map features can be overlaid. Orthophotos represent the primary use of remote sensing imagery.

Orthophotos are now created by scanning aerial photographs and converting them into a raster image file format. A digital terrain model is added as a means of collecting ground points to indicate the changes in elevation. When control points are identified on the aerial photos, stereo models are developed and geometric distortions are estimated. The image is rectified and georeferenced using mathematical models of photogrammetry to remove tilt, terrain, atmospheric and other digital image distortions.

The orthophotoscope, an optical device, was used initially to produce orthophotos. The first instruments designed to produce orthophotographs were by Lacmann in Germany and Ferber in France at the beginning of the 1930’s. After Ferber’s design, a series of designs were built by the Gallus firm and tested by the “Service Geographique de l’Armee” as well as the French Cadastral Service. Neither agency decided to adopt orthophotography.

T-64 orthophotoscope

In 1955, Russell Bean of the Topographic Branch of the U.S. Geological Survey invented the orthophotoscope, which was initially used to make orthophotographs for use by geologists of the same agency. It was the first orthophotoscope used to produce orthophotoquads — rectified air photos. The T-64 model was a modified Kelsh analog stereoplotter that profiled the stereo model while the operator manually raised and lowered the film level based on the terrain height. Successful model designs were produced by the USGS, such as the U-60, T-61, and in 1964, the T-64 model that was put into production by the Kelsh Instrument Company. It was sold to Canada, Greece, and to the U.S. Geological Survey. The operation of an orthophotoscope is similar to tracing correct planimetry from a stereomodel, as it continually adjusts the floating dot to keep it in contact with the terrain surface. After the aperture has moved across the stereomodel along one scan line, it is stepped in a vertical position that is the distance equal to the width of the aperture and the next scan line is exposed.

Current orthophotoscopes also operate by first scanning the entire stereo model by using the floating dot method, or by mathematically computing the parallax difference at each point using a statistical procedure called cross-correlation. The elevation along each scan line computed from the parallax difference is stored in a digital elevation model data file. This file is used to raise and lower the aperture as it moves along each scan line in the orthophotoscope.