An orthophoto or orthophotograph is an aerial photograph that has been geometrically corrected (“orthorectified”) such that the scale of the photograph is uniform, meaning that the photo can be considered equivalent to 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 such as ArcInfo or MetaMap can display the orthophoto and allow an operator to ‘digitize’ or place linework, text annotations or geographic symbols, such as hospital or fire stations. In addition, there are a few software products that can process the orthophoto and produce the linework automatically within a certain percentage of accuracy.
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 the orthophotographs to be used in their primary role as a backdrop onto which map features can be overlaid. Orthophotos represent the primary use of remote sensing imagery.
Orthophotos are now created by scanning an aerial photograph and converting it into a raster image file format. A digital terrain model is then added as a means of collecting ground points to indicate the changes in elevation. When control points are identified on the aerial photos, stereomodels are developed and geometric distortions are estimated. The image is rectified and georeferenced using mathematical models of photogrammetry for the removal of 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.
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. This was the first orthophotoscope that was used to produce orthophotoquads (rectified air photos). The T-64 model was a modified Kelsh analog stereoplotter that profiled the stereomodel 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. Geogeological 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.
An air photo is acquired by a large format camera that is mounted in a stabilized position with the camera pointing straight down at the earth’s surface to record the images in flight lines. A pair of photos is used for determining the relative difference in elevation. During a flight line, each photograph overlaps the previous one by 65%, which is called an endlap. This overlap allows the viewing of a stereomodel, which is a three-dimensional model of terrain elevation. A parallel flight line captures images with sidelap of about 25%.
Aerial photographs appear in various formats and sizes. A 240 mm (9 inch) format specifies a square photograph 240 millimeters on the side, and a 35mm format specifies a film with an imaging area that is 24mm by 36mm. Camera systems based on 35mm and 70mm film are considered to be small format. Cameras that use 240 mm film are considered to be large format. Large format cameras are often used to take aerial photographs that will be used for the development of an orthophoto. These cameras are taken onto specialized aircraft that are designed for taking photographs, and they are able to support aerial photography with precise positioning and flight control.
Digital Conversion Process
In the digital process, the development of orthophotos must go through a process called differential rectification or orthorectification, which is a point-by-point correction of the scale and relief displacements normally resulting from variations in elevation between the aircraft and the topography over which the aircraft flies. The aircraft is flown at normal range heights using a standard wide-angle camera. The orthophotograph is created with a stereomodel of the terrain, the same approach adopted in plotting a conventional line map from aerial photographs using a stereo-plotting machine. The process applies the corrections to photographs prior to digitization, in which a photograph is scanned to produce a digital raster image. Aerial photographs are produced into digital orthophotos by dividing the given area of a photograph into very small, equal sized pixels. The geometric correction of aerial photographs requires the calculating of distortion at each point, and then shifting the image to the proper location. To produce an orthophotograph, the overlapping photographs are set up similarly to using a stereomodel, in which the plotting machine is replaced by a narrow opening, by which only one of the images is admitted through filtering to be recorded onto a film sheet. Instead of continuously following individual features, such as roads, field boundaries, streams, as done by a measuring mark, the aperture is made to transverse the stereomodel systematically in a series of parallel tranverses, forming a raster pattern to ensuree that all details present in the stereo model is recorded orthogonally on the film below.
The digital photograph is registered when each pixel is placed in its precise geographic position by a program that takes into consideration the camera location, the orientation of the camera platform, and the heights of all points in the grid of the area photographed. In the photogrammetric method, it “combines ground-surveyed measurements with measurements taken on aerial images to provide precise, orthographically correct coordinate locations.” This process requires the measurement of the photo coordinates and their combination of x, y, and z coordinates. Photo coordinates can be measured using a ruler or calipers, but they are usually measured using digital methods. The displacement is then calculated for each point in the raster image, and distortion is removed for each raster cell.
Multiple photographs can be analyzed, corrected and mosaiced all at once by a process called “bundle adjustment”, in which interrelated sets of equations are used to find a globally optimum set of corrections across all photographs. If a photo is black and white, each pixel is assigned a single numerical value corresponding to its light intensity. Color orthophotos are handled in an analogous way by transforming a vector of light intensities for different color bands into a single number. After the model has been scanned, the film is then developed as a negative orthophotograph, and it would usually be brought to the required scale by appropriate scaling or setting of the stereo-model, and can enlarged or reduced by the user if the scaling has not been adjusted after being scanned.