Land & Mapping Services provides clients with custom air photography, orthophoto production, digital photogrammetry, and associated surveying services. Mapping and photography can be tailored to your requirements for resolution and budget.
What is a Digital Orthophoto?
A digital orthophoto is a digital image of an aerial photograph in which displacements caused by the camera and the terrain have been removed. It combines the image characteristics of a photograph with the geometric qualities of a map. They are often used as a backdrop in a GIS and as the base for creation or update of vector mapping data.Unlike a standard aerial photograph, relief displacement in orthophotos has been removed so that ground features are displayed in their true ground position. This allows for the direct measurement of distance, areas, angles, and positions. Also, an orthophoto displays features that may be omitted or generalized on maps.
Digital orthos may be produced at a variety of resolutions. Resolution is the minimum distance between two adjacent features or the minimum size of a feature represented in the orthophoto. Resolution is determined by pixel size. Pixel size may range from 6 inches to 2 meters.
Processing Steps
Digital orthophotos require several types of inputs to produce an orthogonally rectified image from the original perspective image. These inputs are the following:
- 1) the unrectified image scanned from the diapositive,
- 2) a digital elevation model with the same area of coverage as the digital orthophoto,
- 3) the image and ground coordinates of photo identifiable ground control points, and
- 4) calibration information about the sensor collector device.
These five inputs are used to register the image file to the scanner and to the sensor platform, to determine the orientation and location of the sensor platform with respect to the ground, and to remove the relief displacement from the image data.
For more information on the processing steps used to create digital orthophotos, try USGS Standards for Digital Orthophotos.
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