The digital orthophoto process transforms a vertical aerial photograph into the equivalent of a traditional map. Yet it retains the advantages of a photograph—visually displaying actual cultural and land features, and the built environment, rather than representing those features using symbols and lines.
How AeroMetric Produces an Orthophoto
1. Horizontal Corrections: The first step is to determine the exact camera orientation at the moment of exposure during the flight mission—the x, y, z of the camera lens center, plus the effect of aircraft pitch, roll and yaw upon the camera. These elements are derived directly with the use of Airborne GPS and IMU technology, or by means of ground control points analyzed photogrammetrically. This first step results in an image that is free of lens distortions, and is horizontally connected with a standardized scale.
2. Vertical Corrections: The second step involves using a DEM to vertically correct each pixel for the effects of relief displacement. This method depends on knowledge of the camera projective geometry, and the relationship between actual feature height and a predetermined vertical point of reference. The resultant image is an ortho-rectified image map that can be referenced to any known ground system.
3. Radiometric Balance: The final stage in the process involves radiometric and tonal balance correction so that all images blend well, and are naturally and aesthetically pleasing. This can be a challenge when imagery is collected in different lighting conditions on different days.
To help solve this radiometric challenge AeroMetric's own innovative tone-balancing software is used as the first step to providing uniform imagery. We then creatively select from our suite of commercial software programs to stitch together several individual orthophotos and generate seamless orthophoto mosaics.
The quality of a digital orthophoto is dependent upon the quality of each step in the process. For example, it is essential to start the process with good imagery and accurate input data, rather than proceeding, and trying to repair flawed imagery later. Because of our attention to quality outputs, AeroMetric will provide you with accurate and visually appealing orthophotography, and do so in your preferred data format.