Digital orthophotos are created to both a defined map scale (i.e. 1”=100’, 1”=200’, etc) and Map Accuracy Standard. The individual digital images that make-up an orthophoto are differentially rectified to remove distortions caused by the rotation of the image about the imaging platform’s nodal point (x, y, and z) while in flight, inherent scale differences caused by flight altitude changes, and constantly changing image scale caused by the terrain (i.e. tops of hills are closer to the sensor and of a larger scale than bottoms of valleys that are further away and of a smaller scale).

The scale differences may be slight. However, in order to create an accurate orthophoto, the earth’s surface must be “flattened” to a common plane. In addition, each image’s perspective must be changed from a single point that equates to the camera lens to an orthographic perspective that has an infinite number of true perspectives as is the case with most topographic maps.  The mathematical surface to which images are rectified is the Digital Elevation Model (DEM).

In essence, the individual images are controlled and mathematically “draped” over the DEM, pixel-by-pixel.  Information from the GPS ground control survey, the aerial triangulation solution, and the DEM are combined using Intergraph’s ImageStation OrthoPro software to produce high resolution images and mosaics of these images called Digital Orthophotos.  The Atlantic Group uses ImageStation OrthoPro to complete our orthophoto production workflow, starting with project planning, through rectification, dodging, true orthophoto generation, tone balancing, automatic seamline generation, mosaicking, and quality control evaluation.

Using OrthoPro, our technicians are able to automatically input data from different projections and datums and integrate this data into a single mapping project. OrthoPro is integrated into GeoMedia, Intergraph’s premiere GIS display and manipulation environment, which easy supports integration of the data with images, footprints, and other data.