The Nature of Geographic Information

14. Orthorectification


An orthoimage (or orthophoto) is a single aerial image in which distortions caused by relief displacement have been removed. The scale of an orthoimage is uniform. Like a planimetrically correct map, orthoimages depict scenes as though every point were viewed simultaneously from directly above. In other words, as if every optical axis were orthogonal to the ground surface. Notice how the power line clearing has been straightened in the orthophoto on the right, below in Figure 6.15.1.

Comparison of unrectified vertical aerial image (bends) and orthoimage of same scene (straight)
Figure 6.15.1 Comparison of a vertical aerial photograph (left) and an orthophoto.

Relief displacement is caused by differences in elevation. If the elevation of the terrain surface is known throughout a scene, the geometric distortion it causes can be rectified. Since photogrammetry can be used to measure vertical as well as horizontal positions, it can be used to create a collection of vertical positions called a terrain model. Automated procedures for transforming vertical aerial photos into orthophotos require digital terrain models.

Since the early 1990s, orthophotos have been commonly used as sources for editing and revising of digital vector data.