Chapter 9. Ortho-rectification of aerial photos

Table of Contents

9.1 The goal of the ortho-rectification
9.2 The camera model and the internal orientation
9.3 The external orientation
9.4 Camera model of compact digital photo-cameras
9.5 The ortho-rectification process
9.6 The effect of the applied elevation model
9.7 Making of digital anaglif images
9.8 Rectification of the photographed documents and maps

The basic geometry and distortion are considerably different from the ones of the maps and map-based raster datasets. Maps are made to show the downscaled version of the landscape in a plane, projected all map objects to this, not depending on their vertical position. The distortions of the photographs are completely different. Here projection is central, the perspective distortion is characteristic, because of the optical realization (Fig. 53).

Line-art of a distorted airphoto seen from the camera

Fig. 53. Characteristic distortions in an aerial photo.

Though geo-reference can be assigned to pixels of photos with any orientation, and the geo-scientific value of surface photos and landscapes is also significant, in this chapter we discuss the aerial photos, mostly taken from aboard of aircrafts. These images approximate the map-like representation of the target, and their fit to standard coordinate systems is of great value in the geoinformatic analyses.

9.1 The goal of the ortho-rectification

The goal of the ortho-rectification is to resample the pixels of an aerial photo to a coordinate system that is interpreted in a selected horizontal surface (practically in a level surface of the region of the airphoto). This coordinate system should be defined in the geographic information system, according to the above chapters (e.g. a map projection plane).

Geo-referring the aerial photos, two different distortion effects should be corrected:

  • The perspective distortion, which is the result of the geometry of the photograph taking.

  • The distortion effect of the relief and/or the surface.

Up to now, just because of the planar model characteristics of the maps, the vertical geo-reference was neglected in the rectification, here this simplification is no more possible. And this is not even impossible; it is very important, which terrain or elevation model is used. In the aerial photos, the soil, the real physical terrain surface is invisible in many place, it is covered by the vegetation or the artificially built objects. It is our decision, based on the available data and the terrain, how to take into account the elevation of the terrain itself, the different, vertically extent objects and vegetation foliage.

There are some auxiliary information, needed to ortho-rectify the image:

  • The camera model and the internal (or in other term: interior) orientation data,

  • The external orientation data, and

  • A terrain or elevation model, covering the area of the photograph.