Aerial photographs cannot be precisely overlaid on a reference map, even if one knows the geographic position of the centre of the photo and its scale [Example 1].
This is because the effects of a possible mis-verticality of the view axis (due to lateral movements of the plane), lateral drift of the plane due to side winds, geometric distortions due to the lens, and (above all) parallax distortions due to the relief, must be corrected to be able to better fit a photo to the reference map [Example 2].
When all photos covering the region of interest are corrected and georeferenced, it is very simple to merge them to produce a continuous, seamless mosaic [ Example 3]
This next example is a controlled mosaic of more than 200 photos covering the Walvis Bay Municipality and Lagoon (in Namibia) . Redish colours in the southern part of the image correspond to evaporation pounds of a salt refinery.
The last example is a mosaic of some 50 photos covering the "domaine du Sart-Tilman", campus of the University of Liège (Belgium).
In 2008, we produced what is so far our largest mosaic, covering some 4000 sqkm with more than 1000 images at 0.50m resolution, for a mineral prospection project in Southern Namibia.