AUTOMATIC ORIENTATION AND MOSAICKING OF ARCHIVED AERIAL PHOTOGRAPHY USING STRUCTURE FROM MOTION

Abstract. Aerial photography has been acquired regularly for topographic mapping since the decade of 1930. In Portugal there are several archives of aerial photos in national mapping institutes, as well as in local authorities, containing a total of nearly one hundred thousand photographs, mainly from the 1940s, 1950s and some from 1930s. These data sets provide important information about the evolution of the territory, for environment and agricultural studies, land planning, and many other examples. There is an interest in making these aerial coverages available in the form of orthorectified mosaics for integration in a GIS.

The orthorectification of old photographs may pose several difficulties. Required data about the camera and lens system used, such as the focal distance, fiducial marks coordinates or distortion parameters may not be available, making it difficult to process these data in conventional photogrammetric software.

This paper describes an essentially automatic methodology for orientation, orthorectification and mosaic composition of blocks of old aerial photographs, using Agisoft Photoscan structure from motion software. The operation sequence is similar to the processing of UAV imagery. The method was applied to photographs from 1947 and 1958, provided by the Portuguese Army Geographic Institute. The orientation was done with GCPs collected from recent orthophototos and topographic maps. This may be a difficult task, especially in urban areas that went through many changes. Residuals were in general below 1 meter. The agreement of the orthomosaics with recent orthophotos and GIS vector data was in general very good. The process is relatively fast and automatic, and can be considered in the processing of full coverages of old aerial photographs.