Ortho-rectified, oblique, aerial photography for verifying and updating spatial data

This paper explains how ortho-rectification of frame camera, aerial photography is fairly complex, and the difficulties associated with rectifying oblique aerial photos (OAPs) from small format photography have been so problematic that the approach has been little used and seldom described within the literature. Recent advances in photogrammetric software, however, have now made conventional, frame camera photogrammetry more convenient and more easily able to deal with the ortho-rectification of space-borne and air-borne sensors as well as hand-held film and digital cameras. Consequently, it is now possible to use oblique photography to correct vertical photographs - provided that we address issues related to image distortion and ensure that the reliability of extracted features is sufficient for their use within Geographic Information Systems (GIS). This will be demonstrated by showing that oblique photographs taken with a hand-held, 35 mm camera during a reconnaissance flight over a weed infested area can be ortho-rectified using modern software, thereby clarifying this approach’s status as a useful aid for delineating infestations for the purpose of say, ground truthing, verification of remotely sensed image analysis or simply contributing towards more comprehensive aerial mapping. The example used here may seem trivial but it actually illustrates, clearly, how a spraying pattern, while possibly too refined to show in a satellite image, can be mapped as a reference for monitoring weed infestations. This example is not intended to be a rigorous application of the technique, but simply a demonstration of its possibilities whenever there is a need to quickly or frequently update existing spatial data and environmental records in a much more cost-effective way than would be possible through deployment of the much more expensive, conventional, aerial survey methods.