HIERARCHICAL AERIAL TRIANGULATION OF OBLIQUE IMAGE DATA

Abstract. Oblique imaging sensor systems are becoming a standard for photogrammetric applications such as city modeling, with increasing need for frequently updated geospatial data in urban areas. The ground processing software has to consider and support this develop-ment, providing a highly performant workflow for projects that can consist of tens or hundreds of thousands of images in different view angles: nadir (vertical), left, right, backward, forward. This is a challenge also for the oblique aerial triangulation (AT), both the large number of images to process and the high amount of overlap in-between them, where more than 50 image measurements for a given ground point are common.

At the same time, the large overlap even in a single view generally provides stable sub-blocks for each viewing direction and, therefore, allows for separating the oblique AT by view and performing a hierarchical approach: The nadir block is triangulated first, updating the exterior orientation parameters of all nadir images and providing adjusted ground points. This nadir reference can be used to tie the oblique views’ sub-blocks in subsequent AT runs to eventually provide adjusted exterior orientations for all images, with comparable accuracy to an integrated AT – but providing a significant performance gain. The hierarchical AT is a three-step approach: 1. adjust-ment of the nadir imagery, 2. adjustment of each oblique views independently by cardinal direction (East, West, North, South) while tying it to the nadir reference, and 3. computation of combined statistics and report.

The paper details our AT approach and evaluates its parameterization for different oblique data sets, comparing results and performance to a fully integrated AT.