Collection efficiency in oblique mapping with the UltraCam Osprey 4.1

20 January 2022

UltraCam Osprey 4.1

NADIR AND OBLIQUE IMAGE CAPTURE

3D City modelling, Smart Cities, urban planning, Digital Twins of our environment – all these data intensive terms have a common starting point. Oblique imagery. Vexcel Imaging has a long and rich history in providing well-designed photogrammetry-grade oblique camera systems. Based on the successful 3^rd generation UltraCam Osprey M3 Premium, we announced the first 4^th generation camera, the UltraCam Osprey 4.1, in 2020 that benefits from all lessons learned to date and that features a significant amount of new technology. Read more about the camera here. The camera concept, with its well-balanced nadir and oblique focal lengths and its backside-illuminated CMOS sensor technology, generates a stunning level of efficiency, while establishing a new benchmark for aerial image quality.

UltraCam Osprey 4.1

NADIR AND OBLIQUE IMAGE CAPTURE

Key specifications for oblique aerial collection efficiency

The camera design is built around a 288 MP panchromatic photogrammetry-grade nadir cone and four oblique cones that deliver 150 MP images. Custom designed lenses allow this well-tuned CMOS-based camera concept to create superb quality, both radiometrically and geometrically.

The camera geometry is the foundation for the efficiency of an oblique camera system. Factors such as pixel size, length-to-width ratio of the imagery, focal length of the oblique and nadir cones, as well as the installation angle of the oblique cones, frame rate, flight speeds and motion compensation are all key parameters and need to be optimized against each other to create a highly efficient camera system. Having that well-defined focal length ratio between the nadir and oblique cones is of critical importance to ensure a comparable ground sampling distance (GSD) in both nadir and oblique imagery.

When collecting imagery for an area of interest (AOI), the goal is to achieve full oblique coverage for no gaps in the oblique imagery. From a flight planning and efficiency perspective, it is best to use a sensor with a very large footprint, flying at high speeds and to have minimum side overlap to be super-efficient. On the other hand, a certain percentage of overlap is needed to deliver high-quality photogrammetric products. Therefore, a good compromise, in terms of overlaps versus efficiency, sets the stage for the actual flight planning.

The footprints in the image above are the output of a UCO 4.1 flight plan with a 60 % side overlap and 5 cm GSD. The units are in meters so the extent of the footprints can be understood.

The necessity to achieve full oblique coverage in the AOI makes it more than obvious that the side overlap of the forward and backward oblique cones (blue boxes) determine the flight collection efficiency of an oblique camera system. In other words, a very wide nadir footprint is not helpful in terms of efficiency. With a 60 percent side overlap configuration, the UltraCam Osprey 4.1 creates a comfortable forward and backward oblique footprint overlap of 25 percent.

Furthermore, the flightlines need to be extended to allow the forward and backward oblique cones to fully cover the AOI. Similarly, for the left and right looking oblique cones, additional flightlines must be added. Both points are clearly visible in the image below, where the blue shaped area is the AOI for Florence, Italy. Flightlines are shown in red.