When talking to data acquisition companies that are just now looking to transition from film to digital sensors, the topic of base-to-height ratio sometimes still comes up. This is often because some sensor manufacturers overemphasize b/h ratio to hide shortcomings of other factors in their camera design. While b/h ratio is not unimportant, it is not the only determining factor with respect to vertical height accuracy and has lost its significance in digital photogrammetry. There are other factors in the digital camera world that have an even greater impact:
- The overall geometric modeling of a digital camera is important. It contains the high-quality geometry calibration and the stability of the camera. B/h achieves nothing if the camera is not calibrated and geometrically stable.
- The PAN channel radiometric dynamic range is also an important consideration. The more that details are visibly clear and sharp in the PAN channel, the better and more accurate the matching results will be. If the PAN channel has a low dynamic range, it lacks detail and you cannot measure precisely what is not clearly and sharply visible in the image. All UltraCam systems feature adynamic range of 7,600 grey values!
- A fast frame interval almost completely eliminates disadvantages of a lower b/h ratio. In today’s digital era, you can go to a huge forward overlap (80%, 90%) without additional costs (film, development, scanning, handling), allowing multi-ray matching and the use of more than two images for measurement. This drives vertical accuracy and adds robustness.
- A PAN channel for the matching. Some cameras achieve matching using an RGB Bayer pattern sensor only. Bayer pattern simply means that the interpolation between individual red, green and blue pixels is already done on the CCD and leads to lower vertical (and horizontal) accuracy in photogrammetric measurements. All UltraCam systems feature a panchromatic channel!
- Have completely parallax-free images. Parallax introduces geometric error into each image which depends on flight height and the variation of the height in each image. To get parallax-free images you need an accurate camera model which allows fitting the camera geometry into the co-planarity condition. Modern digital photogrammetry solves this through software. This approach is well-known as software-leveraged hardware.
When evaluating digital sensors, be sure to evaluate the camera with these considerations in mind. At the end of the day you want a system that surpasses, not matches, the film sensor you are replacing.
– Jerry Skaw, Microsoft UltraCam Team