Calibrating digital photogrammetric airborne imaging systems using a test field

Abstract

Airborne photogrammetry is a fundamental technique for producing reliable, geometrically accurate, high-resolution geospatial information. Passive and active digital imaging is rapidly replacing film imaging in photogrammetric data capture. Optimal use of this new technology enables fast data capture, high accuracy, a high level of automation, and a huge increase in applications. The reliability, accuracy, and efficiency of airborne photogrammetry are based on calibrated, high-quality sensors and rigorous processing. The calibration processes of the digital photogrammetric airborne imaging systems are under development. Central challenges in the development of the calibration are the extensive variation in digital systems, the need for radiometric calibration, and the necessity for accurate system calibration. Test field calibration is a potential approach for determining the system calibration. The hypothesis of this study was that system calibration in a test field is necessary for digital photogrammetric airborne imaging systems and that the calibration should involve geometry, spatial resolution, and radiometry. The hypothesis was proven by developing a methodology for the system calibration in a test field and by empirically investigating the need for and feasibility of the system calibration. In the empirical study, data sets from three first-generation commercial digital photogrammetric large-format sensors, Leica Geosystems ADS40, Intergraph DMC, and Microsoft UltraCamD, were used. Theoretical evaluations were performed together with the empirical evaluations. The Finnish Geodetic Institute's permanent test field in Sjökulla, augmented with some portable targets, is a prototype photogrammetric test field. The results proved that the construction of a test field for geometric, spatial resolution, and radiometric calibration was feasible. A permanent test field can be an efficient, highly automated, and reliable tool for system calibration. The empirical evaluation showed the great geometric and radiometric potential of the systems, but it also revealed problems. The laboratory calibration of the evaluated systems was either insufficient or invalid in airborne conditions. The test field calibration was necessary to provide the missing or invalid parameters and to assess the measurement capability of the systems. The results also showed that calibration of the systems in a test field was feasible. It appeared that in order to obtain the utmost accuracy, the calibration process of the evaluated systems should include the laboratory, test field, and self-calibration. A product level validation of geometric accuracy also appeared to be necessary to ensure the highest reliability. This was the first study to model and demonstrate the simultaneous geometric, spatial resolution, and radiometric test field calibration for digital photogrammetric airborne imaging systems. The study had several results: it proved the need for and feasibility of test field calibration and presented new empirical results concerning system performance and test field calibration; it provided recommendations for the calibration process of the systems and for the construction of photogrammetric test fields; and it identified many additional research topics.