RADIOMETRIC CALIBRATION OF DIGITAL PHOTOGRAMMETRIC CAMERA IMAGE DATA

Since a few years, digital frame cameras are being used for the recurrent conventional photogrammetric mapping tasks within Flanders (Belgium). As a result, area-wide, but mixed-sensor-type (Ultracam, DMC, …) time series of digital airborne imagery are emerging. Since only recently, a trend can be observed that this imagery is increasingly used for environmental mapping tasks, such as agricultural acreage estimation, dry matter productivity estimation and mapping of vegetation stress. The algorithms generating such products often need quantitative measurements (i.e. at-sensor radiance or at-surface reflectance instead of a dimensionless digital number). Also, in applications where data from different sensor systems has to be fused, it often becomes inevitable to transform the imagery from digital number to radiance or reflectance. However, in operational photogrammetric environments, only little effort has been put into utilizing the attractive radiometric properties of digital photogrammetric cameras for quantitative remote sensing. In this paper, a typical operational situation is presented: the radiometric calibration of an older image set (Vexcel UltracamD images of 2004) by means of multiple recent in-situ hyperspectral measurements (17 spectra measured in the same period of the year of image acquisition and with the same weather conditions, but in 2008). Several possibilities are evaluated using the empirical line method combined with absolute radiometric preprocessing (i.e. atmospheric BRDF, target BRDF, haze correction), as well as relative radiometric preprocessing (i.e. histogram matching). The different pre-processing methods are not only tested on a stand-alone basis, but the effect of various combinations is also investigated.