【文章內(nèi)容簡介】 which exist in most closerange datasets (strongly oblique viewing angles, unknown imaging geometry and sensor positioning) usually prevent georeferencing and currently constrain quantitative analysis.The application of digital photogrammetry techniques within volcanology has been generally confined to producing digital elevation models (DEMs) from conventional aerial imagery (Baldi et al. 2000。 Kerle 2002) or, more recently, from satellite data (Stevens et al. 2004). On a smaller scale, closerange and oblique photogrammetry has been successfully used for geomorphological research (. Chandler and Brunsden 1995。 Lane et al. 2001) and a putervision based approach has been recently developed for determining volcano topography from sequences of oblique images (Cecchi et al. 2003).Here, we show how closerange digital photogrammetric techniques can be used to bine image data acquired from arbitrarily positioned thermal and visible cameras in order to overe some of the data processing problems inherent in using oblique viewpoints and paratively low resolution thermal sensors. Automatic photogrammetric and image matching techniques (Papadaki 2002) are employed to obtain topographic data, and to rectify and georeference visible and thermal images. Determination of the observed surface and camera orientations allows the relevant atmospheric attenuation factor to be calculated for each pixel of the thermal images. We illustrate the application of these techniques using groundbased images of active lava channels and flow fronts on Etna volcano.Data acquisitionMount Etna, in Sicily, Italy, has a recent history of both effusive and moderately explosive activity. After about a year of relative quiescence, on 7 September 2004, a small lava vent opened at ~2,800 m ., at the base of the SE crater. Within a week, two lower vents at the head of the Valle del Bove (at 2,650 and 2,250 m, Fig. 1) had also formed, from which lava flowed east into the valley. Fed at relatively modest effusion rates (~3 m3 s?1。 Burton et al. 2005), the channelfed flows were cooling limited (Guest et al. 1987) and (during the period of fieldwork) were km long, with active flow fronts at the break in slope on the valley floor, south of Monte Centenari (Fig. 2a).Whilst a considerable portion of the flow field developed on steep topography exposed to rock fall hazard, the flow fronts were relatively accessible. Hence, ground based images could be acquired of the flow front and distal channel regions and the data used in this paper were collected on 27 September 2004.Visible images were acquired with a Canon EOS 300D (30722048 effective pixels) fitted with a fixedfocus 28 mm lens, the imaging geometry of which had been precalibrated in a laboratory (Robson et al. 1999). Thermal images were obtained with a FLIR ThermaCAM S40 which provides a horizontal field of view of 24176。 (similar to that of a 50mm lens on a digital SLR with an APS dimensioned sensor). The sensor is a 320240 uncooled microbolometer focal plane array, with a spectral range of –13 μm. The imaging geometry of the thermal camera is nontrivial to calibrate, since targets with a thermal signature are required. Hence, although preliminary calibration has been carried out to refine the imaging geometry model (corrections are dominated by the first order radial distortion term), variations due to required on site focusing adjustments have not yet been fully accounted for. Geo graphical location of Etna volcano and the Valle del Bove. The final extent of the 2004–2005 lavas is shown in black on the main map and their extent on 27 September 2004 is shown in the inset sketch. The asterisks surrounding the flow fronts demonstrate the positions of some of the photogrammetric control targets used.The photogrammetric technique used on Etna requires a minimum of four known threedimensional target positions to be observable within each image in order to reliably estimate the camera orientation. Artificial control targets constructed from metal foil were deployed in the field。 some large (~50 cm) flat targets were augmented by smaller (~5 cm) ‘ball’ targets to improve precision at short viewing distances. Target positions were coordinated using GPS (ProMark X) augmented, for relatively close targets, with targettotarget slope distances made using a tape measure. These measurements were included as observations in the photogrammetric network adjustment along with the GPS data.Fig. 2 Visible (a) and thermal (b) images of the active flow front and channels on 27 September 2004, looking approximately north. For scale, the active flow front is ~35 m wide. The visible image shows an inactive flow front and partially drained channel (which had been active during the previous day) to the right of the active front. The thermal image is overlain with a perspective view of the triangulated surface model derived from the visible images.Photogrammetry and resultsThe photogrammetric software employed (VMS, Robson and Shortis, ) provides a general case geometric solution, suitable for application to networks of convergent oblique imagery. Measurements of each identifiable control target image are used along with the threedimensional target coordinate data, to provide camera orientation starting values for each image. These initial values are th