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EXTRACTION OF DIGITAL ELEVATION MODELS USING PHOTOCLINOMETRIC METHODS Jacquelyn A. Cordova and José M. Hurtado, Jr. Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 799868, email@example.com Aside from retrieving rock samples and radiometrically dating them, the most widely used method for dating the surface of planetary surfaces is through crater counting. This method is based on the simple observation that older surfaces are more heavily cratered than younger surfaces. It assumes a relationship between the rate of impacts over time and the size and distribution of the resulting impact craters. Because this method relies on the population statistics of a large number of craters, it can only be applied to areas large enough (and sufficiently old enough) for a statistically adequate number of craters to have accumulated. Given image data of some minimum spatial resolution, this therefore limits the size of geologic features that can be dated by crater counting. In addition, crater counting assumes that the impact flux is known, while, in fact, it is not tightly constrained. Furthermore, it is desirable to be able to date individual landscape features, such as single impact craters. To address all of these issues, a possible alternative to crater counting is morphologic dating, which has been used on Earth to date fault scarps and cinder cones. This method focuses on the changes that occur to the cross-sectional shape of a crater as it erosionally degrades over time. To do this, detailed, high-resolution topographic data from impact craters are necessary. We use a method of extracting this topographic data from images called photoclinometry. Photoclinometry is a process in which a two-dimensional, high-resolution image is converted into a Digital Elevation Model (DEM) by computationally modeling the geometric interaction between illumination and an irregular surface that yields the image of that surface. We have successfully created a MATLAB implementation of an existing “shape-from-shading” algorithm that rapidly that extracts DEMs from imagery of the Moon. We are currently using the best data available, images from the Lunar Reconnaissance Orbiter Camera (LROC), which have a resolution of 0.5 meters per pixel. We are currently modeling craters from Balmer Basin, an impact basin on the nearside of the Moon, and from the Highlands Feldspathic Terrane. Each model, with respect to their location on the Moon will be shown using overlaid images, and KML files viewable in Google Moon. The purpose of this research is to compare the morphologies of small impact craters, excavated in target materials of different types (e.g. mare basalt vs. highlands). In addition, the crater-degradation for impacts of a wide range of sizes in both basaltic and feldspathic targets will be investigated. At the Fall 2010 AGU Meeting, another group presented a study, which uses profiles extracted from LROC NAC stereo images to study the relationships between crater morphologies, ages and target materials. Using LOLA topography with ~1m vertical resolutions, they calculated depth/diameter ratios for selected craters. We will compare their findings with our own to ascertain the accuracy and efficiency of each method.
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