Abstract
Abstract. We have generated sub-millimetre-resolution DEMs of weathered rock surfaces using SfM photogrammetry techniques. We apply a close-range method based on structure-from-motion (SfM) photogrammetry in the field and use it to generate high-resolution topographic data for weathered boulders and bedrock. The method was pilot tested on extensively weathered Triassic Moenkopi sandstone outcrops near Meteor Crater in Arizona. Images were taken in the field using a consumer-grade DSLR camera and were processed in commercially available software to build dense point clouds. The point clouds were registered to a local 3-D coordinate system (x, y, z), which was developed using a specially designed triangle-coded control target and then exported as digital elevation models (DEMs). The accuracy of the DEMs was validated under controlled experimental conditions. A number of checkpoints were used to calculate errors. We also evaluated the effects of image and camera parameters on the accuracy of our DEMs. We report a horizontal error of 0.5 mm and vertical error of 0.3 mm in our experiments. Our approach provides a low-cost method for obtaining very high-resolution topographic data on weathered rock surfaces (area < 10 m2). The results from our case study confirm the efficacy of the method at this scale and show that the data acquisition equipment is sufficiently robust and portable. This is particularly important for field conditions in remote locations or steep terrain where portable and efficient methods are required.
Highlights
Rock breakdown describes a range of geomorphic processes that transform rock masses into soil or regolith and unconsolidated rock materials
Our experiment suggests that there is no statistically significant difference in the accuracy of digital elevation models (DEMs) generated from prime and zoom lenses, we find that the use of the prime lens will yield lower errors compared to a zoom lens for SfM photogrammetry
The results indicated that SfM photogrammetry offers several advantages over the traversing micro-erosion meter (T/MEM), allowing for the measurement of erosion at different scales on rock surfaces with low roughness while providing a means for identifying different processes and styles of erosion
Summary
Rock breakdown describes a range of geomorphic processes that transform rock masses into soil or regolith and unconsolidated rock materials. It plays a vital role in climate control via atmosphere–lithosphere interaction, biogeochemical cycling, and landform evolution on a planetary scale (Goudie and Viles, 2012). To better understand the weathering processes, high-resolution (sub-millimetre to millimetre) microtopographic data are necessary for in situ measurement of small-scale weathering features (Viles, 2001). Many small-scale (millimetre to centimetre) breakdown features are ambiguous, and it remains challenging to distinguish between similar-looking features (e.g. aeolian pits vs dissolution pits) and to establish a clear link between weathering feature form and the formative process.
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