Photogrammetry is a technique today widely applied to generate virtual images of three-dimensional objects that can be viewed from different angles, at different magnifications, calibrated for accurate documentation and measurement of three-dimensional objects. The technique applies the concept of Structure from Motion (SfM), where software combines overlapping photographs taken at different angles under constant lightning conditions. The photographer moves around, taking different images of an object, which the software then analyzes to create a virtual map, first a "sparse cloud" of overlapping image points, ultimately a "dense cloud", i.e., an image in virtual space, a detailed representation of the original, real-world object. The technique is now widely used for documenting architectural artifacts, museum objects, even for creating 3D virtual worlds of modern computer games. Monitoring dynamic change in lichen communities over time has in the past been limited by the challenges to accurately image these microscopically small organism. At a microscopic scale, even relatively flat, tightly appressed crustose and foliose lichens are still three-dimensional objects. Focus stacking is increasingly used to overcome depth-of-field and diffraction limitations, accurately representing even minute three-dimensional structures. Nevertheless, documenting entire lichen communities in sufficient detail for accurately measuring structural change remains a challenge. Photogrammetry has the potential to overcome these deficiencies. Particularly crustose lichens have the reputation of growing extremely slow. Documenting population dynamics therefore requires accurate measurements at sub-millimeter scale. This is particularly relevant for assessing biodeterioration of monuments caused by saxicolous lichens. Combining photogrammetric monitoring with geological analytical techniques has the potential to document long-term how different lichen species affect their substrates differently, in different microhabitats, on different substrates, in different regions and climates. Here we present preliminary results of combining these techniques to document the impact on lichen biodeterioration on lithic monuments at cultural heritage site, both in Arizona (USA), and Peru.
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