A Composite Method to Determine Sample Volumes and Anisotropic Peat Shrinkage during Evaporation Experiments Based on Photogrammetry.

Soils rich in clay and organic matter content often show distinct shrinkage and swelling properties over the drying and wetting cycle. The volume change affects the soil water release characteristic and therefore can be of importance in mass balances. We present a composite method to determine sample volumes and (anisotropic) shrinkage/swelling characteristics of soil samples. We tested the photogrammetric method on peat samples of different sizes during modified evaporation experiments (drying cycle). With approximately 10%, irrespective of sample size, shrinkage was conceived to be considerable. The method aims at the 3D reconstruction of objects out of the 2D geometric information of two or more images. The result is a set of XYZ coordinates of an arbitrary number of points of an object’s surface. Advantageous is that it is precise, reliable and a wide range of evaluation techniques exist to calculate volumes from a set of points (e.g. through fitting geometric shapes or generating convex hulls). Moreover, the method is functional over a scale range of several orders of magnitude (micro meters to tens of meters). For this reason, shrinkage/swelling characteristics of soils may be analysed in great spatial detail. Additionally, it is contactless which is of decisive advantage for soils with weak internal stability, as in samples with high organic matter content and many horticultural substrates. Further, it is low cost by using off-the-shelf photo cameras. It can be applied in the laboratory and due high portability and minimum equipment requirements in the field alike. Lastly, and a reason which sets it aside from other established methods, is that it can be used effectively in circumstances where samples are equipped with many measuring devices, for only points on the surface of an object are measured. While photogrammetry is well-established in many scientific disciplines, we advocate its use in soil science.