ACQUISITION CRITERIA FOR UAV (DRONE) STRUCTURE-FROM-MOTION (SFM) AND PHOTOGRAMMETRY OF BATHYMETRY IN VERY SHALLOW WATER

Techniques for photogrammetric orthophotomosaic and 3D structure-from-motion (SfM) model generation using UAV (“drone”) platforms are now frequently used in industry (e.g., agriculture, mining, construction). These techniques are becoming equally prevalent in certain fields of the Earth sciences as their utility gains wider appreciation and acceptance. One specific application of UAV SfM is the analysis of bathymetric features in very shallow water (less than one meter). UAV technology is particularly well-suited to this task because (i) the spatial dimensions of field areas of interest are often relatively small (less than several square kilometers); (ii) features of interest within a field area are typically decimeter scale and require high-resolution imagery; (iii) site reconnaissance using water craft (inflatables, kayak, etc.) can cause disturbance or damage due to turbulence and bottom contact; and (iv) shallow-water sites are often temporally variable, and time-lapse UAV photogrammetric surveys are now possible with a high degree of spatial repeatability. In addition to UAV-photogrammetry specific acquisition parameters—front lap and side lap percentages, flight altitude, flight ground speed—other issues are specifically critical in shallow-water bathymetry surveying. Sun angle, camera angle, wind speed, acquisition geometry, terrain surrounding the water body, and certain SfM processing parameters are all important for a successful survey. Results from controlled experiments will highlight the relative importance and sensitivity of these extra acquisition criteria. Additionally, a case study of modern microbial-mat formation in abandoned salterns will be presented.