Validation of geodetic seafloor benchmark stability using structure‐from‐motion and seafloor pressure data

One commonly used method to measure and detect centimeter-scale changes on land is structure-from-motion (SfM) photogrammetry, wherein sets of digital images are used to generate three-dimensional spatial models. Subcentimeter accuracy useful for geodetic studies is achievable when these surveys are conducted on land or from the air. This method has made its way into marine environments by way of scuba, remotely operated vehicle, and autonomous underwater vehicle-based surveys largely in the context of mapping. Repeated SfM photogrammetry surveys could be used to monitor changes in the stability of seafloor benchmarks and key sites as the interest in and need for marine geodetic measurements continues to grow. These studies require centimeter-level accuracy or better, and some rely on accurate placement, positioning, or relocation of geodetic monuments, benchmarks, and instruments. We assessed the accuracy of the photogrammetric method for this purpose by simultaneously conducting a remotely operated vehicle-based SfM photo survey to produce three-dimensional spatial data, and a precise pressure survey to accurately measure height information. We found that the SfM survey agreed with the pressure survey height within its uncertainty of +/- 6.0 cm while using a standard, off-the-shelf camera adapted for underwater use. This level of relative accuracy would allow us to detect changes in geodetic benchmarks or instrumented sites between repeated surveys where large changes or the accumulation of small changes is expected within the scene over several years. We believe that centimeter-level accuracy is obtainable with adjustments to the photogrammetry survey parameters, a higher-resolution camera, and inclusion of additional coded targets.