Structure-from-Motion (SfM) photogrammetry can be used with digital underwater photographs to generate high-resolution bathymetry and orthomosaics with millimeter-to-centimeter scale resolution at relatively low cost. Although these products are useful for assessing species diversity and health, they have additional utility for quantifying benthic community structure, such as coral growth and fine-scale elevation change over time, if accurate length scales and georeferencing are included. This georeferencing is commonly provided with “ground control,” such as pre-installed seafloor benchmarks or identifiable “static” features, which can be difficult and time consuming to install, survey, and maintain. To address these challenges, we developed the SfM Quantitative Underwater Imaging Device with Five Cameras (SQUID-5), a towed surface vehicle with an onboard survey-grade Global Navigation Satellite System (GNSS) and five rigidly mounted downward-looking cameras with overlapping views of the seafloor. The cameras are tightly synchronized with both the GNSS and each other to collect quintet photo sets and record the precise location of every collection event. The system was field tested in July 2019 in the U.S. Florida Keys, in water depths ranging from 3 to 9 m over a variety of bottom types. Surveying accuracy was assessed using pre-installed stations with known coordinates, machined scale bars, and two independent surveys of a site to evaluate repeatability. Under a range of sea conditions, ambient lighting, and water clarity, we were able to map living and senile coral reef habitats and sand waves at mm-scale resolution. Data were processed using best practice SfM techniques without ground control and local measurement errors of horizontal and vertical scales were consistently sub-millimeter, equivalent to 0.013% RMSE relative to water depth. Survey-to-survey repeatability RMSE was on the order of 3 cm without georeferencing but could be improved to several millimeters with the incorporation of one or more non-surveyed marker points. We demonstrate that the SQUID-5 platform can map complex coral reef and other seafloor habitats and measure mm-to-cm scale changes in the morphology and location of seafloor features over time without pre-existing ground control.

中文翻译：

---

无需地面控制的水下数字图像的准确测深图

运动结构 (SfM) 摄影测量可与数字水下照片一起使用，以相对较低的成本生成高分辨率的水深测量和正射马赛克，分辨率为毫米到厘米。虽然这些产品可用于评估物种多样性和健康，但如果包括准确的长度尺度和地理参考，它们在量化底栖群落结构方面具有额外的用途，例如珊瑚生长和随时间的精细尺度海拔变化。这种地理配准通常提供“地面控制”，例如预安装的海底基准或可识别的“静态”特征，安装、调查和维护可能既困难又耗时。为了应对这些挑战，我们开发了带有五个摄像头的 SfM 定量水下成像设备 (SQUID-5)，一艘拖曳式水面车辆，配备船上勘测级全球导航卫星系统 (GNSS) 和五个刚性安装的向下看摄像机，可以看到重叠的海底视图。相机与 GNSS 和彼此紧密同步，以收集五张照片集并记录每个收集事件的精确位置。该系统于 2019 年 7 月在美国佛罗里达群岛进行了现场测试，水深从 3 米到 9 米不等，覆盖各种底部类型。使用具有已知坐标、机加工比例尺和两个独立站点调查来评估可重复性的预安装站点来评估测量精度。在一系列海况、环境照明和水清澈度下，我们能够以毫米级分辨率绘制活的和衰老的珊瑚礁栖息地和沙波。数据使用最佳实践 SfM 技术处理，没有地面控制，水平和垂直尺度的局部测量误差始终为亚毫米，相当于相对于水深的 0.013% RMSE。调查间可重复性 RMSE 在没有地理配准的情况下约为 3 厘米，但可以通过结合一个或多个非调查标记点提高到几毫米。我们证明了 SQUID-5 平台可以绘制复杂的珊瑚礁和其他海底栖息地，并在没有预先存在的地面控制的情况下测量海底特征形态和位置随时间的 mm 到 cm 尺度变化。调查间可重复性 RMSE 在没有地理配准的情况下约为 3 厘米，但可以通过结合一个或多个非调查标记点提高到几毫米。我们证明了 SQUID-5 平台可以绘制复杂的珊瑚礁和其他海底栖息地，并在没有预先存在的地面控制的情况下测量海底特征形态和位置随时间的 mm 到 cm 尺度变化。调查间可重复性 RMSE 在没有地理配准的情况下约为 3 厘米，但可以通过结合一个或多个非调查标记点提高到几毫米。我们证明了 SQUID-5 平台可以绘制复杂的珊瑚礁和其他海底栖息地，并在没有预先存在的地面控制的情况下测量海底特征形态和位置随时间的 mm 到 cm 尺度变化。