3D models created from ROV videos and their corresponding subtransect dimensions

Underwater video transects have become a common tool for quantitative analysis of the seafloor. However a major difficulty remains in the accurate determination of the area surveyed as underwater navigation can be unreliable and image scaling does not always compensate for distortions due to perspective and topography. Depending on the camera set-up and available instruments, different methods of surface measurement are applied, which make it difficult to compare data obtained by different vehicles. 3-D modelling of the seafloor based on 2-D video data and a reference scale can be used to compute subtransect dimensions. Focussing on the length of the subtransect, the data obtained from 3-D models created with the software PhotoModeler Scanner are compared with those determined from underwater acoustic positioning (ultra short baseline, USBL) and bottom tracking (Doppler velocity log, DVL). 3-D model building and scaling was successfully conducted on all three tested set-ups and the distortion of the reference scales due to substrate roughness was identified as the main source of imprecision. Acoustic positioning was generally inaccurate and bottom tracking unreliable on rough terrain. Subtransect lengths assessed with PhotoModeler were on average 20% longer than those derived from acoustic positioning due to the higher spatial resolution and the inclusion of slope. On a high relief wall bottom tracking and 3-D modelling yielded similar results. At present, 3-D modelling is the most powerful, albeit the most time-consuming, method for accurate determination of video subtransect dimensions.水下视频样带已成为海底定量分析的常用手段。然而，准确确定调查区域仍存在一大难题：水下导航可靠性欠佳，且图像缩放无法始终补偿因透视与地形起伏带来的畸变。根据相机配置与可用仪器的不同，会采用不同的海底表面测量方法，这导致不同搭载平台获取的数据难以进行比对。基于二维视频数据与参考比例尺的海底三维建模，可用于计算子样带的尺寸参数。本研究聚焦子样带长度，将通过PhotoModeler Scanner软件构建的三维模型所得数据，与水下声学定位（超短基线，USBL）及底跟踪（多普勒测速仪，DVL）方法获取的数据进行对比。本次测试的三种配置下均成功完成了三维建模与比例尺校准，且研究发现，因基底粗糙度导致的参考比例尺畸变，是测量不精准的主要来源。在崎岖地形环境下，声学定位普遍精度不足，底跟踪方法的可靠性也较差。得益于更高的空间分辨率以及对坡度的考量，通过PhotoModeler评估得到的子样带长度，平均比声学定位方法得到的结果高出20%。在高起伏岩壁环境中，底跟踪与三维建模方法得到的结果较为相近。当前，三维建模虽最为耗时，却是精准测定视频子样带尺寸的最优方法。