I've spent years using photogrammetry to document underwater caves and wrecks. One recurring surprise is that a model can show the position of objects relative to one another with great precision, yet still have no reliable geographical position or orientation. In Mallorca we transferred a surveyed surface point down a six-metre cave shaft using a climbing rope and string. At Lipari, 60 metres of water and current meant a GPS float did not reliably identify the point beneath it. On the Mars wreck we could match model features to a surveyed multibeam sonar map. In Brazil I made the mistake of placing three reference markers almost in a straight line, and we also discovered the total station had not been tied to a known geographical point. The lesson for me was to decide what accuracy the project actually needs, then design the whole reference chain before diving. An internally accurate model is not automatically a georeferenced one. Has anyone here tackled similar underwater survey or positioning problems? I wrote up the examples in more detail here: https://johnkendallgue.substack.com/p/how-do-you-know-where-you-are-underwater-2c6   submitted by   /u/JohnKendallDiving   to   r/techdiving [link]   [comments]