Tidal Streams and Photogrammetry

A New Target

Last week The Shipwreck Project surveyed and confirmed another sonar target. Before diving and upholding a long-held Project ritual, the lump was christened “The Block Of Cheese” on account of what it looked like on the sonar. Here’s a screen grab of what we were looking at:

Sonar target known as The Block of Cheese

The Scan

The target was quickly confirmed as a pontoon of some sort, and was scanned. After a few hours of processing using Agisoft Photoscan Pro the model, orthophoto and digital elevation model were complete. Here’s the model:

And the orthophoto:

Orthophoto of the pontoon.

Which turned out very nicely. Check out the lobster who calls the pontoon home:

The lobster has stayed still long enough to appear in multiple frames, thus being preserved in the orthophoto.

And finally, the digital elevation model:

The digital elevation model of the pontoon.

All this for a rusty pontoon?

Everything has a tale to tell. One day, whilst trawling the National Archives we may well turn up an old photograph of a pontoon that matches ours. With the level of detail recorded, we have more chance of confirming a match. But the data teaches us more than just history. Adding GPS data helps us understand more.

A Sonar Match

When comparing the sonar image with the orthophoto it became apparent the sonar data was a very close fit:

A great match between sonar results and reality. Tidal Stream The digital elevation model yields even more, revealing a pattern of how the tidal stream flows over the pontoon. In this image top is due north:

DEM in hillshading view. Note how the scour appears on the north east corner, and deposits are accumulating to the south west.

The darker blue colours to the north east reveal a scour in the seabed, and the lighter colours to the south and west show a buildup of sand. It appears the tidal stream is stronger and more consistent in one direction, in this case when flowing from the north-east.

But its a rusty pontoon – why bother?

When we start to apply this to the bigger wrecks the process is known and understood, so we see things like the pontoon as an exercise in learning technique and efficiency. And because we can because we love the knowledge and history this kind of object reveals, even if its a rusty pontoon.

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