Now You See Me

A significant amount of our cultural heritage is preserved out of sight and hidden. Divers can explore its depths but the sea guards its secrets and does not give them up lightly.

But with the right skills and kit we can not only explore what is lying on the seabed but preserve what we find using photogrammetry and this is something AccuPixel director Simon Brown has worked with for many years.

We are now working on a project to take a closer look at a section of the Dorset Coast known as The Chesil. Stretching for 18 miles from West Bay in the West to Portland and up to 15m high the Chesil is a shingle bank ship’s graveyard. It also has a personal connection…with friend and fellow wreck enthusiast Grahame Knott living just yards from the beach in a house that survived a devastating storm in 1824.

Chesil Beach during a winter storm – the kind of event that would drive sailing ships ashore

Over the centuries hundreds of ships have run aground in stormy seas, particularly in the age of sail, and it’s these shipwrecks we are interested in.

One wreck is known to be frequently dived. The Royal Adelaide ran aground in a storm in 1872 with a loss of 6 lives and today the remains lie close to the beach. But we think the story begins further offshore with anchors and other smaller and isolated remains.

UWIS on the deck of the vessel Sea Searcher heading to the Chesil

For this trip we decided to look at two sites about 80m apart. One was a cluster of three anchors and mast section, and one was a very large and isolated anchor lying to the west.

80m might not sound too far…but with no reference and visibility limited to 3m (on the day) there was no chance of locating the target, so we planned to deploy and use UWIS tracker with the Valtamer Alltab tablet preloaded with the GPS positions of the targets to assist navigation.

The first target was quickly located and surveyed shooting 800 images, with the tablet recording key GPS points for orientation.

Processing in 3DF Zephyr delivered 100% image alignment and a very clean and accurate model:

Closer analysis and interpretation was delivered by the orthomosaic and digital elevation model:

With the first target scanned UWIS guided us to the second target:

Shot in 2012 the image records a very large anchor with a diver for scale

What we had not factored in is just how active and dynamic the Chesil really is.

Normally the seabed is coarse gravel but as we closed in on the target this was replaced by sand…and 10 years later the anchor is now buried under a sand bank!

The UWIS diver track and first target were analysed using our favourite GIS tool Global Mapper with a vector overlay created from the elevation model. The largest anchor is just over 4m long:

The UWIS tablet was a vital tool here and saved considerable time using two separate dives. Underwater visibility was a disappointing 3m but we swam directly to the mark and the orientation of the mast tells us how the site aligns to the beach.

Investigations are ongoing and the reasons these admiralty pattern anchors are clustered together remains unknown at this point.

But we do know the Chesil is a very dynamic place and the next step is to run the sidescan sonar along the beach, and then compare the results with previous data to see what else has been covered up…or uncovered.

chesil beach sunset
Grahame enjoying a cider on the Chesil at the end of a productive day

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