Friday, 18 January 2013

Foot casts and photogrammetry in the Isle of Wight

I managed to get out into the field for a couple of days in early December with the intention of recording (using photogrammetry) some of the footcasts on the beach at Hanover Point on the Isle of Wight. I'd spent some time at the National Oceanography Centre in Southampton where I'd given a scientific illustration and photogrammetry workshop and discussed current and future projects with a colleague at the University of Southampton.

It is possible the stretch of beach of the Isle of Wight at that runs from where the chalk cliffs meet the Vectis Formation at Compton Bay, along through the Wessex Formation eroding out at Brook and Brighstone Bays until the end of the Vectis at Cowlease Chine is my favourite place in the world. I can't afford to get there as much as I'd like, but it's never too far from my thoughts and I've spent many happy hours over the last two decades looking for dinosaur bones and other fossils with my wife and dog, and now it's the site of my research and has become even more important.

3D mesh of an ornithopod foot cast at Hanover Point, IOW.

Once on the beach I met my UoS colleague and a local collector from the island and we headed off to look over the site. The cliff at Hanover Point is famous for its foot casts, which originate from the crevasse splay sandstone that is interbedded with the green, red and grey marls that make up much of the Wessex Formation in this section of coastline. The sandstone outcrops on the beach north of Hanover Point and then rises through the cliff as you proceed south-east; at this point we're close the axis of the Brighstone anticline and a mile down the coast the beds dip in the other direction and disappear under the beach.

3D mesh of a large ornithopod footprint, Brook Bay, IOW. This cast is especially
deep and shows distinct edges crated by the ungual phalanges as they sank into the substrate.
Note it goes vertically down (we are looking at the base of the print).

There are many loose blocks on the beach and a fair few of these show some evidence of dinoturbation; the number runs into several tens over the length of the exposure and the whole underside of the bed probably represents a bioturbated surface. Traditionally these prints have been interpreted as being made by ornithopods but theropod, and possibly thyrephoran and sauropod prints can be found here too and some casts contain more than one print. All of the prints are individual and drop out of the lower surface of the sandstone bed as the softer sediment it sits on erodes from underneath it. The area is managed by the National Trust and it is against the law to remove any casts without permission; the site is widely visited by students and other interested parties specifically to view the prints, and this makes them a valuable resource for education and outreach.

I collected data on several casts and we inspected the full exposure before heading off to another site. A couple of the prints are shown here as 3D meshes processed in Photoscan and rendered in Cinema R13. Also here are the results of an experiment I tried whilst on site, where I attempted to record the crevasse splay sandstone in-situ in the cliff face. This was my first attempt at recording an outcrop using photogrammetry and the results speak for themselves, affirming it is possible to get good data on exposures of this size using this method over more expensive technologies such as Lidar. The above animation was created from data processed in Photoscan and rendered using Cinema R13.

You got to love photogrammetry!

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