One way to ‘collect’ a massive specimen: simple photogrammetry in the field using a mobile phone

Inspired by the recent excellent series of articles by Trevor Watts discussing the types of Mid-Jurassic dinosaur footprints to be found along the Whitby coast (Deposits, Issues 46, 47, 48 and 49), when recently working in the area I (NL) made sure that I would have the time to walk the beaches from Saltwick Bay to Whitby. I also timed my work to make sure I could make use of the low tides early in the morning at first light. As well as the usual ammonites, belemnites and plant fossils, I found a handful of single footprint casts (most too heavy to attempt to move) and some very nice fallen slabs of claw marks and partial trackways – also mostly too big to move. One slab in particular stood out among the others at the bottom of the Ironstone Ramp in Long Bight (Figs. 1 and 2) – a ‘double trackway’ from what look like two quite different beasts walking in parallel – although they were possibly formed at different times. In the form of raised footprint casts rather than actual indented footprints, the specimen included five good prints in the left track and four, possibly five prints, on the right track – so each track contained a ‘full set’. Although the tracks look superficially quite different from one another, both appear to be attributable to theropod dinosaurs.

The slab was not buried in the sand, but sat on some other fallen blocks so could be seen in its entirety. However, at about 1.2m x 1.0m x 0.2m, it must have weighed in excess of 500kg. To ‘collect’ it, one option was to break it up and take it away in pieces in several visits and put it back together again at a later date, but that would have been less than ideal to say the least and small fragments might have been lost. A preferable option would have been to carry the complete slab all the way back to Whitby in one piece, requiring a group of people walking along the beach at low tide. The first five metres or so would have been a difficult scramble over all the other fallen blocks while carrying the heavy weight, but it would have been possible to do this relatively safely by using beams of wood to lever the slab carefully onto a strong pallet, then sliding the beams inside the pallet and carrying it away ‘stretcher’ fashion – if enough people and time had been available. However, ‘Storm Doris’ was about to hit and the chances were that the specimen would be broken into pieces by the next day. Even if it survived the storm, it was possible that, although most people finding it would admire the specimen, maybe take photos and then leave it where it was, someone else might take a hammer and chisel to it and try to remove the nicer of the raised footprints on the left side of the block.

Fig 1
Fig. 1. The slab from the Saltwick Formation containing the two trackways found at the base of the Ironstone Ramp in Long Bight, east of Whitby. (N Larkin for scale.)

The best option – which did not preclude returning at a later date with a gang of people to collect the specimen for the local museum if it survived the storm – was to take as many photos as possible there and then to record the specimen. This would involve not just taking the pictures with something included for scale (as well as the obligatory ‘selfie’) but taking photos from as many angles as possible from approximately the same distance, including around the back and underneath as far as possible, specifically so that a photogrammetric 3D digital model could be made of the specimen at a later date. The problem was that the only camera I had on me was a rather old ( about 2013?) and cheapish mobile phone with limited memory. So I took 65 photos with this. I returned the next day to find that the specimen had thankfully survived Storm Doris and I took 81 more photos, this time with a proper digital camera (a Canon Powershot SX50 HS).

Fig 2
Fig. 2. One of the prints from the track on the left.

Not only did this mean that the double trackway was well recorded before it got damaged, but it also meant that two digital photogrammetric models could be built, one from each set of images. It would also be interesting to compare the two models to see if the old mobile phone camera would provide something useful or whether this sort of photogrammetric work can only be achieved with a ‘proper’ digital camera. To ensure the best chance of good digital models being made from these images, I sent the photos to Steven Dey, a photogrammetry, laser-scanning and 3D-printing expert at ThinkSee3D based in Oxfordshire, with whom I have worked on many palaeontological projects before. Below, he describes the processes of making the 3D digital models, as well as giving top tips for taking good photogrammetry photos in the field.

Photogrammetry (also known as SfM – Structure from Motion) generates digital three-dimensional models from multiple photographs taken from different positions and angles around an object. An advantage of photogrammetry compared with other methods, such as laser scanning, is that the ‘scanning’ part of the process is very cost effective as all that is needed is a digital camera to capture the data and even a mobile phone camera will do. This means it is an ideal method for use in the field, when out looking at large specimens and geological features. It is even possible to take accurate measurements of the resulting digital 3D specimen and to measure features, particularly useful if the specimen itself was difficult to access. To ensure this, place a scale bar or an object of known size (a geological hammer, hand lens or trowel for example) on the specimen in a few photographs to act as a reference and this will allow the digital 3D model to be made with a relevant scale. You can also put a colour scale in the scene, so the colours of the phototexture can be colour balanced if you want accurate colour reproduction.

Informal comparative trials have shown that the accuracy of virtual 3D models of fossils produced with photogrammetry can be equal to or even better than other scanning methods, but this accuracy is dependent on four main influences: (1) the quality of the photography; (2) the quality of the light in the environment; (3) the surface quality of the specimen; and (4) the processing of the images into a 3D model.

  1. The quality of the photography

Most considerations are common to all photography, such as reducing camera shake (a tripod can help), good focus and an appropriate exposure to minimise over-exposed or under-exposed areas. However, some considerations are particularly relevant to the photogrammetry process, such as ‘depth of field’. Oblique angled photographs across a specimen, especially a long object, can be partly out of focus due to a limited depth of field if care is not exercised, so avoid overly oblique shots. ‘Shutter priority’ can be a useful setting on a digital camera for photogrammetry, as this allows the ‘f’ number to be tuned to maximise the depth of field.

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