Choosing a field site: The hunt (Part three)

In another portion of the same field trip, some students were required to do a survey around the town of Hunstanton, about an hour’s drive from the earlier stop of Overstrand. Hunstanton is regionally known for its chalk cliffs, which my supervisor had previously suspected might be a good site candidate. So, after dropping off the students, we went down to investigate its potential as a study site.

The aerial image (once again courtesy of GetMapping) reveals a sharper cliff, far more distinct than what appeared to be a set of landslides at Overstrand. Looking within the shadows reveals numerous rockfalls and slips, which was thought could make for an interesting (and challenging!) environment for photogrammetric mapping. The tide in the image is almost at its highest, a search reveals that it shouldn’t be a big issue for conducting a survey as it stays relatively far out, but this will obviously be influenced by focal length and sensor size of the cameras we use!

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Aerial image of Hunstanton and its cliffs, courtesy of GetMapping

The Hunstanton cliffs are extremely well documented, and another set of cliffs with conservation status. One of the earliest records revealed is an 18th Century essay, ‘Towards a topographical history of the county of Norfolk’, with numerous mentions of the Hunstanton cliffs. The cliffs consist of three distinct strata denoted by their striking colour differences (colour contrast!). The bottommost layer is rusty brown Carstone, which contrasts with the striking Hunstanton Red Chalk Formation and white/grey topmost Ferriby Chalk Formation. It’s a popular area with fossil hunters, being a very productive area for ammonite fossils.

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Strata at Hunstanton, taken from the GCR site description

This definitely ticked the colour contrast box in a big way, and appeared to be very accessible – there’s a carpark located at the top of the cliff allowing an easy path to the strand. The reconnaissance mission revealed one very interesting 50 m or so stretch where a landslip had occurred, according to a passer-by, in the early nineties.

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An overview of the cliffs of Hunstanton, including the aforementioned landslip

Vegetation has taken hold in upper parts of the landslip to create a natural scene with an almost unnatural amount of colour contrast, things were shaping up nicely! One of the only downsides we foresaw was a large amount of traffic through the area, but this wasn’t a large enough of a deterrent to add it to the list.

This set the stage nicely – two sites within an hour of each other, with good options for accommodation and well accessible. Now all that was required was careful planning and enough time to ensure the whole thing was done correctly! A checklist and research design was made of the most important equipment needed to answer the questions posed. I accessed the UK LiDAR inventory in order to make some coarse resolution 3D models of the areas of interest, and used a nifty CloudCompare function to actually place virtual cameras in the scene. This came in useful for planning what focal length lens would be required for data acquisition!

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Overview of the initial plan for the photogrammetric survey at Hunstanton, showing camera positions

I actually spent some (probably too much!) time fiddling around with a Blender plugin which simulates a physical camera, so you can play with the focus distance/aperture, but nothing much came from it. Something on the back-burner.

Stay tuned for part four, where I detail the fieldwork plan.

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Choosing a field site: A photogrammetrist’s view (Part two)

The idea of a colourful coastal cliff seemed well fleshed out and rational, so I honed in on my options. Short of Devon, Seven Sisters near Brighton seemed like a natural choice. Having gone there on a fieldtrip which was included in one of my first blog posts, it is quite iconic with a good scale range available at different parts of the coast. The downsides include the tricky access, coarse grain beaches (instability!) and exposure to the elements – there is no hiding on the south coast!

A second fieldtrip in the department posed an interesting opportunity to investigate options in Norfolk. Norfolk, to paraphrase my supervisor (who was quoting someone else), is ‘incredibly flat’. Good for accessibility in that case!

On one of the days, the student’s on the field course were assigned a task, so I ducked away on a reconnaissance mission to Cromer, a coastal town inside the area of outstanding natural beauty in Norfolk. We headed out to the beach, a popular surf spot, to have a look at how appropriate it would be for imaging. The cliff itself wasn’t perfect (heavily vegetated and shallow sloped), the level of the tide would have rendered it almost impossible to conduct a study on the beach. There were coastal defences located in awkward positions which would have limited our freedom of where to image, and it was also quite busy. It wasn’t quite what we were looking for, but looking up and down the coastline, we could see the potential.

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Coastal defences typical of the Norfolk coast

Thus, we headed a ways down the coast to Overstrand, a smaller town with good access to other parts of the coastline. Here we were in luck. A quick peek at an aerial image (Courtesy of GetMapping) reveals some interesting landslide features, and more coastal defences.

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An overview of Overstrand town (Towards the top left) and the cliffs

Thankfully, when we arrived down at the beach, we realised the coastal defences were set back far enough that from a surveying perspective they would not be an issue.

A search established the conservation status of the cliffs, which would likely become convenient later on, as there was likely some literature on their history. Indeed, there are a smattering of papers on modelling the effects of climate change on the coast, geological surveys of the cliffs and effects of coastal protection on their erosion, to give a few examples. The short site description pretty much fits the bill as per part one of this blog series – ‘vegetated, soft cliffs with ruderal plant communities developing on newly exposed sands’. The colour contrast was quite good, with golden sands contrasting the muddier upper portions and partial vegetation. It included an interesting gully feature also, which would undoubtedly push the photogrammetric software to its limits.

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The gully bisecting the survey site

So, a good potential for a site! However, in an ideal world we would have two sites to study in order to demonstrate the variables we are testing in independent cases. Thus, we added access to a second site to our selection criteria and started planning to do both surveys on back to back days over a weekend. However, one site was still lacking, so it was back to looking at good potentials within a reasonable distance of Overstrand.

Choosing a field site: A starting point (Part one)

Initial thoughts

One of the most important decisions within a PhD involving fieldwork is the correct selection of a study site. Indeed, there are many long-term studies where the study site is static and all the fieldwork revolves around it (The Supersauze landslide, or any of the Smithsonian field sites spring to mind).

Given the questions I have been posing throughout my PhD about how image quality and network geometries can effect photogrammetric accuracies, it was decided a field campaign should be run in an attempt to properly address them. This lead to thinking about what an ideal fieldsite would look like and broadly the properties it should have. I split these originally into two camps:

  1. Practical

Accessiblility

With all the gear we had planned to bring in, this was top of the list. The closer we could get to the field site, the more we could practically achieve within the time frame

Exposure

More in environmental sense, given the sensitive nature of using equipment on tripods, one of the main goals was to have everything static for the duration of each survey

Accommodation

We needed an area from where we could head out early in the morning and be within reasonable distance of the target site

Through-flow

Carrying out a photogrammetric survey with a crowd moving through the scene has obvious drawbacks. While this conflicts with accessibility (As easier access usually means more people), it was a consideration nonetheless

Tides

More to do with the timing, though access to seafronts where tides could potentially have an effect were a consideration

  1. Academic

Correct scale + conditions

One of the main reasons to do fieldwork as opposed to performing everything in lab conditions, is that we can practically get an intuition for how the technology is performing in the field. This includes having to deal with rain/wind/exposure differences in practice

Colour contrast

For my blog’s followers, you’ll know I’ve spoken about colour contrast before. One of the prevailing themes to be tested, we wanted a scene with ideally vegetation, mud/sand and exposed rock

Viewing geometry

Given a UAV was not to be used for this work, we were looking for a scene we could capture images from the direction of the surface normal (Perpendicular to surface).

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A surface and it’s normals

Thus, cliffs were a natural consideration

Refining the selection

Taking account of all the criteria, I took to the ordnance survey maps to try and see the options available. For a moment I thought about how to phrase it as a database query, I think it’d be a pretty nifty query to be able to do, but practically difficult. Something like ‘I want a 100 m stretch of cliff which is partially vegetated and easily accessible within 1 hours drive of a town with a hotel’. For fun, I thought I’d check if google was up to the task.

It lead me to a rambler’s guide to the Devon coast, and what looked like some seemingly suitable cliffs to use as experimental subjects.

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Cliffs near Ilfracombe, Devon

Devon is quite a way from London, but was certainly within the bounds of possibility. In part two I’ll elaborate on thinking which sprung from this, leading to a final selection.