Seeing in the dark

Last night, Mike and I were locked inside a forestry plantation in Cwmcarn. It seemed certain that we were going to spend the night in our cars. This is not an unusual situation for people who work in remote places at night and its not the first time we have had to drive around in the dark on bumpy tracks looking for access gates.

We have been commissioned by Natural Resources Wales to do a thermal aerial survey of one units they want to restock with Sitka and Norway spruce. We are trialling a technique to survey for the presence of nesting birds, particularly nightjars- who have a particular liking for upland clear-fell areas. It is a great project using slick technology and it has clear practical applications in conservation and industry. 

Facilitation of conifer plantation might seem like a counterintuitive conservation strategy to maintain populations of nightjar, and perhaps in some respects it is. The first way that it is a positive conservation strategy is that, if successful, the location of nesting nightjars will be identified and protected for the duration of their breeding season. Illegal or not, killing a brood of nightjars by running a mulcher over them, isn’t going to help the population.The second way is, regardless of how you might feel about conifer plantations in the uplands, at various periods in their timespan they are valuable habitats for nightjars, not just after they have been cut down but also in the early growth phases. I am less keen on the main period of stand growth, but I suppose we need timber.

the black spot is the cold (bright yellow) landing pad and the white blob is a (black) labrador

detail is sparse at 15m height

Thermal imaging is like having superhuman vision- or like being a spy in MI5 and it is very tempting to fly around all night looking for anything that has a heat signature.  It also has its downfalls that make the whole enterprise slightly more complicated than a standard aerial survey. Thermal imaging is most effective when there is a temperature differential between the (warm-blooded) creature and the surrounding environment. This happens best at night when the temperature drops- it works even better at dawn when the ground has really cooled and there are no residual hot pockets. Working at those hours really eats into your rest time (that seems to matter more to me as I get older!- although I fell asleep on the minibus today- I wasn’t driving, happily). The other downfall is that the thermal sensor is not as high resolution as cameras- so you need to fly at low heights to get good detail. Flying at low heights is bad because it requires more flight time to cover an equivalent area. Also there are more obstacles like trees at low heights.

We are yet to find a nightjar nest, not because the technique isn’t effective, but because they are few and far between. Its only a matter of time. Watch this space..

The Final map of the island

Now, the Severn Estuary doesnt usually look like that but the land image is exactly as it was last Friday.

The 3d Model enables full navigation of the island.

The next stage is to isolate the vegetation layer from the ground layer.

Making a Model

After a hot shower and a hot cycle of your seagulled clothes, its time to process the images and make a model. 

Thankfully, there are some very clever people on this planet who make normal people (like me) look very clever with minimal understanding of the dark arts of spatial processing. The little that I do understand is that:  

1. All images should be geo-referenced. This is automatic if you are using standard commercially available drones.
2. There should be a good overlap (80%) of all images. If you are brave enough to send your drone into the sky on a pre-programmed flight path (easier said than done), this is simple- providing you have enough memory (you usually need many gigabytes). This does mean you get many similar photos like these filling up your hard drive:

 

3. You should take all the images on the same day and ideally within a short period. This is to do with image correction and light conditions- there is only so much that a computer can do to deal with shadows. Its quite striking the difference in light between overcast and sunny day.

Once you have the images, all you need is access to a programme that can stitch the images together accounting for position, height and angle of the camera and other visual distortions (in a process called ‘orthorectification’).

The process involves a complex pixel-matching exercise that produces a three dimensional cloud of points, which is called a point-cloud. The point cloud measures the distance between each pixel so the spatial arrangement of the pixels is known.

If you zoom in, it looks rough and messy, but if you zoom out, it looks familiar.

 
The cloud of points is then processed to create a smooth digital surface model or DSM. This happens automatically once you upload your images and select the outputs. Sounds too simple? It is that simple. The trouble is that the programmes capable of undertaking these tasks requires a subscription which costs thousands of pounds. It is troublesome if you cant afford to pay for it- but it is impressive.

Derelict and Deserted?

In the middle of the grey and moody Severn Estuary, the buildings are crumbling and the battlements are empty. There are stories of monks, and cholera, and the ghosts of the long forgotten causing mischief.  From a distance, it may look empty but this place is anything but deserted. 

Aside from the rare plants, slow worms and unusual human inhabitants, the island is packed with sounds and smells. If you stray off the beaten track, the air erupts into life, and large determined creatures fill the air, with shrieking cries whizzing past your head. 

This is Flat Holm Island and it is the nesting site of thousands of lesser black-backed gulls and herring gulls. 

This image is one of 1500 photographs I took on friday morning, and within this shot is probably a thousand nests. With the Cardiff Harbour Authority and the Flatholm ringing group, I am helping to map the nest distributions and model the vegetation to help inform the ongoing management of this challenging place. This image was taken at 75m from ground level with the majestic Inspire 2, a rather large and well-equipped drone. The gulls were quite accommodating on this trip and didn’t seem to notice the drone quietly snapping their home from the sky. There was one single gull who wasnt so keen. With about £5k under my (remote) control, I was keen to avoid any altercations and had to abandon the survey a couple of times to keep a good distance from this curious beast.

The next time I go, I will be trialing the use of a thermal camera to see if we can count the gulls in the dark. I hope the gulls wont mind that either.

As an aside, whenever I go to Flatholm, I camp. I camp because of some of the stories I have heard about the less conspicuous inhabitants. I'd sooner sleep with the gulls than 'old Pete' and his meddling.