Drone mapping

Drone mapping

Drone mapping

For a few months now I’ve been experimenting with the (non-commercial) use of a small camera drone for environmental monitoring. The quality of the resulting images and the unique perspective it offers have been pleasantly surprising. Its potential applications at relatively low cost are impressive I think.

Red Hill Henge

The site of this scheduled Ancient Monument lies east of the A3 in Surrey and just north of Junction 10 on the M25 motorway.

Map of Red Hill Henge

Red Hill Henge scheduled Ancient Monument (1007905). © Crown Copyright and database right 2018.

‘Making the countryside pay’

In late 2017 Surrey County Council, in collaboration with Surrey Wildlife Trust and their contractors, decided that this Site of Special Scientific Interest (SSSI) would be a suitable venue to pilot their commercial felling operations in the county.

The felling licence issued by the Forestry Commission allowed them to extract 30% on 2 occasions over a 10 year period. Upon visiting the site however the impact at ground level looked far more extensive than I would have expected.

Stacks of felled timber

Felling operations on Wisley & Ockham Commons SSSI

Concerned that the contractors may have exceeded their remit and damaged the Ancient Monument in the process I mapped this area to the north of the site by flying the drone in parallel lines at a height of 60 metres with the downward facing camera set to take pictures at intervals of a few seconds.

Cloud view

With the help of DroneDeploy’s internet cloud software I then stitched the resulting 139 overlapping images together to create an orthomosaic map layer.

Drone map

Area mapped by drone at Red Hill Henge in Surrey.

Orthomosaic map imagery

Orthomosaic detail from drone imagery at Red Hill Henge.

Tracks left by the contractor’s heavy machinery can be seen running across the circular ‘henge-like’ earth bank feature.

Switching to the Normalised Difference Vegetation Index (NDVI) imagery analysis clearly shows the difference between live green vegetation and felled woodland.

NDVI map

Normalised Difference Vegetation Index (NDVI) analysis of felled woodland.

NDVI map

Normalised Density Vegetation Index analysis of felled woodland.

Extra dimension

To help peopIe unfamiliar with the site to visualise the monument in context I then generated a 3D model from the visual point data using DroneDeploy’s online competitor, Pix4D.

3D model of ancient monument

3D model of Red Hill Henge generated from drone imagery.

For terrain features and dense canopy this technique works well, but it struggles to accurately represent isolated trees and finer features. Without resorting to an expensive aerial LiDAR platform it would be difficult to achieve better results however.

Bright Spark

The little drone done good on this occasion, and with minimal disturbance to a historically and environmentally sensitive site ironically!

DJI Spark drone

DJI Spark drone. Photo: Ralph Clark.

 

Amphibian Kama Sutra

Common toads in amplexus

Common toads (Bufo bufo) spawning

After emerging from hibernation in early spring, toads migrate back to their breeding ponds.

Males fight to secure mates and often outnumber the females at some sites. When this happens the female may be grasped by several males in a position known as ‘amplexus’, as they compete to be in the best position to fertilise her eggs.

Common toads in amplexus

Group of toads in amplexus

Common toads in amplexus

Spawning might look fun but can also be life-threatening!

Common toads in amplexus

Toad sex is apparently a moveable feast!

The common toad is widespread in Surrey, however populations appear to be declining. Where toad migration routes cross busy roads there can be many fatalities, and local conservation groups police ‘Toad Crossings’ at dusk to help them safely across.

Common toad crossing track

This toad was late to the party, but managed to avoid oncoming traffic

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Harvest Mouse Survey

Harvest mouse at night

Harvest mouse (Micromys minutus) captured on survey after dark

Last week I joined Surrey Wildlife Trust on a harvest mouse survey in wetland habitat beside the River Wey.

Harvest mice are tiny rodents 5cm in length. Their remarkable prehensile tails add an extra 6cm. They live in long tussocky grassland, reedbeds, hedgerows and around woodland edges, building a spherical nest of tightly woven grass, high up amongst the stems. We found several examples at this site:

Harvest mouse nest

Harvest mouse nests are woven from a mixture of living and cut grasses which provides excellent camouflage

Nests are the most obvious sign of harvest mouse activity since the animals are especially active around dawn and dusk and rarely spotted in the thick vegetation.

Dozens of rodent-friendly traps of different designs were set in the thick wetland reeds and grasses. Surrey Wildlife Trust staff and volunteers returned three times each day over the course of a week – at dawn, midday and dusk – to ensure that no trapped animals were left for any longer than necessary.

Rodent trap

Rodent trap baited with peanut butter, bird seed and dried blowfly larvae

After retrieval from the trap harvest mice are transferred to a transparent bag for weighing and sexing:

Harvest mouse in bag

Harvest mouse recovered from trap

Weighing harvest mouse

Harvest mice weigh only 5g on average

Once the details of each rodent have been recorded a small patch of fur is trimmed to identify any animals which are subsequently re-captured.

Harvest mouse being marked

Marking harvest mouse – a delicate operation!

The highest number of individuals captured was 12 one morning, making this the most abundant site for harvest mice in the county at present.

Harvest mouse in the hand

Harvest mice require very careful handling by trained experts

With thanks to Surrey Wildlife Trust staff and volunteers.

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Buglife: Glow Worm Survey

Buglife are using my image in their postcard campaign to publicise a glow worm survey in Scotland this summer:

Glow worm postcard

Scottish Glow Worm Survey postcard

Glow worms are widely distributed in the UK, with records from the south of England up to the north of Scotland, but there have been very few Scottish records in recent years.

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Butterfly Conservation Moth Report

Butterfly Conservation: Moth Report 2013

Butterfly Conservation: Moth Report 2013

Butterfly Conservation used my image of a nocturnal moth trapping event to illustrate their latest report:

Unsurprisingly the report concludes that UK moth numbers are in decline, documenting a 40% drop in total abundance in the south of England over the last 40 years.

On the bright side many more moth species have colonised Britain in recent decades than have become extinct.

This photograph was taken at a recent moth trapping event on Chatley Heath organised by Surrey Wildlife Trust. The original image below:

Moth Trap Assembly

Skinner Trap on Surrey heathland at night

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Suburban Oak

Suburban Oak

The ancient Crouch Oak in Addlestone, Surrey, UK

The Crouch Oak once marked the perimeter of Windsor Great Park and is thought to date from the 11th Century, making it in excess of 900 years old.

This ancient tree now looks a little incongruous as a modern residential estate has grown up around it.

Night Oak

The Crouch Oak at night with passing traffic

It’s interesting to compare depictions of this tree from the 19th Century and in 1904 as wooden fences are replaced with metal railings and a dusty track becomes a surfaced road. The oak also enjoyed an additional limb in those days.

In more recent times the Crouch Oak survived an arson attempt in 2007 and is now fitted with metal grilles across its hollow openings to prevent a repeat.

Oak Urban Effects I

Metal grille stops burning objects being thrown inside

To see the Crouch Oak for yourself follow the map below to the bend in Crouch Oak Lane:

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Dartford Warbler Winter

Dartford Warbler Snow Drift

Dartford warbler (Sylvia undata) foraging on snow covered heathland

In the severe winter of 1962/63 the Dartford warbler (Sylvia undata) was almost wiped out in Britain. The national population dropped to just 10 pairs.

But in good breeding seasons, and with suitable habitat, its numbers are capable of bouncing back, thanks to repeated nesting and high survival rates among its offspring.

On the Surrey lowland heaths Dartfords have been doing well, re-colonising Thursley Common last year for the first time since a devastating fire in 2006.

During the recent freezing winter weather I photographed this Dartford warbler foraging in the snow on Chobham Common:

Dartford Snow Flurry

Dartford Snow Dipper

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