Drone mapping using a 5 inch DJI Spark drone.
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.
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.
A felling licence issued by the Forestry Commission permitted 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.
Felling operations on Wisley & Ockham Commons SSSI
Concerned that the contractors may have exceeded their remit and damaged the Scheduled Ancient Monument in the process I mapped an area to the north of the site, flying the drone in parallel lines at a height of 60 metres with the downward-facing camera set to take pictures at intervals every few seconds.
With the help of DroneDeploy’s internet cloud software I then stitched the resulting 139 overlapping images together to create an orthomosaic map layer.
Area mapped by drone at Red Hill Henge in Surrey.
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.
Normalised Difference Vegetation Index (NDVI) analysis of felled woodland.
Normalised Density Vegetation Index analysis of felled woodland.
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 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.
The little drone done good on this occasion, and with minimal disturbance to this historically and environmentally sensitive site ironically.
DJI Spark drone. Photo: Ralph Clark.