Webinar: Mapping River Habitats From the Air

CIEEM held a webinar today on mapping river habitats from the air, and the use of drones for field surveys. The webinar started with a quick poll to determine how many of the audience had previously used aerial data to map habitats. The results came back with 45% of the audience having used this method before, and 55% having not used this before – a pretty even split!

David Campbell, the Divisional Director for remote sensing at APEM, presented on how it’s possible to map habitats from the air using surveys such as satellites and drones. He started by explaining traditional methods of mapping fish habitat, including bankside walkover with OS paper maps, using a GPS to locate specific points. This would be a visual interpretation of habitats and different groupings i.e. salmon and fry, which would be noted in the field and digitised when back in the office. Information and data can then be collated to calculate areas of different habitat such as juveniles.

David is involved in aerial imagery and mapping from the air, which offers an alternative to traditional methods. The main platforms include satellite, airborne, and UAV/Drone. These offer different resolutions and spectral bands (colour, near infra-red, thermal imaging), and should consider repeatability and cost budgets.

  • Satellite – low resolution (30cm expensive, cheaper is 1-2m), useful for land assessment, and wetted width, but limited in terms of habitat mapping in channel
  • Airborne – bespoke surveys, ultra-high resolution (up to 2cm), detailed habitat mapping, can be carried out whenever convenient i.e. looking at particular river conditions or flow levels.
  • Drones – good for small areas and close inspection, however have limited range – operator needs to be a certain distance away from the drone (no more than 3km). Beyond this, aerial imagery would be more appropriate.

There are also different data types including colour and near infrared imagery. Thermal data can help contribute towards the classification of the type of habitat in the river. Topographic data is elevation data from imagery or LIDAR.

An airborne survey involves a series of straight lines with overlapping imagery in forward motion (direction aircraft is flying). Forward and lateral images overlap to create orthomosaic data. This data can later be processed with a software package. David explained that software had been developed specifically for looking at habitat from imagery. The software considered aspects such as wetted area, dry area, vegetated area, grain size and  channel depth to help interpret the habitats present.

Image roughness can be used as a proxy for grain size data, and can be calibrated with reference sites of known condition, to produce grain size maps. This can also be applied to depth data, where calibration reference data can be used to generate a depth map from the image. Output data and maps can be used to specify what type of habitats are present.

River temperature surveys or thermal surveys are sometimes done from the air if there is specific reason for it. For example, discharge from a reservoir showed cold water being discharged into a river. Even in summer, the discharge temperature was much lower than water already in the channel. This effected the suitability of fish habitat, as different fish species require certain temperatures in order to inhabit an area. A mapping investigation was able to look at how far the temperature of this discharge impacted downstream, and the location of a cross over zone, i.e. where the physical habitat exists but the temperature is inappropriate for fish to inhabit. Furthermore, this technique can be used to look at how the temperature of the main channel is influenced by tributary inputs, by mapping the temperature at confluence points.

This shows the significant overlapping advantages of both imagery for physical habitat mapping, and thermal mapping for temperatures of the channel, and how this impacts on fish residence. These techniques can also be used for catchment diffuse pollution mapping and invasive species mapping i.e. pennywort, Himalayan Balsam, and knotweed.

Following his presentation, David then gave examples through a 3D Live Demo. He showed how Salmonid fish habitat could be captured along the river banks using aerial imagery, and explained how sometimes it can be useful to use aerial survey when site visits are not possible due to access issues or permissions.

As well as mapping habitats, this technique can be useful for looking at other features including pollution inputs, drainage and artificial inflows, input channels, as well as noticing pressures i.e. bank poaching and its potential local impact on habitats and connectivity.

These habitats, features and pressures can be digiitised straight from the imagery, and maps can be hosted online for fisheries teams and other stakeholders to view.