Measuring Biodiversity Net Gain, and the question of whether Drone vs. Satellite data is viable, has been a major development of Biodiversity Team Leader Abby Robinson’s work in the last 12 months at CSX.
As of February 2024 the introduction of the Biodiversity Net Gain (BNG) required developers to mitigate any vegetation habitat loss through creation or enhancement, either on the proposed development site or offsite elsewhere. These Biodiversity Net Gain policy requirements have led to the progression in accuracy of measuring and mapping habitats, as well as the creation of a new environmental market.
This is an opportunity for landowners to enter the private market, and earn a new revenue stream by offering developers a way to mitigate their habitat losses off-site.
A developer must demonstrate their BNG baseline and commitments to reaching the required 10% biodiversity gain to the Local Planning Authority (LPA). Whilst a landowner delivering offsite units must demonstrate their BNG baseline, and progress of the biodiversity recovery project as part of the monitoring program. Consequently, this has led CSX to develop systems to enable earth observation data to become a tool to measure, map and report on BNG projects.
Satellite imagery is a populist way of viewing earth observation data, as it is accessible by anyone e.g., Google Earth. Satellite data offers conveniency as it can be accessed pretty much at any time, from almost anywhere. However, when applying this method to BNG, many limitations arise.
As part of the BNG agreement with the Local Council a monitoring plan must be put in place, to confirm proactivity towards delivery by the developer or landowner. Generally, a monitoring plan requires an assessment for the first three years of the project, then again on the 5th, 10th 15th, 20th, 30th years. Some LPAs may request more frequent reporting.
A starting issue is that the majority of satellite data takes months, or years to update. Moreover, due to for example weather constraints it is highly likely that BNG recordings cannot be taken at the chosen time. Furthermore, this poses a problem for time of year reporting; the ideal survey time is in the spring/summer months, developers and landowners could be penalised for not meeting targets.
The next challenge with the satellite data that can be obtained is the resolution of it, and whether the free access data at 10m resolution is realistically usable, or the costly 25cm to 2m resolution data is viable for analysis and commerciality requirements.
Image 1: A section of field taken from satellite. In this image, it is difficult to classify the habitat; it could be distinguished as cropland or grassland.
CSX’s solution is that RGB drone imagery, of at least 0.75cm and higher resolution, offers a much more detailed record and understanding of the site. The higher resolution means that the data is more meaningful. Furthermore, in comparison to satellite data, drone surveys can be carried out on demand, making the imagery more relevant.
Image 2: The same section of field taken from a RGB Drone flight at 0.75 GSD. In this image, it is much more easier to distinguish this field as a cropland. 
At CSX Carbon, we have explored how we can use RGB drone imagery for the BNG assessment. When mapping existing and proposed habitats, the area measurement is much more accurate. For example, fences that divide land parcels are much easier to see and measure on analysed drone imagery. Furthermore, we can measure canopy area of all individual trees more precisely.
Understanding how CSX can use the drone imagery to aid condition assessments has been an area of system and process development focus through the launch of our BNG services. Many of the condition criteria can be fulfilled via a whole site drone survey e.g., bare ground coverage, hedgerow width and height. As this can be done across the whole site, it reduces the need for extrapolation from a small plot.
To enhance the analysis CSX also use drones to capture imagery of ground vegetation quadrats at a much higher resolution. This means that each pixel is equivalent to approximately 0.05cm (0.05cm Ground Sampling Distance). Through this, individual species can be captured and earth observation data can accompany the BNG assessment.
There is an opportunity for drone data to underpin ecological assessment reasoning, and therefore, support BNG delivery. Without meaningful data landowners or developers could be wrongfully penalised, BNG cannot be effectively enforced, and LPAs cannot approve and monitor all projects successfully.
It is clear that earth observation data is going to be vital for BNG delivery, and to ensure that it is successful and efficient then a higher level of detail will be required. Good quality data will be paramount for evidence and audit trail to measure, monitor and report biodiversity recovery. Making measuring Biodiversity Net Gain with drone surveys an important element of successfully delivering this new policy.
Image 3: The same section of field captured using the RGB drone but flown at a much lower height, making the resolution 0.05 GSD. From this image, it can be confirmed that this is cropland – temporary grass and clover lay. It is clear to see individual species on this image e.g., white clover Trifolium repens rye-grass Lolium.
