The Case for Including Slope in BNG
Despite the growing use of sophisticated tools and datasets in biodiversity assessments, one key factor is often overlooked: slope. We’ve incorporated slope considerations into our FRIDAS checklist because it is such an important and often neglected aspect of accurate habitat measurement. The problem is two-fold: it is both geometrical, affecting how we calculate the true surface area, and ecological, influencing the characteristics and function of habitats. Here’s why getting slope right matters so much.
Most assessments still measure habitat area as if the ground were perfectly flat. That’s fine on level terrain, but as soon as you’re dealing with a slope, the real surface area is larger than what the flat map shows. For example, at a 30-degree incline, the surface area is around 15% bigger than the flat estimate, and at 60 degrees, it can actually be double. Since habitat units are fundamentally calculated based on area, underestimating area means underestimating biodiversity value.
Luckily, the mathematics behind correcting for slope is simple. You can estimate the real surface area by dividing the flat area by the cosine of the slope angle. That means a 1ha patch on a 30 degree slope is actually about 1.15ha, and on a 60 degree slope, it’s 2ha. On steep terrain, this correction could mean baseline habitat areas are being under-reported by 15% to 100%. This can truly affect the accuracy of baseline habitat calculations and can throw off compensation targets, habitat creation plans, and ongoing monitoring efforts.
This issue becomes especially important when dealing with habitats that have higher distinctiveness and condition scores. Because Biodiversity Net Gain units multiply area by these metric values, even small underestimations in area due to ignoring slope can result in disproportionately large errors in the final biodiversity unit calculations.
Slope is also a factor ecologically. Steeper slopes influence drainage, erosion risk, soil makeup, and exposure to sun and wind, all of which shape the habitats and determine which species can thrive. Ignoring slope can lead to proposals that don’t truly reflect ecological realities. Many experienced ecologists do take slope into account in their assessments, often drawing on field experience and expert judgement. For example:
A senior botanist might flag the risks of soil erosion on steep slopes when proposing species-rich grassland.
A habitat restoration specialist might redesign woodland planting zones to follow contour lines and mitigate runoff.
A GIS consultant might incorporate slope layers into habitat feasibility models when creating habitat opportunity maps.
Despite good practice by practitioners, the problem lies in the fact that such considerations are not embedded within the metric. There is currently no formal requirement to declare how slope or terrain has been factored into BNG decisions, leaving room for inconsistency, oversight, or error. In short, professional judgement helps, but it is not a failsafe.
So why isn’t this correction standard practice yet? Even though national LiDAR data is widely available and GIS is common in ecological consulting, slope correction isn’t built into the official BNG calculator. It’s not currently a requirement, and if a project doesn’t have GIS expertise onboard, it often gets skipped. There’s also a misconception that slope correction is too technical or only needed for extreme terrain, but in reality, it’s an accessible adjustment, especially when automated through GIS tools.
We believe slope correction should become a standard part of best practice. Not every site needs a detailed topographic model, but where terrain is sloped or stepped, getting the area right brings big benefits. To help with this, we’ve integrated slope considerations into the FRIDAS checklist, our BNG centric GIS standard that ensures robust and consistent data inputs in BNG assessments. It’s a handy tool for anyone looking to incorporate slope and other critical factors into their workflows.
