26th May 2023

A Senior Ecologist once brought up a very interesting point last week: how are we accounting for slope with our Biodiversity Net Gain (BNG) calculations?

Of course, ecologists and landscape architects know what habitats grow on different slopes, this will inevitably affect drainage, sunlight, etc. but what about from a spatial point of view?

For BNG calculations, a geometry measurement is taken in hectares and multiplied by metrics in the Natural England Metric Calculation Tool. Sounds simple, but using a 2D vantage point alone might mean we are underestimating area measurements and ultimately our BNG unit score.

For example, take a Textomur Wall on a landscape design that’s to be planted as a grassland. From above we see a polygon that measures 0.1476 ha and 0.28 BNG Units. however, the design states that same polygon measures 0.1854 ha. How can there be such a difference in measurements of the same polygon?

The answer is slope. The depth of a polygon on an incline is smaller in 2D than 3D. The Textomur Wall has an incline of 60 degrees, therefore the calculated area of a polygon will be smaller than the true measurement.

By accounting for the 60 degree incline the GIS polygon measurement has increase to 0.1813ha, increasing the BNG score of that polygon to 0.35 units. That’s a 25% difference in the BNG score, now image that extrapolated across an entire site…

Needless to say, I’m going to be including incline to my GIS Standard with a request for ecologists and landscape architects to highlight areas that need this measurement added. This could be a potential BIM-GIS integration for extracting more accurate areas and inclines. I also plan on assessing the need for using elevation data and slope analyses for BNG baselines too to ensure our geometry calculations are as accurate as possible. A bit more work needs to be done around the significant threshold of incline, but this is a good start.

This is why I love this field. Biodiversity protection is not only an ecological problem to solve, but also geographical, architectural, mathematical, and computational. It’s going to take a village to reach our BNG targets.