Slope: Enhancing Accuracy in BNG Assessments

In the final instalment of our FRIDAS? series, we explore Slope—an often overlooked but crucial element in enhancing the accuracy of your Biodiversity Net Gain (BNG) assessments.

Why Slope Matters

Slope is an important factor that can significantly affect the accuracy of habitat quality assessments. Even though it isn’t currently part of the Biodiversity Metric, considering the slope of the land can give you a more accurate picture of both the size and quality of habitats. Without this consideration, habitat area measurements might be underestimated, and BNG scores could be skewed.

A common issue in habitat assessments is the discrepancy between the true surface area of a sloped habitat and the 2D area seen in satellite imagery. The area recorded when a habitat is on a slope should reflect the actual surface area—as if measured using a trundle-wheel, rather than the 2D area suggested through satellite imagery, which won’t reflect the full extent of the surface.

?? Example: Consider a hill that forms a triangle. The actual length of the hill as measured on the ground (using a trundle-wheel) would be 5m, even though flat satellite imagery would indicate only 4m. The correct approach is to use the 5m length to inform the true area of the habitat, ensuring accurate BNG calculations.

By integrating slope-based area adjustments into BNG assessments, we can prevent the underestimation of habitat size, improving both the accuracy and ecological relevance of conservation planning.

?? Jonathan Gatward has done extensive work in this area, emphasising the importance of surface area-based habitat calculations for more reliable BNG assessments.

How Slope Supports BNG Goals

Accurate Area Measurement:

Habitats on slopes are typically represented in 2D as smaller than they actually are. By accounting for slope, you can ensure that area calculations reflect the true three-dimensional extent of the habitat. This leads to a more accurate BNG unit score.

Improved Ecological Relevance:

Slope is not just about measurements—it also affects habitat dynamics. For example, steeper slopes may impact vegetation growth, soil quality, or water drainage, all of which influence the overall health of the habitat. By considering slope, you ensure that your habitat assessments take into account these real-world conditions.

Enhanced Data Completeness:

Including slope information adds another layer of detail to your data, which can help prevent overlooking significant ecological factors that could impact your BNG calculations. This contributes to more robust, comprehensive assessments.

Checklist Question for Slope

• What is the approximate range of slope covered by your data?

If the range is significant, then consider how the terrain might affect your habitat measurements

Best Practices for Managing Slope Data

1?? Incorporate Trigonometric Functions: If possible, use trigonometric functions in your GIS tools to account for slope in your BNG multiplier. This ensures that the impact of slope on habitat area and quality is properly integrated into your calculations.

2?? Use 3D Measurements: When assessing habitats on slopes, shift from 2D measurements to 3D models to capture the true extent of the habitat. This helps provide a more accurate and ecologically relevant picture of the site.

3?? Document Slope Considerations: Since slope isn’t part of the official Biodiversity Metric, it’s important to document how slope was considered and incorporated into your assessments in the supporting materials.

4?? Consult Topographic Data: Ensure that you have access to reliable topographic data, such as Digital Elevation Models (DEMs), to accurately calculate slope across your habitat polygons.

Common Challenges

• Overlooking Slope Impact: Not considering slope can lead to inaccurate habitat size and, ultimately, incorrect BNG scores.
• Lack of Topographic Data: Without access to good topographic data, accurately accounting for slope can be difficult.
• Inconsistent Data Reporting: If slope is not consistently considered across datasets, it can cause discrepancies and reduce the overall quality of the BNG assessment.

FRIDAS? in Action

The Slope element of FRIDAS? highlights the importance of considering topographic features in habitat assessments, ensuring that habitat areas are measured more accurately and reflecting true ecological conditions. By incorporating slope into your assessments, you can improve the quality and reliability of your BNG calculations and contribute to better conservation planning.

Looking Ahead

With the completion of our FRIDAS? series, you are now equipped with insights on how each element—from Format to Slope—enhances the precision, consistency, and comprehensiveness of your BNG data.

?? Follow us for ongoing updates, tips, and best practices to enhance your BNG assessments.

?? Learn more about the FRIDAS? checklist here: https://www.ecospatialsolutions.com/fridas