The UK Centre for Ecology & Hydrology (UKCEH) has produced the most comprehensive map of hedgerows across the length and breadth of England, with the aim of guiding the planting and restoration of these vulnerable habitats. Spatial Data Scientist Josep Serra Gallego, one of the UKCEH researchers involved in the analysis, explains how they harnessed LIDAR (Light Detection and Ranging) remote sensing data released by the Environment Agency.
Why are hedgerows important?
"Hedgerows are more than just strips of woody vegetation than separate our agricultural fields. They are traditional features of the agricultural English countryside and have a key role in landscape management, biodiversity enrichment and carbon sequestration, among other vital environmental services.
"Approximately half of Britain’s hedgerows were lost between the 1940s and 1990s, mostly in England, due to development and agricultural intensification. In recent years, a lack of maintenance has resulted in gappy hedges or overgrown lines of trees.
"The UK Centre for Ecology & Hydrology (UKCEH) has produced a map of hedgerows across England – available via the Environmental Information Data Centre. This shows that a total of 390,000 km of hedgerows (1-6 metres tall) exist on field boundaries across England, enough to go round the world almost 10 times. The UKCEH Land Cover Plus: Hedgerows dataset serves as a baseline for the amount of hedgerow, as well as providing important new detail about the condition of these important habitats.
"The Government’s Environmental Improvement Plan (EIP) sets out the aims to create or restore 30,000 miles of hedgerows a year by 2037 and 45,000 miles a year by 2050. To support this, it is crucial to understand where hedgerows are in the landscape, both by mapping the position of individual hedges on the ground and then understanding patterns at landscape scales, identifying which areas of England have the greatest density of hedgerows.
"The hedgerow dataset will also guide planting and restoration of these vulnerable habitats to support wildlife and mitigate climate change. The new, interactive visualisation of the dataset allows easy and quick exploration of England’s hedgerows, at landscape and local scales, in both 2D and in 3D."
This dataset is part of the UKCEH Land Cover Map series.
What data did you use to create the hedgerow map and why?
"Previous estimates of hedgerow length and condition were largely based on field or aerial surveys of smaller areas, and then scaled up for regional or landscape estimates. The UKCEH Hedgerow dataset has used airborne LIDAR (Light Detection and Ranging) data from the Environment Agency, provided through the National LIDAR programme.
"The LIDAR dataset provides elevation data of +/-15cm RMSE (Root mean square deviation). The data are captured with a beam of light in the form of a pulsed laser which is fired over the ground, with the time taken to return to the receiver indicating whether there are features there and their dimensions. This is then used to build a model of the land surface.
"To create the hedgerow dataset from the source data we identified vegetation on field boundaries and extracted features that corresponded to the heights of hedgerows. Each minimum 2m long segment of hedgerow or woody linear feature is classified, from a minimum height of 0.5m to the feature maximum; based on original 1cm vertical resolution and 1m horizontal resolution, summarised into 0.5m, 1m or 2m height bands to a maximum band of 6+ m tall. These classifications were then colour-coded by height class (blue or green for shorter hedges and red for taller tree lines).
"We excluded certain land cover types – urban, suburban, mountains, moors, heath, coastal and open water – because these areas contain very few hedges, so any detected linear features were very likely to be something else, like rocks, drystone walls and urban infrastructure. We then used the JASMIN super-computer to process the vast amount of data from across England.
"The model was validated by ‘ground truthing’ against hedgerow and tree-line features in 38 test squares of 1 km2 each. The results showed 96% accuracy when comparing total length of hedgerows across all 1km x 1km squares surveyed."
How did you display the data, what methods and techniques?
"We used a two-stage approach to create the dataset, and then to display it in a useable way: initially, the map uses 50 km x 50 km tiles to display hedgerow density. As the user zooms in, larger tiles give way to smaller units, gradually shifting from 50km to 25km, then from 25km to 10km, and finally to 5km tiles.
"We built our visualisation in ESRI’s ArcGIS Online platform. To add contextual data alongside the hedgerows, we added basemaps from World Imagery provided by Esri, Maxar, Earthstar Geographics, and the GIS User Community.
"It is presented as a vector dataset; each line on the map representing a section of hedgerow with attributes describing its height and characteristics. However, to ensure that the data is accessible to a wide, and non-GIS, audience, we wanted to provide a user-friendly and engaging way through creating a map tool that would simplify the handling of this information and allow users to explore the hedgerows of England quickly, easily and interactively.
"The best way to enable this, we believed, was by providing maps showing both spatial distribution and exact location of hedgerows in England, and in both 2D (more familiar to spatial data specialists) and 3D (more familiar to the way people see their local landscapes) formats."
Who is the intended audience?
"The hedgerow map is potentially useful to a wide range of audiences; policymakers can use it to assess hedgerow stock at various levels, while conservation organizations can identify fragmented or deteriorating hedgerows to guide restoration and improve habitat connectivity.
"Farmers and landowners will benefit from an easy way to monitor and analyse their habitats, identifying where hedgerows can be planted or restored to attract subsidies and participate in agri-environment schemes. Researchers can explore the role hedgerows play in our landscapes, and the public can see the state of hedgerows in local rural areas, exploring and acquainting themselves with one of the most iconic components of the English countryside."