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Data Surgery

Identifying tree canopy: a free alternative to Bluesky's National Tree Map

    • 23 July 2018 03:35PM
    • Visits: 37
    • Discussions: 2
    • Responses: 2
    • Registered: 20 July 2011 12:56PM
    • Last active: 24 August 2018 12:32PM

    Bluesky's National Tree Map (https://www.blueskymapshop.com/products/national-tree-map) is an impressive and useful dataset, purportedly providing the canopy extent of every tree in England and Wales. It's pretty expensive though. I was excited, therefore, when I realised that the recent Aerial Photography for Great Britain agreement (APGB, https://www.apgb.co.uk/) provides us in the public sector with the raw materials to produce an equivalent, free of charge.

    Under the APGB we can access colour infrared imagery, which enables us to calculate the normalized difference vegetation index (NDVI, https://en.wikipedia.org/wiki/Normalized_difference_vegetation_index). This allows us to distinguish between vegetated and non-vegetated surfaces. We can also access a digital terrain model and a digital surface model, which together can be used to calculate the height of objects, such as trees, on the land. Vegetation taller than a certain height is tree canopy.

    I have developed a script tool in ArcGIS that carries out this process, and have run it for The Mersey Forest (a large area of Merseyside and North Cheshire) with some success. If you would like a copy of the script tool, or any further information, feel free to contact me at tom.butlin@merseyforest.org.uk.

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