Information on hedges, small woods and individual trees is not yet available directly from the National Forest Inventory. It is possible to map these small features using Environment Agency LiDAR imagery, although until very recently the Chew Valley was not completely covered by LiDAR data.


Light Detection and Ranging, or LiDAR, is a technique for measuring distances very accurately with a laser. By putting the laser on an aircraft and firing it at the ground the height of ground features can be measured very accurately from the timing of the return. A bit like radar but with light. Of course you also need to know exactly where the aircraft was in space when the laser was fired. This works well with vegetation canopies because some canopies are semi-transparent and photons map be reflected back from all levels of the canopy and potentially from ground also. The Environment Agency has a LiDAR campaign and make height models available for every square metre of England. For each square metre they generate: the ground surface (Digital Terrain Models, DTM), the surface (Digital Surface Models, DSM), and the ‘first return’ (First Return DSM).

Using the LiDAR surface height (DSM) and the terrain height (DTM) it is possible to get a reasonable indication of tree height. Below is a shaded DSM and DTM of part of Lords Wood in Publow-Pensford parish. The DTM “sees” right through the tree canopy to the surface to reveal historic mine workings.

This is illustrated nicely in the graphic below which shows cross-section profile across the wood. The undulating terrain height is shown in red with the surface (canopy) height in green. Subtracting the terrain height from the surface height reduces the surface height to sea level where we can directly compare one to the other.

With some further processing we can exclude buildings and infrastructure and and Woodland leaving just other vegetation comprised of hedges, small woods and even trees in gardens. This is illustrated below in blue for the whole parish. In this particular parish there is a substantial amount of small woods and hedgerows remaining, perhaps reflecting the complex nature of the terrain and geology hereabouts.

To do this analysis requires both the DSM and the DTM. Unfortunately these are not yet both available for the whole of the Chew Valley so this remains a work in progress. However, the full DTM has been released and the DSM should follow soon.

This type of analysis tells us where hedges and small woods are but it says nothing about their composition.