Why LiDAR Belongs in Every
Heritage Impact Assessment
Most Heritage Impact Assessments draw on the same three sources: the Historic Environment Record, historic Ordnance Survey mapping, and aerial photography. Each is genuinely useful. None of them can see through woodland canopy, detect a ploughed-out earthwork, or pick up a relict field boundary that nobody ever wrote down. LiDAR can do all three, and it belongs in the standard HIA toolkit, not treated as an optional extra for only the largest or most sensitive sites.
What LiDAR Actually Reveals
LiDAR yields two core outputs. The Digital Terrain Model (DTM) strips out vegetation to expose the bare earth surface. The Digital Surface Model (DSM) keeps that vegetation layer, which matters because it preserves subtle relief under tree canopy that the DTM removes along with the trees themselves. Applying hillshade from multiple directions, sky view factor and slope analysis to both models exposes:
enclosures and field boundaries no longer visible at ground level
ridge and furrow cultivation earthworks
building platforms and associated access tracks
water management features such as leats, ponds and mill races
boundary banks and ditches degraded almost to nothing on the surface
None of this needs ground access, a dry summer for crop marks, or a specific season for shadow marks. The data already exists for most of England through the Environment Agency’s Open Data programme, and the whole process runs from a desk, no site visit required.
Why It Changes the Assessment
The value of LiDAR isn’t only what it finds on its own. It’s what happens when you set it against the HER, the historic maps and the aerial photographs. Where all three agree, confidence in a feature’s presence and character goes up substantially. Where LiDAR shows something the HER has never recorded, that’s a genuine new identification, not a discrepancy to explain away. Where the HER records a feature LiDAR can’t see, that’s worth investigating too, since it might mean ploughing has flattened the feature since the last survey, or that the original record was inaccurate.
Either way, an HIA that only draws on documentary sources is testing one line of evidence against itself. Adding LiDAR supplies independent corroboration, and independent corroboration is what makes a significance assessment and impact conclusion defensible in front of an LPA conservation officer or at appeal.
The Cost of Leaving It Out
NPPF Chapter 20 requires the LPA to understand heritage significance before deciding, not after. An HIA that skips LiDAR isn’t necessarily wrong, but it’s working from a smaller evidence base than it needs to, and that has two practical consequences. Weaker assessments invite more scrutiny and more requests for further information, which costs the applicant time. Features missed at the desk stage tend to turn up during groundworks instead, where they become a watching brief finding or a stop-work notice rather than a line in a report, and considerably more expensive to deal with.
Building LiDAR into the HIA from the start avoids both outcomes.
Strata Heritage integrates LiDAR analysis as standard in every DBA and HIA, alongside HER research and GIS-based spatial analysis. If you’re scoping heritage input for a site anywhere in Shropshire, the Welsh Borders or the West Midlands and want to know what LiDAR would add before you commission the work, get in touch at j.hancock.watts@gmail.com
