Lidar and aerial photography in desk based assessment

Lidar and historic aerial photography are the two evidence sources that most often change the conclusion of an archaeological desk based assessment, and they are also the two most often reduced to a single sentence saying the site was inspected. An assessment that lists the flights consulted by sortie number and date, and states the lidar capture date and resolution with the visualisations applied, is doing something a county archaeologist can check. One that says "aerial photographs were examined" is not.

For an architect commissioning the work, the difference matters because a properly read dataset can move trench positions off the developable footprint.

What each dataset shows

Lidar gives you a measured surface. An airborne laser records ground elevation at high density, and a terrain model built from the last returns strips out vegetation and buildings to leave the ground itself. What survives as a lump or a hollow shows up: ridge and furrow, hollow ways, house platforms, quarry pits, moats, park boundaries, military earthworks, field banks and lynchets.

Historic aerial photography gives you a moment in time, repeatedly. Vertical survey sorties, many flown for military or mapping purposes, cover most of England from the 1940s onwards, and oblique photographs taken by archaeologists concentrate on known sites. They show features that have since been levelled, and they show cropmarks and soilmarks that leave no surface relief at all.

The two are complementary rather than alternatives. Lidar finds what is still standing as topography. Aerial photographs find what has been flattened, and they date when the flattening happened.

Getting hold of coverage

Environment Agency lidar covering England is published under an open licence and downloadable by tile, with national terrain model coverage at metre resolution and finer data in some areas. Check the capture date on every tile you use, because a survey flown after a site was stripped for a previous development is worthless for archaeological purposes and a survey flown before it is evidence of what was lost.

Historic aerial photography is held in national and local archives. Historic England holds the largest collection, local Historic Environment Records hold prints and mapped interpretations for their areas, and the county archaeologist can usually say what has already been mapped. Where Historic England's aerial investigation and mapping work has covered the area, that interpretation is already in the record and repeating it is wasted fee.

Order early. Archive searches and print or scan supply are not instant, and on a programme where the desk based assessment sits on the critical path, a three week archive wait is a three week delay.

Visualisation is half the interpretation

Raw lidar tells you very little. What you see depends entirely on how the model is rendered, and a single hillshade from one direction hides every feature aligned with the light source.

Standard practice is to render several ways and compare:

• Hillshades from at least four azimuths, because features parallel to the illumination disappear.
• A multi-directional or principal component composite that combines them.
• A local relief model, which removes the broad landform and leaves small amplitude features visible on a slope.
• Sky-view factor or openness, which works well for hollows, ditches and quarry edges.
• A slope model, which picks out breaks of slope that a hillshade softens.

An assessment should state which visualisations were produced and at what resolution. That is the part a reviewer uses to judge whether the absence of features means absence of evidence or absence of effort.

Earthworks, cropmarks and soilmarks

Three different mechanisms, three different conditions for visibility.

Earthworks are surviving relief and show on lidar and on photographs taken under low sun, typically in winter. Grazed pasture preserves them and arable ploughing destroys them, so a site under permanent pasture is the best prospect and a site in continuous cultivation since 1950 the worst.

Cropmarks form where buried features change the depth of soil moisture available to a growing crop. Ditches hold more water and produce taller, greener growth; walls and metalled surfaces produce stunted, parched growth. They appear in dry early summers on free draining soils over gravel, chalk and sand, and barely at all on heavy clay. So cropmark evidence is a function of the weather in the year the photograph was taken, and a negative result from a wet summer proves nothing.

Soilmarks appear on bare ploughed ground where the plough brings up material of a different colour. They are the shortest lived of the three and the easiest to confuse with geology.

Where the data stops working

Woodland is the obvious limit. Lidar penetrates a deciduous canopy in leaf off conditions well enough to model the ground beneath, and dense conifer plantation defeats it, producing a terrain model that is partly canopy. Check the capture season before relying on a woodland result.

Urban and previously developed land is the second limit. Buildings, hard surfaces, made ground and services remove the surface evidence entirely, and the terrain model shows the modern platform.

Then there are the false positives, which do more damage to an assessment's credibility than a missed feature. Periglacial stripes and polygons on chalk and gravel look convincingly like enclosures. Land drains produce regular linear patterns. Pipeline easements, tramlines, fertiliser spreading, old hedge lines shown on enclosure maps and levelled twentieth century features all generate marks. Natural drainage channels look like ditches. The discipline is to check every candidate against geological mapping, historic Ordnance Survey editions and the modern field pattern before calling it archaeology.

Resolution sets a floor as well. A metre grid will not resolve a single posthole or a narrow gully, so a negative lidar result says nothing about the presence of small cut features.

Recording the sources properly

Chartered Institute for Archaeologists standards require sources to be listed, and for these two datasets that means specifics.

For photography: the collection, the sortie or flight reference, the frame numbers, the date of each flight, the scale or altitude, and whether vertical or oblique. For lidar: the survey name, the capture date, the resolution, whether the model used is terrain or surface, and the processing and visualisations applied.

Listing them lets somebody else reach a different conclusion from the same evidence, which is the point. It also protects the author when a later evaluation finds something the assessment did not predict, because the record shows what was available to be seen.

Turning the results into a fieldwork target

The payoff for doing this properly is a smaller, better aimed evaluation. Where remote sensing identifies a probable enclosure across one corner of a site, the sensible next step is geophysical survey over that area followed by trenches placed on the anomalies, rather than a uniform trenching percentage spread across the whole field.

Set that argument out in the assessment explicitly, with the mapped features and the proposed targeting on the same drawing. The county archaeologist is being asked to agree a scope, and a scope supported by mapped evidence is easier to agree than one justified by area alone. Where an archaeological field evaluation follows, the mapping becomes the baseline against which the results are judged.

The same remote sensing work also feeds the wider heritage case. Earthwork evidence of a designed landscape or a levelled park boundary belongs in a Heritage Statement, and where standing structures are involved the note on recording a building before demolition or conversion covers the parallel obligation.

Archaeological assessments are documents councils expect from a suitably qualified author, and professional review before submission is strongly advised. Enter your postcode on PlanScope to see which heritage and archaeology reports your site is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.