Using LIDAR terrain data for a levels and drainage check

Before you commission a topographical survey, you can answer most of the levels questions on a site from free national LIDAR data. It will tell you which way the ground falls, where surface water collects, whether the site drains towards a neighbour, and whether there are earthworks worth asking an archaeologist about. It will not give you the invert level of a manhole, and it is not a survey.

Used as a desk check, it is the cheapest hour you will spend on a plot.

Where the national terrain data is published

The Environment Agency has flown airborne laser survey over England for decades, first for flood modelling and latterly under a national programme that gives complete coverage. The data is published as open data on the Defra data services platform, free to download and reuse under the Open Government Licence with attribution.

You will find several products. A composite terrain model built from the best available data at a range of resolutions, a composite surface model, the newer national programme tiles at one metre resolution, and in some areas point cloud data for those who want to process it themselves.

Downloads come as tiles keyed to the national grid. Pick the tiles covering the site and a catchment upstream of it, because a levels check that stops at the red line misses where the water is coming from.

Terrain models against surface models

This distinction decides whether the output is useful.

A digital terrain model represents the bare ground with buildings and vegetation removed by filtering. It is what you want for levels, falls and flow routing.

A digital surface model represents the top of whatever is there, including roofs, tree canopies and hedges. It is useful for shading studies, for checking the height of a neighbouring building, and for seeing hedgerows that the terrain model has stripped out.

Using the wrong one produces confident nonsense. A surface model will show a hedge as a two metre ridge and route water around it. A terrain model will remove the hedge along with the bank underneath it, which is why a flow path that looks open on screen sometimes turns out to be blocked on the ground.

Resolution and what it can resolve

One metre resolution means one height value per square metre, so features smaller than that are averaged away. Kerbs, small walls, narrow ditches and the ridge of a bund can all disappear or be smoothed.

Vertical accuracy is quoted in the metadata, in centimetres rather than millimetres, and it varies with the survey, the vegetation and the terrain. Read the metadata for the tile rather than assuming a figure, and record it in whatever you produce.

The other limit is age. Tiles carry a survey date, and a site that has been stripped, regraded or built on since then will not match. Check the date before you rely on a level, and compare the hillshade against current aerial imagery to spot changes.

One practical note on file sizes. A single one metre tile covers a large area and the files are big, so download only the tiles you need and keep the grid references in your project notes. Most desktop mapping software will open the raster directly without conversion.

Identifying overland flow routes

This is the most useful application for a small site. Load the terrain tiles, generate a hillshade and contours at a close interval, and look at where the ground falls.

The questions worth answering at desk stage are simple. Does the site fall towards the highway, towards a neighbour or towards a watercourse. Is there a hollow where water will pond. Is there a defined valley upstream that concentrates flow onto the site during heavy rain. Does the natural low point coincide with where you were planning to put the house.

Where you have the tools, a fill and flow accumulation analysis will draw the overland flow paths for you, and the result is a good first approximation of the exceedance routing that a drainage strategy has to address. Set it alongside the published surface water flood risk mapping, which is modelled from similar terrain data and is the source the lead local flood authority will quote back at you.

Treat the output as indicative. It informs the Drainage Strategy and the exceedance plan rather than replacing the modelling behind them.

Checking a site for historic earthworks

Hillshaded terrain data is a standard archaeological prospecting tool, because low relief features invisible at ground level show clearly under raking artificial illumination.

What turns up regularly on rural plots: ridge and furrow, former field boundaries and headlands, hollow ways, quarry pits and marl pits, building platforms, moats and fishponds, and the earthwork remains of shrunken settlements. On brownfield sites you may see foundations, railway embankments and cuttings, and the terraces of demolished structures.

Vary the illumination azimuth. A feature invisible under light from the north-west can be obvious under light from the north-east, and running several azimuths is standard practice. Anything you find belongs in the consultation with the county archaeologist and feeds the Archaeological Desk-Based Assessment rather than being interpreted in isolation.

Woodland is the weak spot. Under dense canopy the laser returns fewer ground points, so the terrain model is interpolated across gaps and small features are lost. Treat earthwork interpretation in woodland as a prompt to walk the ground rather than as a finding.

Limits against a measured survey

Be clear about what the data cannot do, because a levels claim in an application that turns out to be wrong is expensive.

It gives no invert levels, no manhole or gully positions, no service runs, no tree positions with stem diameters and no building corner levels to the accuracy a designer needs. It cannot establish a boundary. It does not give you a benchmark you can set out from, and it will not satisfy a highway authority looking at a proposed access profile.

A measured topographical survey remains the document that the design is built on. The terrain data tells you whether to commission one urgently, what to ask the surveyor to extend beyond the site, and whether the plot is worth buying at all.

Presenting the output in an application

Where you use it, show it honestly. Label the extract with the product name, the resolution, the survey date, the vertical datum, which is Ordnance Datum Newlyn, and the licence attribution. State that it is desk-based and indicative, and say what it has been used for.

A clear hillshade with contours, an arrow plan of overland flow routes, and a long section through the site are usually enough at application stage for a small scheme, alongside the measured survey for the detail. In a Flood Risk Assessment the terrain data supports the site description and the residual risk section, and the Environment Agency will expect finished floor levels to come from the measured survey rather than from a raster.

One honest caveat. Filtering artefacts are real, particularly in dense woodland and on steep banks, and a feature that appears only in the terrain model and nowhere on the ground is probably an artefact. Walk the site before you rely on anything surprising.

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