August 2021

Using LiDAR layers for landscape research

Comparing OS six-inch 1900s mapping (left) with LiDAR DTM (right) for Eildon Hill North, near Melrose

Light Detection and Ranging (LiDAR) is an airborne mapping technique, which uses a scanning laser to very accurately measure the distance between the aircraft and the ground. It allows highly detailed representations of relief or terrain models to be generated, often at spatial resolutions of between 25 cm and 2 metres.

LiDAR has many uses, and is particularly valuable for showing subtle variations in relief or elevation on the ground. Slopes and small streams are easy to spot, as well as types of trees and their densities. LiDAR can also be ground based rather than airborne and is often used in self-driving vehicles for showing relief and obstructions. LiDAR data has many archaeological applications due to its ability to show the underlying land surface with vegetation removed and to show up patterns of small variations in elevation which are often not visible to the human eye at ground level.

This page offers brief guidance on using LiDAR layers on our maps website for landscape research.

LiDAR surveys and outputs

LiDAR point cloud for an example area - showing all points
LiDAR point cloud for an example area - showing all points.

LiDAR surveys are undertaken during winter months (approximately November to April each year) when trees are without leaves, to ensure the most accurate survey of the ground. Up to 500,000 measurements per second are made of the ground, measuring height to an accuracy of better than ±15cm. (This is the root mean squared error or RMSE, quantifying the difference between the Ground Truth Survey and the LiDAR data).





LiDAR point cloud for the example area - showing ground points
LiDAR point cloud for the example area - showing ground points.

The survey data is then processed into different outputs for different purposes:




LiDAR Digital Surface Model (DSM) for the example area
LiDAR Digital Surface Model (DSM) for the example area.







LiDAR Digital Terrain Model (DTM) for the example area
LiDAR Digital Terrain Model (DTM) for the example area.



Resolution

For a 1m DSM surface model the point cloud will have an average point density across the survey of at least 1 point per square metre (ppsqm). For 50cm this increases to 4 ppsqm and 25cm resolution is 16ppsqm. The point density is calculated for the DSM and not the DTM, as under vegetation the point density that makes up the DTM will not necessarily have 1ppsqm at 1m resolution. The point density also varies across the scan - increasing where you have overlap from multiple flightlines and can also vary in forward scan direction in areas where the terrain changes significantly.

Colour and hillshade

The DSM or DTM LiDAR data also can be presented in a very wide range of different ways. It is often styled using a colour ramp for altitude, where different heights are represented by different colours or shades. Generally, one colour will blend into the next in a graduated progression. A hillshade is a different visualisation technique which illustrates what a land surface would look like if it were illuminated from a given direction, with the light source illuminating the landscape with a varying degree of brightness, depending on the angle between the slope and the light source at each point on the surface. The hillshade thus enables the more minor or subtle variations in relief to be picked out. The standard English and Welsh LiDAR data on our website has a basic colour ramp for altitude, which is then combined with a hillshade generated by illumination from a single direction.

Comparing OS six-inch 1888-1913 mapping (left) with LiDAR DSM (right) for Maiden Castle hill fort, Dorset

The default Scottish DSM and DTM layers available online also use a colour ramp for altitude, but without a hillshade, so that minor topographic features cannot be seen. We have therefore downloaded and reprocessed the Scottish DTM layers using the Relief Visualisation Toolbox, which is able to generate more useful visualisations from the same source data. We have generated 3 channel (Red, Green and Blue) hillshade images which show how the land surface would look if illuminated from 3 different compass directions at the same time (ie. as if we had 3 suns). The red channel 'shines' from 315 degrees, the green from 15 and the blue from 75. We used RVT's default sun elevation angle (height above the horizon) of 35 degrees. This multi-direction hillshade effect has the advantage of picking out minor landscape features more clearly than a single direction hillshade. (See Acknowledgements).

Comparing modern ESRI Imagery (left) with LiDAR DTM (right) for East Benhar Mine Bing
Comparing modern ESRI Imagery (left) with LiDAR DTM (right) for East Benhar Mine Bing, West Lothian. This conical heap of mine waste clearly illustrates the effects of hillshade colour from 3 compass directions.

Geographic extent

The geographic extent of available LiDAR data continues to grow substantially. By 2021, the Environment Agency had flown over 90% of England at 1 metre resolution, with 1 metre coverage for all areas expected by the end of the year. Natural Resources Wales have LiDAR data at 1 metre covering over 70% of Wales, with full coverage expected at the end of 2022. In Scotland, around 45,000 km2 were available by 2021, which due to some overlaps covers around 40% of the country. Further releases which will expand coverage are underway.

Viewing the LiDAR layers

At present for Scotland the DTM 50cm-1m (2019-2020) layer shows the processed LiDAR data with multi-direction hillshade. The different phases of Scottish LiDAR data that have been released since 2012 can also be viewed separately, which is sometimes useful where they overlap. So to view the best micro-topography for Scotland, choose the DTM 50cm-1m (2019-2020) or one of the separate phases, ie. LiDAR DTM 1m (Scotland) Ph 1.

The layers available in the drop-down lists reflect those which have coverage of the area you are viewing on screen.

Examples of LiDAR for particular sites

LiDAR is excellent for showing archaeological features like Roman forts and their ramparts, comparing them to historical Ordnance Survey maps that often illustrate these features clearly too:

Comparing OS six-inch inch 1888-1913 mapping (left) with LiDAR DTM (right) for Tomen y Mur Roman fort, Merionethshire

It can also be useful to compare LiDAR to modern satellite imagery for earlier pre-Roman hillforts and related features like cultivation terraces:

Comparing modern Bing Imagery (left) with LiDAR DTM (right) for Hownam Rings hill fort, Roxburghshire

Comparing OS six-inch 1900s mapping as an overlay with LiDAR DTM (background) for White Meldon hill fort, near Peebles

Features like old limekilns that not always clearly marked or labelled on maps, can be confirmed by comparison with LiDAR layers:

Comparing OS 25 inch 1892-1914 mapping (left) with LiDAR DTM (right) showing old limekilns (including some within woodland) within Campsie parish, Stirlingshire
Comparing OS 25 inch 1892-1914 mapping (left) with LiDAR DTM (right) showing old limekilns within Campsie parish, Stirlingshire. Note that not all the limekilns picked out by LiDAR were recorded by Ordnance Survey.

The effects of glaciation on local topography, showing the direction of glacial flow across the landscape can be brought out by LiDAR imagery:

Comparing the OS 1:25,000 1937-61 mapping (left) with the LiDAR DTM layer (right) to bring out ‘crag-and-tail’ glacial trends north of Bathgate
Comparing the OS 1:25,000 1937-61 mapping (left) with the LiDAR DTM layer (right) to bring out ‘crag-and-tail’ glacial trends from west to east north of Bathgate.

Contemporary landscape features like quarries, railway cuttings, or motorway embankments can be usefully compared alongside modern satellite imagery:

Comparing Bing Satellite imagery (left) with LiDAR DTM (right) for Dunbar Cement Works and quarry.

Detailed terrain modelling allows soil scientists to see slope changes and landform breaks which indicate patterns in soil spatial relationships:

Comparing LiDAR DTM (left) with Soil Survey (right) in the upper Tweed valley

Historic Ordnance Survey maps showing man-made structures such as canals, quarries, and locks can be understood more clearly by comparison with LiDAR imagery:

Comparing OS 25 inch 1884 mapping (left) with LiDAR DTM (right) on the Somerset Coal Canal

Acknowledgements

We are very grateful to Richard Pearson for testing, downloading and processing the Scottish LiDAR data to generate these results. We would also like to thank Dr Ed Peveler for much advice and for the original idea of doing this work, and Jeremy Booth for solving the problem of how to download the LiDAR data in bulk.

This page has been put together with helpful advice and contributions from Paul Bishop, Richard Pearson and Ian Roy.

Further information


We hope you have found some of this information new and useful. If you have further suggestions or comments, please do contact us at maps@nls.uk.

View our other Research guides.