What is LiDAR?
LiDAR stands for Light Detection and Ranging, and is a form of remote sensing. This type of data capture essentially records the height of individual sections of the ground (and any features existing above it) in order to map the earth's surface. This form of technology has been advancing rapidly in recent years, with 2 m and 1 m resolution datasets now widely available in the United Kingdom, thanks to an open-access portal operated by DEFRA. Independent surveys commissioned over a smaller area using private contractors can capture even greater resolution which has allowed the precise mapping of archaeological features with an accuracy never seen before within the world of remote sensing, especially in the context of planning.
The applications of this technology for the study of the historic environment have been explored and welcomed by Historic England - the leading advisory body for the Cultural Heritage Sector in the United Kingdom.
How Does it Work?
LiDAR uses light in the form of a laser to measure distances between the laser source and the surface of the ground. This is accomplished by calculating how long it takes for the laser to bounce off the surface of the earth, and return to the sensor located at the source of the light beam. Given that the speed at which the light travels remains a constant, the distance that the light has travelled can be calculated using mathematical equations. T
he position of the sensor is accurately tracked through the use of GPS systems which enables the point to be spatially mapped accordingly. By rapidly capturing a multitude of points, a point cloud can be generated which maps the surface of the earth, and any features above it, to provide a detailed impression of the local terrain and any earthworks which may be indicative of archaeological features.
This can be compared to how the individual pixels in a digital photograph are combined to make one complete image. The points are converted as such into a 'raster' image format which can be imported into GIS mapping softwares. It can then be processed in different ways to produce the desired outputs.
Applications
Desk Based Assessments
LiDAR plays a major role in assessments of archaeological potential, such as those conducted within a Desk-Based Assessment. This constitutes a piece of research which utilises existing archaeological, historical, and environmental data to determine the potential for archaeological assets to survive within a given site, as well as assess the impact a proposed scheme could have upon their significance. Through the creation of Hillshades - a tool used to enhance visual perceptions of the LiDAR data - illustrations can be produced demonstrating either the presence or absence of any earthwork features within a proposed development site.
Historic Environment Consultants have sufficient expertise to interpret LiDAR data to identify whether any raised earthworks or ditched features constitute archaeological features, as opposed to natural geological manifestations or even modern disturbances. This requires an adequate understanding of the local geology of the development site which can be provided by online mapping services, such as that published by the British Geological Survey.
Reference manuals play a great role in allowing us to distinguish putative archaeological features from geological features and modern disturbances. One example is Natural Solution Holes caused by the erosion of chalk deposits due to acidic rainwater. These features are often difficult to distinguish from Dene-Holes or mineral extraction sites in areas rich in clay and chalk. Modern field drainage systems are also extremely recognisable as 21st century features, although elsewhere, isolated drainage pipes are less distinguishable unless accompanied by service plans to aid interpretation of these sites.
You may also see LiDAR consulted in the same way within Heritage Impact Assessments which specifically include the assessment of archaeological potential in the project brief. This is most often the case where groundworks are included as part of the development proposals. This can include householder extensions, the laying of services, introduction of new hard landscaping, changes to boundary treatments, or the digging of foundations for a new structure. However, it is important to note that Heritage Impact Assessments will only include the assessment of archaeological potential when both of the following apply:
Assessment of archaeological potential has been included in the project brief. This is often based on the advice of the Historic Environment Consultants assigned to the project and/ or due to the requirements of a local authority.
Your Historic Environment Consultants cover both Built Heritage and Archaeology. Blue Willow Heritage operate in both of these fields, but many consultancies explicitly exclude the assessment of archaeological potential in their reports. It is not appropriate for individuals to conduct assessments beyond their realm of expertise. As such, it is important to pick your design team carefully!
When combined with site surveys in addition to the consultation of cartographic sources and archival research, LiDAR is an invaluable tool for identifying features of archaeological significance. It is a requirement of the National Planning Policy Framework (NPPF) that the archaeological resource within a development site be considered, and that the impact of any proposals upon features of suspected heritage significance be properly assessed. Inadequate assessments of archaeological potential may necessitate further costly archaeological mitigation work that may not have otherwise been required. Identifying any features at an early stage in the design process also works to avoid costly re-designs which will inevitably cause further delays to your project.
Heritage Impact Assessments (HIA)
LiDAR is also an invaluable tool for Heritage Impact Assessments, even where a proposed development does not include any groundworks or features of suspected archaeological significance. Illustrations created from LiDAR data can reduce or even eliminate the element of subjectivity when establishing any impacts to 'views' of a development site. By mapping the surface of the earth - in addition to intervening obstacles such as trees and buildings - we can visually demonstrate how visible a site is within its immediate context and wider landscape.
Simple illustrations included within some of our standard reports include contour illustrations which allow for a better visualisation of the topography of a proposed development site in relation to its surroundings. These contours can be derived from the LiDAR data and are processed in QGIS. In this way, any nearby Listed Buildings, Conservation Areas, or Public Rights of Way can be mapped in relation to the topography of the development site. Any steep slopes with the potential to reduce visibility of the site will be clearly shown on this mapping, as opposed to basemaps which provide no indication of the local terrain.
Where visibility is the key barrier to a proposed development, specialist 'ZTV Model' or 'Viewshed' reports can be commissioned from Blue Willow Heritage. These reports establish which portions (if any) of a development site are visible from a specific location - such as a nearby Listed Building. Visibility can also be assessed on a sliding scale. For instance, an illustration could tell us that, from any given point, only 50% of the site is visible.
This is demonstrated visually through the use of colour ramps. Alternatively, an illustration could establish from which points in a defined study area (eg. 1 km radius) the development site is visible from. Our reports include a method statement to ensure that the conclusions of the report are justified, repeatable, and grounded in the data.
How Are We Keeping Up With the Industry?
Our team attended the Aerial Archaeology Research Group in September (2024) to stay up to date with the latest developments in our industry, with a particular focus on archaeological prospection. Tiffany and Ailsa also presented a poster in the display room on the topic of an 'Integrated Approach to Archaeology and Built Heritage in Planning' which was well received by organisations such as Historic England. Members of the roundtable discussion were strongly of the opinion that, though many archaeological prospection techniques have been proven to be cost effective, this is not well-integrated into the planning system.
As previously mentioned, this actually causes costly delays in the planning process and has sometimes caused friction in the relationship between local authorities and developers. This is especially when requests for the assessment of archaeological potential come relatively late in the process, resulting in time-consuming redesigns. A gap in the expertise of Heritage Consultants was also identified, considering the fact that, whilst access to LiDAR data is now widespread, individuals often lack sufficient expertise to interpret it, resulting in Heritage Impact Assessments being submitted without the assessment of archaeological potential. As such, an urgent call for training was made, with organisations including ourselves volunteering to try help fill this gap in knowledge across our sector.
At Blue Willow Heritage, we provide expert advice on planning in relation to the historic environment. We incorporate LiDAR data into all of our assessments of archaeological potential which are interpreted by our expert team. Should you be in need of a Desk-Based Assessment, or want your Heritage Impact Assessment to show zones of theoretical visibility, then Blue Willow Heritage can help!
If you would like to discuss your project or simply need some impartial, no-obligation advice, then please do not hesitate to get in touch.
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