Aerial Investigation and Mapping
Aerial photographs, and the mapping derived from them, should be an intrinsic part of any assessment of the historic environment.
From 2011 to 2014 over 14,000 previously unknown sites were added to the historic environment record from Historic England aerial survey projects using National Mapping Programme (NMP) standards. The impact of recent research is clear in our wide ranging and exciting NMP projects.
Archaeological research has long benefited from the use of aerial photography, revealing sites that are often difficult or even impossible, to see on the ground. New technologies such as airborne laser scanning usually called lidar, are adding to the toolkit of the aerial surveyor.
Interpretation and mapping of sites brings together information on buried features revealed as seen as cropmarks, soilmarks,or parchmarks or features visible on the surface such as earthworks and structures. The maps and records allow a better understanding of past landscapes to inform research and management strategies.
Transforming our understanding
Aerial photographs held in national and local archives are a huge resource for studying archaeological sites and landscapes. The best way to collate and understand the archaeological information on aerial photographs is to map it accurately and to provide interpretations of the sites and landscapes.
This allows easy access to the information for research and further work using different techniques. The mapping also provides essential information on the extent and nature of archaeological sites to inform the planning process.
To be effective, the information from aerial photographs should be integrated into Local Historic Environment Records.
Archaeological air photo interpretation and mapping provides information for:
- Identification and understanding of buried sites visible as cropmarks and soilmarks as well as those surviving as earthworks or standing structures.
- Identification and understanding of structures and earthworks.
- Assessments of monument condition over time.
- Strategic planning and research to inform mitigation in certain circumstances including major infrastructure projects e.g. road, rail and Growth Areas, aggregate extraction, or threats from coastal erosion, accretion and sea level changes.
- Data to inform management of protected landscapes e.g. Areas of Outstanding Natural Beauty, National Parks, World Heritage Sites.
- Strategic information to inform national heritage protection initiatives e.g. environmental/farm stewardship schemes, national designation.
- Data to inform the local planning process including key information for desk based assessments prior to proposed development work (such as those listed above).
- Data for the application of heritage protection initiatives – identification of regionally and locally important sites, information for farm stewardship through the Selected Heritage Inventory for Natural England (SHINE).
- Information to enhance landscape characterisation studies.
- Information and sources for outreach and training for land managers, local communities etc.
National Mapping Programme standards
The Historic England standard for air photo mapping and recording is applied to projects under the banner of ‘The National Mapping Programme’ (NMP).
The scope of work involving aerial photographs will vary depending on the circumstances and user requirements but standards should be applied to ensure methods and products are suitable for research, planning and protection of the historic environment.
Standard sources for any archaeological mapping project should include the Historic England Archive (formerly the National Monuments Record), the Cambridge University Collection of Aerial Photography (CUCAP), relevant local collections, such as those held by local authorities in Historic Environment Records (HER), and online sources of aerial photographs, such as Google Earth.
Other remote sensed data, such as satellite or lidar (airborne laser scanning) images, should be used where readily available.
Background material should be reviewed from published sources and the data held in the Historic England Archive and local HERs. Other data such as historic maps, soils and geology maps should be used.
The best available base mapping (including height data) should be used for rectification and georeferencing of aerial photographs. This is typically 1:2500 scale and can be derived from Ordnance Survey mapping or specially georeferenced vertical aerial photographs.
Some sources, such as web-based providers of photo mosaics, do not always provide details of locational accuracy. Therefore, it is important to understand and explain the accuracy of your base map, and resultant archaeological mapping.
A landscape view
Mapping from aerial photographs is most effective when all archaeological features are included, not just those of specific relevance to the researcher. Therefore, features will be recorded with a date ranging from the Neolithic, through to near the present, including 20th-century military features.
All archaeological features visible on aerial photographs should be mapped whether they are buried features revealed as seen as cropmarks, soilmarks,or parchmarks or features visible on the surface such as earthworks and structures.
The composite map encourages a layered view of the landscape, where the aerial evidence, and information from other sources and survey techniques, provides glimpses of the changing use of an area over thousands of years.
This is an important viewpoint in terms of heritage protection which considers all known aspects of the past land use in the context of managing future change.
Using aerial photographs in the archives taps into a record of up to 100 years of landscape change including different agricultural land use, urban development and coastal erosion. Mapping from aerial photographs and lidar can therefore chart the condition of monuments over time.
Projects record the basic condition of the archaeological features, for example earthwork, levelled earthwork, cropmark or destroyed monument/demolished building. This enables analysis of the effects of 20th and 21st century farming and development on the archaeological features.
The standard products from air photo projects should be a digital archaeological map with archaeological descriptions and a report. Descriptions of sites should include interpretation of date, function and at least a basic record of the condition such as cropmark, earthwork, or levelled earthwork.
A report should discuss the character, diversity, association and distribution of archaeological sites, visible on aerial photographs, within a project area.
The report should also explicitly explain the methods used and sources consulted and any limitations in the data. Assessment and mapping also identifies why and where there may be gaps in the aerial photograph evidence.
Information on planning projects involving aerial photographs can be found in the MoRPHE Project Planning Note 7 - Interpretation and mapping from aerial photographs and other aerial remote sensed data.
Using archaeological mapping
It is important to understand the nature of archaeological mapping supplied by a professional air photo interpreter, the HER or the Historic England Archive.
Sources used and scope of mapping vary greatly depending on the original project needs. Although most data is now supplied in digital format, this may vary greatly in accuracy depending on how the data was gathered. Even digital data may have different levels of information attached.
New discoveries are being made all the time through aerial reconnaissance and when other new sources, such as lidar, become available. Other investigation techniques add to our knowledge and understanding of past land use. The archaeological map of England can always be updated.
Assess what is required for your project and make the nature and accuracy of sources of archaeological mapping clear when writing reports. In many cases new mapping adapted to the needs of the client will be required to ensure that the results are up to date and fit for purpose.
Historic Places Investigation