Department of Archaeology and Anthropology, University of Bristol. Email: email@example.com (http://orcid.org/0000-0002-3587-9073)
Cite this as: Gray, S. (2015). UAV Aerial Survey - Blackquarries Hill Long Barrow (Data Paper). Internet Archaeology, (38). https://doi.org/10.11141/ia.38.9
The dataset has been deposited with figshare doi: 10.6084/m9.figshare.2275172
Referee statement by Toby Driver
The aim of this project was to conduct a low-altitude aerial survey of Blackquarries Long Barrow (ST77509320) in order to produce a re-usable dataset (Gray 2014c), to within a known degree of accuracy. A second objective was to construct and make available an exemplar UAV survey dataset.
A custom-build hexacopter UAV (Unmanned Aerial Vehicle) was used to collect data, running Ardupilot 3.1 firmware in conjunction with Mission Planner v1.1. The flight was conducted at approximately 25m above ground level at a vertical speed of approximately 2.5m/s. The camera used was a Canon Powershot S110 running CHDK 1.2.0 and a standard intervalometer script. The intervalometer script was set to take a photograph every 5 seconds. The number of photographs taken was ultimately constrained by flight time, limited by the capacity of the battery used to power the hexacopter.
Ground Control Point (GCP) markers were used to support the georeferencing of the photographs. GCPs were placed at intervals of approximately 10m. GCP location was established using a Garmin GLO device. The Garmin GLO is accurate to within 3m (manufacturer's error rate).
The dataset was collected via low-altitude UAV survey of Blackquarries Hill Long Barrow (ST77509320) and comprises metadata records, near-vertical photographs and a derived 3D polygonal mesh.
Known locally as the 'Clump' this structure is generally considered to be a long barrow, possibly Neolithic (National Trust HBSMR n.d.) According to Crawford (Crawford 1925) the structure has been considerably mutilated by stone digging leaving only "amorphous remains".
The dataset is intended for use in comparative analysis of long barrows in similar states of decay. It is also intended to provide an initial point for 'time series' analysis through which deterioration of the site can be assessed. A second driver for creating this dataset was to act as an exemplar for the standardisation of results generated via low-altitude aerial survey.
The dataset includes a great deal of visual information relating to the site. However, realising the potential of this visual information is limited by currently available processing methods and software (e.g. the generation of a 3D point cloud is limited primarily not by the information contained within the image set but by the current generation of photogrammetry software). Therefore it is vital to make such primarily datasets available in full for future re-use. Transparency of data processing methods has been facilitated by the inclusion of unprocessed images and lens calibration photographs. The dataset prioritises open technologies and is accompanied by rich metadata to help ensure maximum data re-usability. The 3D mesh was generated using AGISoft Photoscan 1.0.4 software and is intended to act as an example of a derived data object.
The dataset relates to two guides published by JISC (Gray 2014a) and the Archaeology Data Service (Gray 2014b).
National Trust HBSMR n.d. NT HBSMR Number: 75402. http://www.heritagegateway.org.uk/Gateway/Results_Single.aspx?uid=MNA140&resourceID=6 [Accessed 1 December 2014].
Crawford, O.G.S. 1925 Long Barrows of the Cotswolds, Gloucester: John Bellows.
Gray, S. 2014a Archaeology use case: 3D surveying with a UAV. JISC Digital Media. http://www.jiscdigitalmedia.ac.uk/infokit/3d/uav-survey [Accessed 1 December 2014].
Gray, S. 2014b UAV Survey: A Guide to Good Practice. Archaeology Data Service/Digital Antiquity: Guides to Good Practice. http://guides.archaeologydataservice.ac.uk/g2gp/AerialPht_UAV [Accessed 1 December 2014].
Gray, S. 2014c UAV Aerial Survey - Blackquarries Hill Long Barrow (ST77509320), figshare, http://dx.doi.org/10.6084/m9.figshare.1275172 Retrieved 10:37, Mar 09, 2015 (GMT)
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Toby Driver, Royal Commission on the Ancient and Historical Monuments of Wales
Cite this as: Driver, T. (2015). 'Referee Statement' in Gray, S. (2015). UAV Aerial Survey – Blackquarries Hill Long Barrow (Data Paper). Internet Archaeology, (38). http://dx.doi.org/10.1141/ia.38.9
This dataset describes a low-altitude UAV (Unmanned Aerial Vehicle) survey of Blackquarries Long Barrow near Wotton-under-Edge in the Mendip Hills, Gloucester, carried out at low cost with a custom-built hexacopter in order to test issues of archaeological UAV data re-use.
UAV surveys are becoming widely accessible and very cheap to carry out, meaning that there are few barriers to archaeologists, students and researchers obtaining low altitude aerial photography of a given site and producing further photogrammetric or survey products from the data including digital terrain models. There remain caveats to the low cost and ease with which such surveys can be carried out, not least current restrictions in aviation law regarding UAV flights (comprehensively reviewed in Gray 2014a) and the payload capacity of a given UAV which will govern the weight and therefore the quality of the camera that be carried; a cheaper, lighter camera may still yield reasonable images but these may not be of sufficient quality or resolution to be suitable for more detailed analysis or long-term archiving. Problems of archiving datasets produced from the increasing number of UAV surveys, particularly issues of re-use and forward compatibility of data derived from onboard GPS or IMU systems, are also still faced by some archives but the key lies in thorough metadata documentation. Stephen Gray, who authored both current good practice guides for UAV survey (Gray 2014a; 2014b), is fully conversant with all these issues and has been central to promulgating good practice guidelines for the wider discipline.
Against this background of a rapidly developing methodology in field archaeology and remote sensing, the Blackquarries dataset is refreshingly comprehensive and transparent. Gray has completed a relatively straightforward survey of an archaeological site both as a testing ground for a fully re-usable dataset and as a management tool to provide measurable baseline data for future monitoring, something of interest to all heritage managers. He notes the mass of image data obtained from the survey, and the current limitations of software to realise the full benefits of this dataset beyond the production of 3D point clouds and animations of the 3D surface produced. A range of text, image and data files is provided including flight plans, camera calibration information, Eastings, Northings and altitude information for each image. Some specialist data formats such as GPX files (a transfer format for GPS data) may suffer from issues of future compatibility, but comprehensive metadata should negate this. Data sharing for case studies is becoming increasingly commonplace. Although the Blackquarries Long Barrow may only be of local or regional archaeological interest, the publication of this dataset has wider resonance to UAV survey in Britain and beyond.