Although the recent drought has meant that many historical sites have ‘appeared’ on the ground, as crop or parchmarks (fig.1), it has been 42 years since the last really hot summer in 1976 when this occurred - and clients can’t always wait this long to discover what is below the ground.
Fortunately, with ever improving detection and mapping technologies, getting information from a site area has never been as easy or cost-effective. Two techniques which can help are Ground Penetrating Radar and UAV Photogrammetry.
Above: Digital photograph of parchmarks revealing former formal garden below lawn, see online Daily Mail for more information.
Firstly, what are both survey techniques?
Ground Penetrating Radar: -
The advent of Ground Penetrating Radar (GPR) can be traced back to as early as 1910. After years of development in radio technologies, digital signal processing (DSP) as well as breakthroughs in the software field, GPR has become a highly valuable tool for many varied applications. These include underground infrastructure and resource mapping, mine detection, archaeology & geophysics, and engineering applications.
The technique works by pulsing electromagnetic waves into the ground and measuring the strength and time delay of the returning signal. This allows the approximate depth as well as the density of the buried object to be estimated. In the accompanying photograph a single time slice shows the level of detail which can be obtained from Ground Penetrating Radar. The advantage over a single digital picture (Fig. 1) is that the GPR data can ‘see’ into the ground and provide ‘pictures’ at ever increasing depths, up to about 2m on archaeological sites.
Above: High density Ground Penetrating Radar Results of formal gardens by SUMO Geophysics
Aerial Photogrammetry: -
Photogrammetric Surveys enable rapid data capture, using specialist cameras fitted to Unmanned Aerial Vehicles (UAV's). The methodology is as old as modern photography, dating to the mid-19th century, but it is only in recent years and with the rising popularity of UAV's that this form of photogrammetry has taken to the skies.
For mapping purposes, overlapping digital images are recorded which allow the survey data to be combined. In the past, where a single stereo pair of images were used, nowadays photogrammetry software can analyse oblique (both horizontally and vertically) and convergent images, as well as parallel images, and this helps minimise any systematic errors. Current software even permits the use of oblique images from flights that circle the subject, rather than flying in the more typical overlapping swaths used for aerial mapping image but distance measurements, so that height information can be recorded.
Photogrammetry is used in fields such as topographic mapping, architecture, engineering, manufacturing, quality control, forensic investigation, and geological mapping. Archaeologists use it to quickly produce plans of large or complex sites, particularly on earthwork sites where features are visible above ground. See: Historic England for the Photogrammetric Applications for Cultural Heritage.
Above: This is a photogrammetric model produced by SUMO Aerial-Cam of Brancaster Roman Fort, showing the raised platform of the fort in the main field and the sea marshes beyond.
How are the techniques combined and what are the benefits?
The techniques can be combined to overlay the individual data to obtain a better 'image' of the detected features. Superimposing Ground Penetrating Radar data onto photogrammetry is a good method of visualisising the GPR data within the ‘space’ and gives you points of reference. As well as this, the advantage of having measured height differences (from photogrammetry) is that you can make topographic corrections to the GPR data thus increasing the accuracy of the depth information - GPR software assumes that data are collected from a level surface. See below an example of GPR data superimposed onto photogrammetry.
Above: Photogrammetry data by SUMO Geophysics.
Above: Geophysical data and photogrammetry by SUMO Geophysics.
Are there any limitations?
When carrying out GPR (or any geophysical method – magnetic or electrical) and you wish to use the results with a photogrammetric image it is necessary to establish common ground control points using GPS so that the two (or more datasets) can be georeferenced and tied into the same image mapping. This is especially important if the two survey methods can’t be carried out at the same time because varying weather conditions can hinder the use of UAVs and cause increased costs.
Additionally, Photogrammetric surveys must be tied into GPS fixed ordnance datum (OD) points otherwise they are floating in the air with no accurate height information.