Integrated Geomatic Techniques for Georeferencing and Reconstructing the Position of Underground Archaeological Sites: The Case Study of the Augustus Sundial (Rome)

A large part of the archaeological remains still to be discovered and excavated are not inremote and depopulated areas of the earth but are often beneath urban centres that have buriedthem with centuries of debris and later constructions. Excavating in these contexts is much morecomplex than digging in rural or sparsely inhabited areas because of the constraints imposed byexisting buildings and infrastructure. It should also be considered that within an urbanised area,any archaeological remains are concentrated in the subsoil of the historic centre, which is, therefore,often surmounted by buildings that are more recent than the remains but historical as well, and thus,of considerable value and vulnerability. For this reason, an archaeological excavation in an urban areamust be preceded by a real feasibility study, where the potential risks for the structures above areminimised and accurately quantified. In many situations, as in the case under study, the discoveryof a small segment of a structure is the only clue to reconstruct the development of the remainingpart still to be excavated, which may stretch tens or hundreds of metres away from the measurablepart. As a consequence, an error of a few centimetres in the survey of the excavated part can lead toerrors of metres in estimating the positions of the far parts still to be excavated, and this, in manycases, as in the one under study, must absolutely be avoided. In practice, high-precision geomaticsurveys, in support of the archaeological and historical interpretation of the observable structures,will help to establish the exact locations to possibly continue the excavations, helping the accurateplanning of the excavation itself. Here, we have shown how the various techniques, compared toeach other, have made it possible to reconstruct the location of a short stretch (less than 7 m) of theEmperor Augustus’ Sundial, the only currently visible evidence of a scientific instrument of imposingdimensions (tens of metres in length and height) that served to define some of the characteristics ofthe calendar that we still use today. The portion of the sundial currently observable, according to themost reliable hypotheses, is located approximately at one end of a structure and extends for severaltens of metres. The accurate positioning of the observable parts in a geodetic reference system willenable to identify with certainty the possible areas in which excavation may continue and will alsoallow to accurately reconstruct the principle of operation of the sundial through an approach that could be defined as “reverse engineering” of the scientific instrument itself. The aim of this work isto study and thus define the combination and integration of existing geomatic techniques for thisspecific field of application.