Studying the hinterland of Portus
We have been using several different techniques and integrated strategies as part of our ongoing fieldwork to understand the hinterland of the port and the Tiber delta as a whole,
There are many existing forms of archaeological and topographical data we can use which cover the entire delta area:
Geological data gives us an understanding of the deeper processes that have combined to form the present landscape, in terms of Pleistocene activity and the Holocene development of the Tiber and the progradation of the delta,
Modern remote sensing data such as satellite imagery and LiDAR;
Aerial photographic records dating from the early part of the 20th century, through the years of World War II, and into the 1950s and 1960s.
Together, these enable us to analyse and identify features associated with the development of the delta, pre-Claudian use of the area, the creation and development of Portus and the wider impact of the port on the surrounding hinterland.
These kinds of data have been integrated with the results from several seasons of topographic survey, fieldwalking and extensive geophysical survey around Portus and on the Isola Sacra to provide evidence about the palimpsest (superimposed layers) of archaeological remains in the hinterland to the west and the south of the port. The nature of the different methods of data collection for remote sensing makes comparison across seasons imperative. Ground conditions are such that the quality and effectiveness of air photographs and satellite data can vary from season to season, based on the time of year, rainfall, aridity and a number of other factors. Thus remote sensing not only allows the archaeologist to consider archaeological features from a very broad area, but also different datasets and what a combination of different data might tell us.
For example it has revealed the extensive system of canals and waterways that linked Portus with the Tiber, Ostia Antica and the Tyrrhenian sea. Deep core samples have been taken from some of these by Ferreol Salomon and Jean-Philippe Goiran, and are discussed in a later step. It has also provided evidence for smaller scale features such as the system of small canals which demarcated fields on the Isola Sacra to the south of Portus and would have provided drainage to the area during periods of flooding when the Tiber was in spate.
By using a combination of datasets we have identified an ancient cemetery along the Via Flavia between Portus and Ostia Antica and, more recently, warehouses and a defensive wall associated with Ostia Antica to the north of the Tiber which have changed the way in which we perceive the hinterland of the port.
Another example of the benefits of complementary datasets came in the discovery of the Portus to Ostia canal. This canal on the Isola Sacra was not detected in the topography or satellite imagery, but was clearly visible in Aereo Militare air photographs from the 1950s, due to the vegetation and conditions on the ground when the photographs were taken. Post-war air photos by private commercial organisations also exist. These photos provide high resolution coverage of the Tiber Delta at high and low altitude, showing landuse, archaeological features and areas of seasonal flooding related to the lagoons of the Tiber Delta. We can then digitise different anomalies to create a map of archaeological features across the entire delta area.
In addition to airborne photography, use of satellite imagery has been essential in developing our understanding of the hinterland of Portus. This data includes low resolution digital elevation model files, and low resolution satellite imagery. Data such as that provided by Quickbird and Worldview 2 satellite is also allowing us to look at high resolution imagery as panchromatic (greyscale) data at c. 0.8m pixel resolution, and multispectral data at 1.8m resolution. The panchromatic data allows us to see basic forms of possible archaeological remains, however, the multispectral data allows us to use the different colour bandwidths of the data based on the red, green and blue channels of the imagery, to locate changes in the light spectrum caused by archaeological remains. This imagery has been useful in picking out the pattern of meander deposits close to the Tiber north of Ostia Antica, and locating canals associated with Portus. Again we can digitise anomalies that are present in the dataset to create a map of the archaeology across the hinterland.
The magnetometry in the area picked up the full line of the canal. Other features which were present in the satellite imagery were not visible in the magnetometry due to the nature of the archaeological deposits and a lack of strong magnetism. Therefore the use of many different data sources is crucial to develop a full and nuanced understanding of the hinterland of Portus.
© University of Southampton, 2015