Mediterranean Archaeology and Archaeometry, Vol. 14, No 3, pp. 55-65 Copyright © 2014 MAA Printed in Greece. All rights reserved. HIGH-PRECISION GPS SURVEY OF VIA APPIA: ARCHAEOASTRONOMY-RELATED ASPECTS Giulio Magli1, Eugenio Realini2, Mirko Reguzzoni3 and Daniele Sampietro2 1Faculty of Civil Architecture, Politecnico di Milano, Milan, Italy 2Geomatics Research & Development (GReD) s.r.l., c/o Politecnico di Milano, Polo Territoriale di Como, Como, Italy 3Department of Civil and Environmental Engineering (DICA), Politecnico di Milano, Milan, Italy Corresponding author: Giulio Magli ([email protected]) ABSTRACT Via Appia was built by the Romans around 312 BCE to connect Rome with Capua dur- ing the Samnite wars. The road is an astonishing engineering masterpiece. In particular, the segment which runs from Collepardo to Terracina – 61 km long – is renowned for be- ing virtually straight; however this “straightness” was never investigated quantitatively. As a consequence, the techniques used by the ancient surveyors and their scope – wheth- er it was only practical, or also symbolic – remain obscure. We report here a high- precision GPS survey of the road, performed with a u-blox receiver and further checked with a dual frequency receiver. We give a detailed analysis of the methods used and of the errors, which are shown to be less than 6’. To our knowledge it is the first time that such a long ancient manufactured structure has been surveyed with such a high accuracy. The results lead us to conclude that astronomy was certainly used in the construction of the road and in that of the associated grid, oriented to the setting of the star Castor and to the cardinal points respectively. KEYWORDS: GPS, azimuth estimation, satellite image georectification, archaeoastrono- my, Via Appia, centuriation. 56 GUILIO MAGLI et al. 1. INTRODUCTION ical and/or hydrographical considerations; The issue of the accuracy required in ar- there exist, however, clear and documented chaeoastronomical analyses is a delicate cases (Trousset, 1997) in which the orienta- one. There are, indeed, times when the ac- tion was clearly astronomical (to the sol- curacy required for obtaining sound results stices or to the cardinal points), and an is relatively low and/or the quantity of da- astonishingly accurate case of astronomical ta calls for quick methods of relief; for in- orientation of grid and roads will be pre- stance, for the collection of hundreds of sented in this paper. The centuriation in pieces of data about many scattered dol- question, although already discovered in mens, a combined compass-clinometer cer- the 1970s (Cancellieri, 1990), is poorly tainly suffices (Hoskin 2001). However, in known, while the road is the most famous other cases accuracy must be very good. among the Roman roads. It is Regina This is the case when ancient architectures Viarum, ‘the queen of the long roads’, or as were oriented with astonishing precision, the Romans called it, Via Appia. for instance, the pyramids of Giza (Magli This road, constructed by the Roman 2009, Nell and Ruggles 2014), and/or when consul Appius Claudius around the year they exhibit perfectly conserved and clearly 312 BC to connect Rome with Capua, is an measurable features, as in architectural astonishing work of engineering (Quilici, projects of ancient Greece (Belmonte & 1990). In particular, the first section of the Hoskin 2002) and ancient India (Malville road, leading from Rome to Terracina, is 2000). composed of two straight segments con- Another context in which high-accuracy nected by a short zigzag section aimed to methods of modern survey are always cross the Alban hills at Colle Pardo. The needed is clearly the study of ancient high- first of these segments runs for 26 km. The accuracy surveys. Among these, as is well second crosses the Pontine Marshes, going known, are many of the land-forming straight for as far as 61 km. works carried out by the Roman mensores. To our knowledge, no one has ever tack- These works, called centuriations, were led seriously the problem of the project of regular divisions of huge territories accord- this road. Further, although it is well ing to orthogonal grids of lines. These divi- known –and actually obvious to anyone sions had multiple objectives: assigning the even today– that the path proceeds with lands of new colonies, establishing the Ro- impressive straightness, the accuracy asso- man rule and cadastral control, regulariz- ciated with this “impression” was never ing the hydrology of the area by tracing analyzed quantitatively. For instance, it is canals along the grid, and finally renovat- usually taken for granted in the literature ing or creating ex novo a system of roads. that the segment crossing the Pontine Roman land-forming through centuria- marshes runs with azimuth 135°; that is, tion is a very old system, as it traces back to along the northwest/southeast direction. the early fourth century BC (Settis 1983), Such a feature – if true – would have been and there exist famous examples which of great help in the project of the road. In- show the accuracy of the Roman surveyors deed, astronomy could be used to deter- and their ability to maintain it over vast mine the north celestial pole, and then the territories. Perhaps the most astonishing groma (the instrument of the Roman sur- centuriation is the one carried out in the veyors) could be used first to find the me- territories of modern Tunisia by the 3rd ridian and then to bisect it two times to ob- Augusta Legion (Caillemer and Chevallier, tain the northwest/southeast direction. 1957). However, as we shall see, although the The orientation of the centuriation grids Romans did survey all the associated terri- was in many cases dictated by topograph- tory with a cardinally oriented grid, they © University of the Aegean, 2014, Mediterranean Archaeology & Archaeometry, 14, 3 (2014) 55-65 HIGH-PRECISION GPS SURVEY OF VIA APPIA 57 did not choose the intercardinal azimuth Specifically, the u-blox antenna was posi- for the road. tioned on the rooftop of a car (see figure 1) The aim of the present paper is to pre- driven at an almost constant velocity of 60 sent the first high-precision GPS survey of km/hour from Cisterna di Latina to Torre the Colle Pardo – Terracina segment and of Elena (round trip), with the receiver re- the associated Roman grid, while the com- cording observations at the frequency of 1 plete historical and architectural analysis is Hz. provided in a companion paper (Magli et al., 2013). Many technical difficulties had to be solved and, to our knowledge, this is the first time that an ancient artifact stretched for so many tens of kilometers is analyzed with such a high accuracy. 2. THE GPS SURVEY OF VIA APPIA FROM ROME TO TERRACINA The first section of Via Appia, starting Figure 1. The u-blox antenna positioned on the rooftop of the car. from Rome, runs straight for about 26 km, then the road crosses the Albani hills. The The Latina permanent GPS station was second straight section, which is the main used as the master station in a double- focus of the present work, crosses the Pon- difference GPS positioning, and a Kalman tine marshes and proceeds to Torre Elena, filter was applied in order to reduce the near Terracina, for as far as 61 km. A very measurement noise (for details on the GPS long section of this path, about 41 km, from processing with goGPS, see Realini and Cisterna di Latina to Torre Elena, is cov- Reguzzoni 2013). ered by the modern road, which has been In this way the geocentric Cartesian co- traced along the ancient one. A few visible ordinates and the corresponding geodetic remains, such as Clesippo’s mausoleum in coordinates, i.e., latitude, longitude and Mesa, survive here and there along the ellipsoidal height, respectively, of more modern street, as do scattered blocks of the than 5000 points, with an expected accura- ancient crepidines (platforms) on the west cy better than 1 m, were retrieved. The two side. coordinate systems are related by the rela- The survey of this segment of the Via tions: Appia was performed with a u-blox AEK- 4T GPS receiver processing data with X=( Nh + )cosϕλ cos goGPS software (Realini and Reguzzoni, Y=( Nh + )cosϕλ sin (1) 2013, http://www.gogps-project.org), and Z= N1 −+ 2f f2 + h sinϕ further checked with a dual-frequency ( ) Leica 1200 system GPS receiver processing data with LGO software (Leica Geosys- a tems, 2006). Note that the present research, where N = is the ra- 2 2 apart from the analysis of the Roman pro- 1−−(2f f )sin ϕ ject of the Via Appia, had also the practical dius of curvature of the prime vertical, i.e. scope of investigating the possibility of us- the right, and 2.54cm top and bottom. The ing relatively low-cost GPS receivers, like distance from the surface to the Z-axis the u-blox receiver, instead of the more ex- along the ellipsoid normal, a and f the pensive dual-frequency receivers, for the semi-major axis and the flattening of the purpose of high-precision topographical ellipsoid respectively. In order to estimate and archaeoastronomical surveys. the azimuthal direction of Via Appia, the © University of the Aegean, 2014, Mediterranean Archaeology & Archaeometry, 14, 3 (2014) 55-65 58 GUILIO MAGLI et al. coordinates of these points have been ex- tion from the meridian is smaller than 17 pressed in a local east, north, up (enu,,) km (i.e. δλ ≤°0.15 ), corresponding to a reference system centered in the middle of maximum error in the north definition the survey, close to Borgo Faiti.