Navigation – Plan du site
Landscape and Ressource evolution

Contribution of large scale geophysical survey to analysis of the evolution of the western boundary of the city of Allonnes (Sarthes, France): Integration of Google images, the Napoleonic cadastre and large magnetic surveys

K. Gruel, Michel Dabas, V. Gruel et V. Bernollin
p. 179-181

Texte intégral

1How is it possible to comprehend the anthropogenic evolution of landscapes? A. Levy, in his book about urban morphology, states that “the concept of the layouts’ morphology covers how these layouts are distributed within the space of the whole city according to the various stages of urban growths and their expansion procedures”. Thus, the urban binomial (couple) networks/frames perfectly explains the construction process of the towns’ composition. Mapping of Allonnes’ urban composition demonstrates the importance of this morphological reading. Indeed, it is striking to see how successive waves of urbanization can be oversimplified in this way. From the ancient city to the contemporary one, several phases of urbanization remain legible.

Figure 1: Superposition of Cadastre and magnetic map (scale =-4 to 4 nT/m).

Figure 1: Superposition of Cadastre and magnetic map (scale =-4 to 4 nT/m).

2The expansion projects of the modern town to the west have focused attention on this still agricultural area. “La ZAC de la Buissonnière” is part of the area threatened by the growth of the town. Examination of aerial photographs shows several enclosures in these plots. INRAPs’ archaeological trial trenches, on the other side of the road, have identified a Roman necropolis. Is this route part of an ancient track or not? Does the Roman necropolis continue further to the west? Have we reached the limits of the Western Roman city? What was the nature of the occupation of this area before and after the Roman period? Within the framework of the ANR Celtecophys program, motorised geophysical instruments make possible the development of a method of prospecting, which can be associated with other known surveying techniques. Fifteen hectares were magnetically surveyed in two days. The apparatus used (AMP for Automatic Magnetic Surveying), coupled with a dGPS and onboard computer, allows the map of magnetic anomalies to be viewed immediately and the area to be surveyed may thus be adjusted if necessary as the survey progresses. Combining these geophysical surveys with other sources such as: photographs, aerial surveys, Geoportail (French topographical maps on-line), Google imagery and cadastres, we can analyse the evolution of cadastral frames from antiquity to present day, geocoding them together, including the buildings on the outskirts of the city. Napoleon’s cadastre was georeferenced and vectorized and found to coincide precisely with linear anomalies in the magnetic surveys. So, it is possible to control the topographical accuracy of the Napoleonic cadastre, and only some positions of frame borders need to be corrected. Two land parcels predated the Napoleonic cadastre. A wide double-ditch enclosure was found and can be followed both over the Google images and over the magnetic images. Further west, several enclosures are visible. Following the geophysical survey, a re-examination of aerial photographs, taken at different time of the year, provided additional information about the continuation of these structures in the adjacent plots. Many discrete magnetic anomalies are clearly visible; some of them are anthropogenic and will have to be checked by archaeological excavation. Some may correspond to tombs. However we are outside of the Roman city. The shape of these enclosures is closer to medieval structures or even proto-historic. No surface artefacts have been found to help clarify the chronology. But, while uncertainties remain, it has nevertheless been possible without excavation to show that the area has been occupied since antiquity, to precisely frame the borders of land units and to determine the plans of some of the dwellings.

Figure 2: Interpretation of magnetic map.

Figure 2: Interpretation of magnetic map.

This geophysical survey has been financed partly by the program ANR CELTECOPHYS, partly by the Conseil general  de la Sarthe.

Haut de page

Bibliographie

Levy, A. and Spigai, V., 1992.La qualité de la forme urbaine : problématique et enjeux. IFU, Paris.

Levy, A., 1998. La qualité de la forme urbaine : problématique et enjeux. Rapport, Laboratoire TMU/URA CNRS 1244/IFU/Paris 8.

Haut de page

Table des illustrations

Titre Figure 1: Superposition of Cadastre and magnetic map (scale =-4 to 4 nT/m).
URL http://journals.openedition.org/archeosciences/docannexe/image/1543/img-1.png
Fichier image/png, 162k
Titre Figure 2: Interpretation of magnetic map.
URL http://journals.openedition.org/archeosciences/docannexe/image/1543/img-2.png
Fichier image/png, 215k
Haut de page

Pour citer cet article

Référence papier

K. Gruel, Michel Dabas, V. Gruel et V. Bernollin, « Contribution of large scale geophysical survey to analysis of the evolution of the western boundary of the city of Allonnes (Sarthes, France): Integration of Google images, the Napoleonic cadastre and large magnetic surveys », ArcheoSciences, 33 (suppl.) | 2009, 179-181.

Référence électronique

K. Gruel, Michel Dabas, V. Gruel et V. Bernollin, « Contribution of large scale geophysical survey to analysis of the evolution of the western boundary of the city of Allonnes (Sarthes, France): Integration of Google images, the Napoleonic cadastre and large magnetic surveys », ArcheoSciences [En ligne], 33 (suppl.) | 2009, mis en ligne le 30 octobre 2011, consulté le 17 décembre 2017. URL : http://journals.openedition.org/archeosciences/1543 ; DOI : 10.4000/archeosciences.1543

Haut de page

Auteurs

K. Gruel

UMR 8546, AOROC CNRS/ENS-Paris, ANR CELTECOPHYS (Katherine.Gruel@ens.fr)

Articles du même auteur

Michel Dabas

Directeur scientifique GEOCARTA, Paris (dabas@geocarta.net)

Articles du même auteur

V. Gruel

UMR 8546, AOROC CNRS/ENS-Paris, ANR CELTECOPHYS

V. Bernollin

UMR 8546, AOROC CNRS/ENS-Paris, ANR CELTECOPHYS. CAPRA, Allonnes

Haut de page

Droits d’auteur

Tous droits réservés

Haut de page