Gathering GPR Inspections and UAV Survey in Cultural Heritage Documentation Context
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Gathering GPR Inspections and UAV Survey in Cultural Heritage Documentation Context Alessandro Arato2, Flora Garofalo2, Giulia Sammartano1 and Antonia Spanò1 1(DAD) Department of Architecture and Design, Politecnico di Torino, Torino, Italy 2(DIATI) Department of Environment, Land and Infrastructure Engineering, Politecnico di Torino, Torino, Italy Keywords: UAV Photogrammetry, Ground Penetrating Radar (GPR), Historical Maps, GIS, Cultural Heritage. Abstract: The archaeological researches and more generally the Cultural Heritage (CH) documentation and conservation activities have been favourably disposed to the use of new technologies, with renewed and increasing interest in the use of integrated techniques. In the field of Geomatics the advent of advanced technologies has allowed and facilitated multidisciplinary studies as well as combined approaches to the documentation in various contexts. The production of spatially located data (e.g. from active or passive sensors placed in different system segments, from terrestrial to aerial to satellite position) and their interoperability from different source, with the help of Geographic Information Systems (GIS), were then made easier. The work has the aim of investigating the integration of multiple data derived from aerial photogrammetry products through Unmanned Aerial Vehicles (UAV) survey, from geophysical Ground Penetrating Radar (GPR) prospection technique and analysis of historical maps. An archaeological area in the south of Piedmont (Italy), next to the ancient Roman settlement of Pollentia, has been the test case. The present fulfilled test was objected to exclude ancient presences, although this type of workflow is generally aimed to analyse and compare results in order to formulate some hypothesis about the potential presence of submerged elements or built substructures in the investigated area. 1 INTRODUCTION generally highly significant in this kind of project, have been compared and commented by GIS tools in Adding to the widening of cultural assets typologies final results. subject to the safeguard aims, ranging from ancient to modern heritage, from architectural to landscape, and from mobile assets to intangible cultural heritage, the 2 METHODS increasing challenges of CH conservation has given rise to an incessant specialization of methods 2.1 UAV Photogrammetry involved in the protection projects. This trend has become visible in data collecting and survey, data The UAV photogrammetry has developed very archiving and structuring, and finally in rapidly whereas it is able to fulfil successfully the communication and dissemination processes. Some large-scale maps updating in many fields such as features are increasingly requested to acquisition environmental, CH applications. (Haala et al., 2011) techniques: mainly 3D nature of data, quick acquiring One relevant strong point of UAV systems for and low cost solutions, keeping high accuracies and photogrammetry is the nadiral point of view overall quality. On these bases, the present paper wish connected with the ability to choose the height of to investigate the integration of strengthened flight depending on the requested scale of survey. techniques as Ground Penetrating Radar purposed to The low cost, the high handily with contained search possible submerged archaeological remains, playloads, and the ability to perform autonomous with emergent methods for quick large-scale flights starting from a predefined flight plan, are other mapping, generated from UAV photogrammetry. decisive advantages. Navigation sensors such as Moreover the examination of historical maps, GNSS receivers (Global Navigation Satellite System) 85 Arato, A., Garofalo, F., Sammartano, G. and Spanò, A. Gathering GPR Inspections and UAV Survey in Cultural Heritage Documentation Context. In Proceedings of the 2nd International Conference on Geographical Information Systems Theory, Applications and Management (GISTAM 2016), pages 85-91 ISBN: 978-989-758-188-5 Copyright c 2016 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved GISTAM 2016 - 2nd International Conference on Geographical Information Systems Theory, Applications and Management and inertial sensors (IMU, Inertial Measurement 2.2 GPR (Ground Penetrating Radar) Unit) are conveniently integrated; the effectiveness of Survey the whole system depends from the balance of their accuracy and reliability with the Flight control GPR method is based on the generation of an device. (Rinaudo et al., 2012) electromagnetic (EM) impulse through a signal The product of UAV photogrammetry consists in generator, connected to a transmitting antenna (Tx) a set of images from which it is possible to extract and a receiving antenna (Rx). The GPR systems work point clouds, 3D textured models, Digital Surface in the frequency range between 10 MHz and 3 GHz. Model (DSM) and Orthophoto the processes are The choice of the system frequency is essential and mainly automated by an image matching technique, has to be made according to the scope of the survey. so it’s clear the reasons of increase. In fact, being the frequency inversely proportional to In the next experience the only ortophoto will be the wavelength, lower frequencies imply deeper used, which is absolutely the most automated product signal penetration but lower resolution, while higher that is possible to derive, and this is one of the main frequency allow the increase the resolution but reason of the method choice. reducing the signal penetration. Structure-from-Motion (SFM) systems allow In archaeological researches, GPR survey is used estimating the 3D position of points represented in in order to correlate electromagnetic impedance multiple images, reconstructing the geometry of the contrasts to possible submerged remains. represented object (structure) and the acquisitions The electromagnetic discontinuities (or dielectric position (motion), even when are not available the interfaces), act as signal reflectors, refractors and defined camera calibration parameters. The diffractors, via multiple mechanisms (e.g. Davis and characteristic elements (tie-features) of images are Annan, 1989; Reynolds, 1997). Soil specific automatically preliminarily extracted; in particular characteristics, like structure and texture, mineral they are extracted objects (points) recognizable by the composition, pore-fluid distribution and chemistry, software thanks to the radiometric contrast in influence the signal penetration as well (e.g. Sen et different directions, which identifies unambiguously al., 1981; Friedman, 1998; Cosenza et al., 2003). the pixel. One of the most common algorithms for Each reflection implies an energy loss, and the features identification and description for the received signal is strongly attenuated with respect to association and the consequent orientation of images the constant transmitted signal. Common GPR in photogrammetric software and computer vision is surveys are carried out along profiles, with the aim of SIFT (Scale-Invariant Feature Transform). (Lowe, obtaining 2-dimensional (2-D) sections of the subsoil. 2004; Szeliski, 2011) A radar profile is a composition of all the recorded In the next step of feature matching, points with signals, called traces, and allows to understand the more similar descriptors (recognized by the software subsoil inhomogeneity, or dielectric interfaces. In as homologous) are aligned to make the next step of archaeological context, the analysis of a single profile triangulation - bundle adjustment. is not sufficient, and a 3-D survey must be done to In SFM processes the orientation parameters are map the lateral discontinuities. The results are usually not necessarily known; so, using the rules of the presented in the form of time-slices, horizontal maps epipolar geometry, which regulates relations between at constant time (or depth). the tie points (TP), the acquisition centers and the point 3D position, to direct acquisitions and estimate 2.3 Historical Maps Contribution and internal and external orientation. Their Spatial Reference The success of the GPR survey is highly affected by the prevision of searched remains. The shape of possible submerged building elements related with the period of construction and especially the depth at whom they are expected to be found are very relevant in order to correctly plan the parameters of GPR Figure 1: Two example of unmanned vehicles, the ones survey. Then a panoramic examination of historical used in the further test: hexacopter by Mikrokopter and cartography is usually highly advisable. Ebee by Sensfly. The geometric and projective characteristics and properties of early cartographic representations have been largely studied when digital technologies have 86 Gathering GPR Inspections and UAV Survey in Cultural Heritage Documentation Context spread new perspectives for the visualization in the 3.1 Photogrammetric Survey of Lot same spatial reference of recent maps. (e.g. 2076 in Pollenzo by Drones Livieratos, 2006; Balletti, 2006). Many research projects has directed efforts towards The first purpose of the photogrammetric flight on the the use of GIS techniques of calibration, georeferencing Pollenzo 2076 lot was exactly the fast generation of a and transformation of projection, for enabling the high resolution ortophoto, using aerial photos acquired comparison among historical and actual maps. contemporary to GPS survey in order to keep easily the In brief, the relation among the points coordinates spatial reference of GPR