From TLS survey to 3d solid modeling for documentation of built heritage: The case study of porta savonarola in Padua

It is a matter of fact that 3D visualisation and proper documentation of cultural objects helps to preserve the history and memories of historic buildings, archaeological sites and cultural landscapes, and supports economic growth by stimulating cultural tourism. Preservation, visualisation and recreation of valuable historical and architectural objects and places has always been a serious challenge for specialists in the field. Today, the rapid developments in the fields of close-range photogrammetry, terrestrial laser scanning (TLS) and computer vision (CV) enable to carry out highly accurate 3D models so as to be extremely effective and intuitive for users who have stringent requirements and high expectations. In this note we present the results of the survey and 3D modeling of an ancient gate, Porta Savonarola, located within the remains of the medieval town walls surrounding the historical city center of Padua, Italy. The work has been undertaken within the framework of the project \u201cWalls Multimedia Museum\u201d (WMM) promoted by the local private association \u201cPadua Walls Committee\u201d. The goal of the project was to develop a prototype of an \u201cextended\u201d virtual museum, spreaded along most interesting locations of the town walls. The survey of the ancient gate was performed with a Leica C10 and P20 terrestrial laser scanners. Once the acquired scans were properly merged together, a solid model was generated from the global point cloud, and plans and elevations were extracted from it for restoration purposes. A short multimedia video was also created for the \u201cWalls Multimedia Museum\u201d, showing both the outer and inner part of the gate. In the paper we will discuss all the steps and challenges addressed to provide the 3D solid model of Porta Savonarola from the TLS data

OPEN SOURCE WEB TOOL FOR TRACKING IN A LOWCOST MOBILE MAPPING SYSTEM

During the last decade several Mobile Mapping Systems (MMSs), i.e. systems able to acquire efficiently three dimensional data using moving sensors (Guarnieri et al., 2008, Schwarz and El-Sheimy, 2004), have been developed. Research and commercial products have been implemented on terrestrial, aerial and marine platforms, and even on human-carried equipment, e.g. backpack (Lo et al., 2015, Nex and Remondino, 2014, Ellum and El-Sheimy, 2002, Leica Pegasus backpack, 2016, Masiero et al., 2017, Fissore et al., 2018).<br><br> Such systems are composed of an integrated array of time-synchronised navigation sensors and imaging sensors mounted on a mobile platform (Puente et al., 2013, Tao and Li, 2007). Usually the MMS implies integration of different types of sensors, such as GNSS, IMU, video camera and/or laser scanners that allow accurate and quick mapping (Li, 1997, Petrie, 2010, Tao, 2000). The typical requirement of high-accuracy 3D georeferenced reconstruction often makes such systems quite expensive. Indeed, at time of writing most of the terrestrial MMSs on the market have a cost usually greater than 50000, which might be expensive for certain applications (Ellum and El-Sheimy, 2002, Piras et al., 2008). In order to allow best performance sensors have to be properly calibrated (Dong et al., 2007, Ellum and El-Sheimy, 2002).<br><br> Sensors in MMSs are usually integrated and managed through a dedicated software, which is developed ad hoc for the devices mounted on the mobile platform and hence tailored for the specific used sensors. Despite the fact that commercial solutions are complete, very specific and particularly related to the typology of survey, their price is a factor that restricts the number of users and the possible interested sectors.<br><br> This paper describes a (relatively low cost) terrestrial Mobile Mapping System developed at the University of Padua (TESAF, Department of Land Environment Agriculture and Forestry) by the research team in CIRGEO, in order to test an alternative solution to other more expensive MMSs. The first objective of this paper is to report on the development of a prototype of MMS for the collection of geospatial data based on the assembly of low cost sensors managed through a web interface developed using open source libraries. The main goal is to provide a system accessible by any type of user, and flexible to any type of upgrade or introduction of new models of sensors or versions thereof. After a presentation of the hardware components used in our system, a more detailed description of the software developed for the management of the MMS will be provided, which is the part of the innovation of the project. According to the worldwide request for having big data available through the web from everywhere in the world (Pirotti et al., 2011), the proposed solution allows to retrieve data from a web interface Figure 4. Actually, this is part of a project for the development of a new web infrastructure in the University of Padua (but it will be available for external users as well), in order to ease collaboration between researchers from different areas.<br><br> Finally, strengths, weaknesses and future developments of the low cost MMS are discussed

COMPARISON OF LOW COST PHOTOGRAMMETRIC SURVEY WITH TLS AND LEICA PEGASUS BACKPACK 3D MODELS

This paper considers Leica backpack and photogrammetric surveys of a mediaeval bastion in Padua, Italy. Furhtermore, terrestrial laser scanning (TLS) survey is considered in order to provide a state of the art reconstruction of the bastion. Despite control points are typically used to avoid deformations in photogrammetric surveys and ensure correct scaling of the reconstruction, in this paper a different approach is considered: this work is part of a project aiming at the development of a system exploiting ultra-wide band (UWB) devices to provide correct scaling of the reconstruction. In particular, low cost Pozyx UWB devices are used to estimate camera positions during image acquisitions. Then, in order to obtain a metric reconstruction, scale factor in the photogrammetric survey is estimated by comparing camera positions obtained from UWB measurements with those obtained from photogrammetric reconstruction. Compared with the TLS survey, the considered photogrammetric model of the bastion results in a RMSE of 21.9cm, average error 13.4cm, and standard deviation 13.5cm. Excluding the final part of the bastion left wing, where the presence of several poles make reconstruction more difficult, (RMSE) fitting error is 17.3cm, average error 11.5cm, and standard deviation 9.5cm. Instead, comparison of Leica backpack and TLS surveys leads to an average error of 4.7cm and standard deviation 0.6cm (4.2 cm and 0.3 cm, respectively, by excluding the final part of the left wing)

From survey to fem analysis for documentation of built heritage: The case study of villa revedin-bolasco

In the last decade advances in the fields of close-range photogrammetry, terrestrial laser scanning (TLS) and Computer Vision (CV) have enabled to collect different kind of information about a Cultural Heritage objects and to carry out highly accurate 3D models. Additionally, the integration between laser scanning technology and Finite Element Analysis (FEA) is gaining particular interest in recent years for structural analysis of built heritage, since the increasing computational capabilities allow to manipulate large datasets. In this note we illustrate the approach adopted for surveying, 3D modeling and structural analysis of Villa Revedin-Bolasco, a magnificent historical building located in the small walled town of Castelfranco Veneto, in northern Italy. In 2012 CIRGEO was charged by the University of Padova to carry out a survey of the Villa and Park, as preliminary step for subsequent restoration works. The inner geometry of the Villa was captured with two Leica Disto D3a BT hand-held laser meters, while the outer walls of the building were surveyed with a Leica C10 and a Faro Focus 3D 120 terrestrial laser scanners. Ancillary GNSS measurements were also collected for 3D laser model georeferencing. A solid model was then generated from the laser global point cloud in Rhinoceros software, and portion of it was used for simulation in a Finite Element Analysis (FEA). In the paper we discuss in detail all the steps and challenges addressed and solutions adopted concerning the survey, solid modeling and FEA from laser scanning data of the historical complex of Villa Revedin-Bolasco

ASSESSMENT OF A PORTABLE TOF CAMERA AND COMPARISON WITH SMARTPHONE STEREO VISION

Abstract. Nowadays time-of-flight (ToF) cameras and multiple RGB cameras are being embedded in an increasing number of high-end smartphones: despite their integration in mobile devices is mostly motivated by photographic applications, their availability can be exploited to enable 3D reconstructions directly on smartphones. Furthermore, even when a ToF camera is not embedded in a smartphone, low cost solutions are available on the market in order to easily provide standard mobile devices with a lightweight and extremely portable ToF camera. This work deals with the assessment of a low cost ToF camera, namely Pico Zense DCAM710, which perfectly fits with the above description. According to the results obtained in the considered tests, the ranging error (precision) of the DCAM710 camera increases linearly approximately up to the nominal maximum range in the considered working mode, up to approximately 1 cm. Despite the device allows to acquire measurements also at larger ranges, the measurement quality significantly worsen. After assessing the main characteristics of such ToF camera, this paper aims at comparing its 3D reconstruction ability with that of a smartphone stereo vision system. In particular, the comparison of a 3D reconstruction obtained with stereo vision from images acquired with an LG G6 shows that the stereo reconstruction leads to a much larger point cloud. However, points generated by the ToF camera are more homogeneously distributed, and they seem to slightly better describe the real geometry of the reconstructed object. The combination of such two technologies, which will be investigated in our future work, can potentially lead to a denser cloud with respect to the ToF camera, while preserving a reasonable accuracy

DIGITAL MODEL OFWALLS OF PADUA LOWRELIEF

Photogrammetry has been widely used in the recent years in a number of applications, e.g. cultural heritage, surveying buildings and infrastructures. Despite nowadays its use is quite common, most of the used photogrammetric softwares are commercial. This paper aims at comparing the use of a free Matlab tool that is being developed at the University of Padova mostly for educational purposes with that of a commercial (and widely used) software (Agisoft PhotoScan). Despite the above mentioned free Matlab tool is designed to work for airborne photogrammetric, in this work it is used in a slightly different case: the 3D reconstruction of a low relief of the walls of Padova, which is on the fac¸ade of the church Santa Maria del Giglio, Venice, Italy

Indoor photogrammetry aided with UWB navigation

The subject of photogrammetric surveying with mobile devices, in particular smartphones, is becoming of significant interest in the research community. Nowadays, the process of providing 3D point clouds with photogrammetric procedures is well known. However, external information is still typically needed in order to move from the point cloud obtained from images to a 3D metric reconstruction. This paper investigates the integration of information provided by an UWB positioning system with visual based reconstruction to produce a metric reconstruction. Furthermore, the orientation (with respect to North-East directions) of the obtained model is assessed thanks to the use of inertial sensors included in the considered UWB devices. Results of this integration are shown on two case studies in indoor environments

Comparison of discrete return and waveform terrestrial laser scanning for dense vegetation filtering

Abstract. In this paper we present the results of the comparison between two terrestrial laser scanners (TLS), a discrete return system (Riegl LMS-Z620) and an echo-digitizing system (Riegl VZ-400), employed for the survey of a dense forested area, in the italian Alps. The site is actually undergoing a strong debate among the inhabitants and local government authorities about the exploitation of the area as a huge quarry to produce building material. The dispute originates from the uncertainty about the instability of the underlying mountain slope, which was interested in 1966 by a landslide. The whole area was surveyed with the two laser scanners on February 2011 during the vegetation dormant period. A slight different processing workflow was applied to the collected datasets: the VZ-400 scans were pre-filtered by exploiting the "calibrated relative reflectance" readings and the multi-target capability provided by this laser scanning system. Next, two different spatial filters were applied to both the resulting georeferenced 3D models, in order to eliminate as much vegetation as possible: iterative filter and a custom morphological filter, developed by the authors. Achieved results show that for both datasets, the iterative and the morphological filters perform quite well for eliminating the vegetation, though some manual editing is still required since vegetation does not feature a prevalent growing direction. Furthermore, the comparison between the number of points left in the final DTMs shows that the VZ-400 provided a one order of magnitude denser point cloud wrt. the LMS-Z620. This demonstrates that a TLS with multi-target capability can potentially provide a more detailed DTM even in presence of dense vegetation

On the use of INS to improve Feature Matching

The continuous technological improvement of mobile devices opens the frontiers of Mobile Mapping systems to very compact systems, i.e. a smartphone or a tablet. This motivates the development of efficient 3D reconstruction techniques based on the sensors typically embedded in such devices, i.e. imaging sensors, GPS and Inertial Navigation System (INS). Such methods usually exploits photogrammetry techniques (structure from motion) to provide an estimation of the geometry of the scene. Actually, 3D reconstruction techniques (e.g. structure from motion) rely on use of features properly matched in different images to compute the 3D positions of objects by means of triangulation. Hence, correct feature matching is of fundamental importance to ensure good quality 3D reconstructions. Matching methods are based on the appearance of features, that can change as a consequence of variations of camera position and orientation, and environment illumination. For this reason, several methods have been developed in recent years in order to provide feature descriptors robust (ideally invariant) to such variations, e.g. Scale-Invariant Feature Transform (SIFT), Affine SIFT, Hessian affine and Harris affine detectors, Maximally Stable Extremal Regions (MSER). This work deals with the integration of information provided by the INS in the feature matching procedure: a previously developed navigation algorithm is used to constantly estimate the device position and orientation. Then, such information is exploited to estimate the transformation of feature regions between two camera views. This allows to compare regions from different images but associated to the same feature as seen by the same point of view, hence significantly easing the comparison of feature characteristics and, consequently, improving matching. SIFT-like descriptors are used in order to ensure good matching results in presence of illumination variations and to compensate the approximations related to the estimation process

COMPARING ACCURACY OF ULTRA-DENSE LASER SCANNER AND PHOTOGRAMMETRY POINT CLOUDS

Abstract. Massive point clouds have now become a common product from surveys using passive (photogrammetry) or active (laser scanning) technologies. A common question is what is the difference in terms of accuracy and precision of different technologies and processing options. In this work four ultra-dense point-clouds (PCs) from drone surveys are compared. Two PCs were created from imagery using a photogrammetric workflow, with and without ground control points. The laser scanning PCs were created with two drone flights with Riegl MiniVUX-3 lidar sensor, resulting in a point cloud with ~300 million points, and Riegl VUX-120 lidar sensor, leading to a point cloud with ~1 billion points. Relative differences between pairs from permutations of the four PCs are analysed calculating point-to-point distances over nearest neighbours. Eleven clipped PC subsets are used for this task. Ground control points (GCPs) are also used to assess residuals in the two photogrammetric point clouds in order to quantify the improvement from using GCPs vs not using GCPs when processing the images.Results related to comparing the two photogrammetric point clouds with and without GCPs show an improvement of average absolute position error from 0.12 m to 0.05 m and RMSE from 0.03 m to 0.01 m. Point-to-point distances over the PC pairs show that the closest point clouds are the two lidar clouds, with mean absolute distance (MAD), median absolute distance (MdAD) and standard deviation of distances (RMSE) respectively of 0.031 m, 0.025 m, 0.019 m; largest difference is between photogrammetric PC with GCPs, with 0.208 m, 0.206 m and 0.116 m, with the Z component providing most of the difference. Photogrammetry without GCP was more consistent with the lidar point clouds, with MAD of 0.064 m, MdAD of 0.048 m and RMSE value of 0.114 m