ACCURACY ASSESSMENT IN STRUCTURE FROM MOTION 3D RECONSTRUCTION FROM UAV-BORN IMAGES: THE INFLUENCE OF THE DATA PROCESSING METHODS

The evolution of Structure from Motion (SfM) techniques and their integration with the established procedures of classic stereoscopic photogrammetric survey have provided a very effective tool for the production of three-dimensional textured models. Such models are not only aesthetically pleasing but can also contain metric information, the quality of which depends on both survey type and applied processing methodologies. An open research topic in this area refers to checking attainable accuracy levels. The knowledge of such accuracy is essential, especially in the integration of models obtained through SfM with other models derived from different sensors or methods (laser scanning, classic photogrammetry ...). Accuracy checks may be conducted by either comparing SfM models against a reference one or measuring the deviation of control points identified on models and measured with classic topographic instrumentation and methodologies. This paper presents an analysis of attainable accuracy levels, according to different approaches of survey and data processing. For this purpose, a survey of the Church of San Miniato in Marcianella (Pisa, Italy), has been used. The dataset is an integration of laser scanning with terrestrial and UAV-borne photogrammetric surveys; in addition, a high precision topographic network was established for the specific purpose. In particular, laser scanning has been used for the interior and the exterior of the church, with the exclusion of the roof, while UAVs have been used for the photogrammetric survey of both roof, with horizontal strips, and façade, with vertical strips

INDOOR PHOTOGRAMMETRY USING UAVS WITH PROTECTIVE STRUCTURES: ISSUES AND PRECISION TESTS

Abstract. Management of disaster scenarios requires applying emergency procedures ensuring maximum safety and protection for field operators. Actual conditions of disaster sites are labelled as "Triple-D: Dull, Dusty, Dangerous" areas. It is well known that in this kind of areas and situations remote surveying systems are at their very best effective, and among these UAVs currently are an effective and performing field tool. Indoor spaces are a particularly complex scenario for this kind of surveys. In this case, technological advances currently offer micro-UAV systems, featuring 360° protective cages, which are able to collect video streams while flying in very tight spaces. Such cases require manual control of the vehicle, with the operator piloting the aircraft without prior knowledge of the status quo of the survey object and therefore without prior planning of flight paths. A possible benefit in terms of knowledge of the survey object could lay in the creation of a 3D model based on images extracted by video streams; to date, widely tested methods and techniques are available for processing UAV-borne video streams to obtain such models. Anyway, the protective cage and the need to use, in these operating conditions, wide-angle lenses presents some issues linked to ever-changing image framing, due to the presence of the cage wires on the field of view. The present work focused on this issue. Using this type of UAVs, video streams have been collected in different environments, both indoors and outdoors, testing several procedures for photogrammetric processing in order to assess the ability to create 3D models. These have been tested for reliability based on data collection conditions, also assessing the level of automation and speed attainable in post-processing. The present paper describes the different tests carried out and the related results.</p

HBIM METHODOLOGIES FOR THE ARCHITECTURAL RESTORATION. THE CASE OF THE EX-CHURCH OF SAN QUIRICO ALL'OLIVO IN LUCCA, TUSCANY

Abstract. In the last decade, in the field of conservation of historic buildings, several research projects have shown the potential of applying BIM technology to architectural heritage. However, the use of BIM for historic buildings (HBIM) is still evolving. This paper presents an application of Building Information Modelling targeted to the development of a restauration proposal for the ex-church of San Quirico all'Olivo in Lucca, Tuscany. Following a brief review of the state-of-the-art of BIM applied to architectural heritage, the paper shows the results of a study that included 3D architectural survey with Structure-from-Motion methodology, critical analysis of historical archival and bibliographic sources, analysis of the conservation status of the building, proposal for its conservation and enhancement.HBIM methodology has been critically applied to all the phases of the project. This study also explores the possibility of organizing the BIM model into temporal phases, integrating documentation in a structured and easily accessible way. In our study, we also chose to link the 3D point cloud to the model, in order to increase the level of information; the 3D survey, therefore, is both the starting point for modelling, and represents a source of information within the model, to be recalled when required.</p

RANGE AND IMAGE BASED MODELLING: A WAY FOR FRESCOED VAULT TEXTURING OPTIMIZATION

In the restoration of the frescoed vaults it is not only important to know the geometric shape of the painted surface, but it is essential to document its chromatic characterization and conservation status. The new techniques of range-based and image-based modelling, each with its limitations and advantages, offer a wide range of methods to obtain the geometric shape. In fact, several studies widely document that laser scanning enable obtaining three-dimensional models with high morphological precision. However, the quality level of the colour obtained with built-in laser scanner cameras is not comparable to that obtained for the shape. It is possible to improve the texture quality by means of a dedicated photographic campaign. This procedure, however, requires to calculate the external orientation of each image identifying the control points on it and on the model through a costly step of post processing. With image-based modelling techniques it is possible to obtain models that maintain the colour quality of the original images, but with variable geometric precision, locally lower than the laser scanning model. This paper presents a methodology that uses the camera external orientation parameters calculated by image based modelling techniques to project the same image on the model obtained from the laser scan. This methodology is tested on an Italian mirror (a schifo) frescoed vault. In the paper the different models, the analysis of precision and the efficiency evaluation of proposed methodology are presented

Orientation of archive images on 3D digital models of painted vaults: an interesting tool for restorers

Over the last years, the area of conservation and enhancement of cultural heritage has benefited from new surveying methodologies. In particular, laser scanners and the latest evolutions in digital photogrammetry and processing software based on structure from motion and multi-view stereo algorithms allowed to achieve 3D models of the status quo whose features include precision suitable for restoration scales and high-quality textures. Besides methodological and technical issues, another key requirement for any intervention on this kind of buildings is their full, in-depth knowledge, which includes building design, modifications’ timeline, current conditions, present-day and historical causes of degradation etc. In this view, surveys are a valuable asset for the investigation of historical sources. The present work investigates historical photographic sources, focusing on the possibility of orientation by means of photogrammetry techniques, which provides an additional texture layer as a support tool for historical analysis. The paper displays a case study featuring the availability of a reference survey, which allows validation of the results, and two other cases highlighting the importance of this kind of documents to restoring teams

COMPUTATIONAL VISION IN UV-MAPPING OF TEXTURED MESHES COMING FROM PHOTOGRAMMETRIC RECOVERY: UNWRAPPING FRESCOED VAULTS

Sometimes it is difficult to represent “on paper" the existing reality of architectonic elements, depending on the complexity of his geometry, but not only in cases with complex geometries: non-relief surfaces, can need a “special planar format” for its graphical representation. Nowadays, there are a lot of methods to obtain tridimensional recovery of our Cultural Heritage with different ranges of the relationship accuracy / costs, even getting high accuracy using “low-cost” recovery methods as digital photogrammetry, which allow us easily to obtain a graphical representation “on paper”: ortho-images of different points of view. This can be useful for many purposes but, for others, an orthographic projection is not really very interesting. In non-site restoration tasks of frescoed vaults, a “planar format” representation in needed to see in true magnitude the paintings represented on the intrados vault, because of the general methodology used: gluing the fresco on a fabric, removing the fresco-fabric from the support, moving to laboratory, removing the fresco from the fabric, restoring the fresco, gluing back the restored fresco on another fabric, laying the restored fresco on the original location and removing the fabric. Because of this, many times, an unfolded model is needed, in a similar way a cylinder or cone can be unfolded, but in this case with a texture included: UV unwrapping. Unfold and fold-back processes, can be especially interesting in restoration field of frescoed vaults and domes at: chromatic recovery of paintings, reconstruction of partially missed geometries, transference of paintings on surfaces, etc

Low-altitude UAV-borne remote sensing in dunes environment: Shoreline monitoring and coastal resilience

UAV systems, fitted with either active or passive surveying sensors, can provide land-related measures and quantitative information with low costs and high resolution in both space and time. Such surveying systems can be quite valuable in defining geometrical and descriptive parameters in coastal systems, especially dune ecosystems. The present work is based on a survey of the dune system at the mouth of the Fiume Morto Nuovo in the San Rossore Estate (Pisa) and focuses on comparing LiDAR with UAV- and airplane-borne photogrammetry, as well as the respective 2D and 3D cartographic output, in order to assess topography changes along a stretch of coastline and to check their possible use in defining some ecological resilience features on coastal dune systems. Processing of survey data generates a Digital Surface Model (DSM) or Digital Terrain Model (DTM) and an orthophotograph, checked for accuracy and image resolution. Comparison of these products against those available in public access cartographical databases highlights differences and respective strengths