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Stereophotomap and Stereopace for the management of cultral heritage: An example
Elena Albery,Sergio Dequal
DIGET - Politecnico di Torino
C.so Duca degli Abruzzi, 24, 10129 - Torino
Tel. (+39)011/5647700, (+39)011/5647601, fax 011/5647601
E-Mail: email@example.com, firstname.lastname@example.org
A new photogrammetric system for the management of cultural heritage has been conceived at the Politecnico di Torino and developed in cooperation with "Nikon Italia" and "Menci Software". The system is based on a photogrammetric station, named Stereospace, and on an "image database" named Stereophotomap. Stereophotomap could be considered as the 3D evolution of the traditional ortho-photomap. It is made of a set of aerial and/or terrestrial photogrammetric blocks and allows surveying and recording of monuments and sites, without any preliminary restitution.
Stereospace allows the user to explore the Stereophotomap and "navigate" all over the stereo-models. The user can move freely from aerial to terrestrial 3D views in a unique geo-referenced environment, and can easily measure co-ordinates of points, distances and areas.
In order to describe the system in practice the authors show how to navigate inside the Basilica of Assisi, investigate the paints and have a complete documentation of the façade.
: image acquisition in digital form and orientation of the models are the required preliminary operations. Stereophotomap and Stereospace offer new opportunities in documentation, measuring, conservation and cultural heritage management in general.
Surveying techniques play a fundamental role in the study and knowledge of cultural goods for their cataloguing and restoration.
Traditional survey and representation methods have always had the problem of subjective interpretation of the objects.
Interpretation of the form, metrically approximate graphic representation and attention to only same physical characteristics in function of particular needs and applications has often led to partial documentation. These surveys could be used for studies and research that are different from those they were originally planned for.
A new photogrammetric system for the management of cultural heritage has been conceived at the Politecnico di Torino and developed jointly with Nikon Instruments S.p.A and Menci Software S.r.l.. The system is based on a photogrammetric station, named Stereospace, and on an "image database" named Stereophotomap.
Stereophotomap and Stereospace allow one to read the space and the objects without any subjective interpretation. They give a complete knowledge of space and complex shapes. The users can move around the objects, study the context and measure each part using traditional photogrammetric procedures and new digital image processing techniques.
An example of Stereophotomap of the Basilica of Assisi has been carried out that is consultable with Stereospace.
Stereophotomap is made up of a set of aerial and/or terrestrial photogrammetric blocks and allows surveying and recording of monuments and sites, without any preliminary restitution. It could be considered a 3D evolution of the traditional ortho-photomap.
Sterophotomap can be observed in 3D and appears as a photographic image that contains all the metric information, like a traditional map, in a single 3D reference system.
Stereophotomap can include both aerial and terrestrial images at the same time, in an unique geo-referenced environment. This could be an interesting and powerful tool when the object of interest is an architectonical or historical building inserted in its contest. Stereophotomap represents a new way of recording photogrammetric surveys, separating the different competencies required by a metric survey and its representation.
The Stereophotomap production process follows the standard procedures used in a traditional photogrammetric survey: image acquisition, survey of a control network and photogrammetric triangulation. Digital images can be directly acquired with a digital camera or, indirectly, by using a classical camera and a photo-scanner. The orientation parameters are obtained through photogrammetric triangulation, where a bundle block adjustment procedure is recommended.
Images, orientation parameters and other data are organised in a logic structure through a software named SV Master. At the end of this phase, any operator, even if an unskilled, seek information, measure elements of the object and carry out a historical-artistic analysis.
The sets of images that constitute the Stereophotomap of the Basilica of Assisi are:
Figure 1 - Basilica of Assisi - Scheme for the 3D navigation
The resulting drawings obtained by a traditional architectural survey allow measurements of geometric elements (co-ordinates of single points, distances, areas, volumes…) and require a simple ruler as a measuring instrument. In a more modern approach, the results of a survey are recorded in digital form and the required measuring instruments for reading the co-ordinates of the points are appropriate CAD softwares (e.g. Autocad, Microstation, etc.). In the same way, a stereophotomap requires a specific instrument that is suitable for a correct and complete use of this new type of "map": the Stereospace. This allows the user to explore the stereophotomap and "navigate" all over the stereo-models. The user can move freely from aerial to terrestrial 3D views in a single geo-referenced environment, and can easily measure point co-ordinates.
The features of this instruments are: easy movement inside the whole photogrammetric block without problems in passing from model to the next, easy exchange from aerial to terrestrial images, stereoscopic collimation of points even by unskilled operators thanks to the refinement that is offered by the system, calculations of distances, areas and volume and basic procedures for the restitutions of plots, profiles and DEM. Furthermore, the quick and precise measurement of points co-ordinates and the simultaneous vision of different models with different scales and/or epochs are also possible.
Stereospace uses many of the automatic procedures offered by digital photogrammetry in order to obtain maximum efficiency and to foresee its use by unskilled operators. Stereospace is based on low cost PC, equipped with a stereovision device: an active polarised filter on the screen and passive polarised glasses or LCD glasses.
The images should be recorded at 800/1200 dpi resolution in order to obtain a good visualisation of the details. When a model is observed, only the common part of the images (about 70%) is charged in the RAM, saving space for other purposes.
The standard configuration of the PC is:
Figure 2 - Organisation of the screen
A second window contains a "graphic index" In addition, the operator can activate one or more stereoscopic windows simultaneously , that could contain:
Figure 3 - Hot-spot joining different blocks and images taken at different dates
This software allows the user to move inside the whole project space. A graphic index gives a general view of the selected partition of the Stereophotomap, the user clicks on a point and the software automatically loads the stereo-pairs of interest, a 3D window appears and the floating mark follows the movements of the mouse.
The movement inside the Stereophotomap is continuous: the user does not see the change of stereo-pairs during the navigation in any direction. When required, the system shows special symbols on the images, highlighting the availability of other strips or blocks. Clicking on the symbol it is possible to load and explore in a continuous way, for example, the Basilica facade, or Giotto' s frescos, or the vaults always referred to the same absolute reference system.
The stereoscopic collimation and the co-ordinate measurements of single points can be carried out by using one of two available procedures. When the operator moves in height, the first procedure moves both images (Helava procedure). The second moves only one image (Inghilleri principle). This last solution is suitable for an unskilled operator: they can monoscopically collimate the chosen point on the fixed image and then eliminate the x-parallaxe by just looking at the homologous point on the other image.
Figure 4 - Co-ordinate recording of the same selected points
In both cases, the system uses matching algorithms, pixel and sub-pixel correlation, in order to refine the collimation and to obtain the required accuracy in measuring X,Y,Z co-ordinates.
The co-ordinates of the selected can be recorded in an electronic sheet (for example Excel).
The data organisation follows a hierarchical structure. Images and data that describe an object (e.g. a town or a monument) are recorded in a directory named "Project". The project can be subdivided "sub-projects" that contain images and data of a portion of the object, also at different dates or at different scale.
The following information are linked to each image: strip name, focal length, taking point co-ordintes and rotation values w,j, k.
Figure 6 shows the organisation of the data in the Assisi Project. This hierarchical structure allows the user to choose which part of the Stereophotomap he wants to explore, and optimises the way of archiving data. The user can "navigate" inside the Basilica and, at the same time, he knows the co-ordinates of a point in a local system (e.g. internal wall) and in the facade reference system or in the cartographic system (e.g. aerial block).
Figure 5 - Assisi data structure
All the data are organised in this logical structure by means of a software named SVMASTER. Every sub-directory (e.g. facade, inside wall, vault, etc.) contains images, data and parameters for the transformation of the co-ordinates. Each sub-project contains the following folders: "Data", "Reference", "Link". The "data" folder contains general information on the data (metadata) and vectorial and raster files, that are necessary to build the index window in Stereospace. The "Reference" folder contains data for the organisation of the reference systems. It is possible to link image blocks oriented in different reference systems through the definition of parameters of a spatial rototranslation. The "Link" folder contains all those parameters that are necessary for consulting the Stereophotomap as a continuum.
Stereophotomap represents a new way of recording architectural photogrammetric surveys, while Stereospace is a user friendly instrument that is able to enquire the Stereophotomap. In architectural applications, these instruments allow one to "navigate" around a building with the possibility of continuous measuring point co-ordinates, distances, areas and so on. The operator has access to the information offered by images, without any preliminary selection or restitution. Stereospace uses the well known PC platform and Windows environment. Any user can explore the Stereophotomap even if he has no photogrammetric experience. No knowledge of measurement procedures is required: experts that have build the Stereophotomap, have solved these problems before. The cost of production of a Stereophotomap is only 30% of a traditional digital map cost. The possibility of connecting aerial and terrestrial images is an additional advantage of the system and a powerful investigation tool for architectural applications. Stereophotomap and Stereospace offer a new way to represent and survey the cultural heritage.
Figure 6 - "navigation" on the vault
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