Gabriele Garnero
Dipartimento di Economia e Ingegneria Agraria, Forestale e Ambientale, Facoltà di Agraria, Università di Torino, Via Leonardo da Vinci, 44, 10095 GRUGLIASCO (TO), tel. +39(0)11 6708601, fax, +39(0)11 6708609, E-mail topog@aerre.it

Ursula Zich
Dipartimento di Scienze e Tecniche per i Processi di Insediamento, Facoltà di Architettura, Politecnico di Torino, Viale Mattioli 39, 10100 TORINO, tel. +39(0)11 5644367, fax, +39(0)11 5644399, E-mail zichu@araxp.polito.it

Abstract

The conjectural philological survey of urban centres, devised at the end of the Seventies by a research group headed by Prof. A. Cavallari Murat of the Turin Politecnico is based on a perceptive analysis of the entities that made up the urban texture of an urban centre.
The information obtained, coded according to standard UNI 7310/84, is in the form of a graphical representation in which the various themes are rendered through colouring, point symbols, different line styles, and a certain number of labels. The above-mentioned UNI standard admits a single subjective interpretation of conditions that are deemed pathological for the urban texture through a two-colour representation. With the use of GIS-based tools it becomes possible to create two closely interlinked information structures: a database and a map-generating system. The database has the potential of storing all relevant information that may be of use, enabling the researcher to browse through the data, submit queries and handle the outcome of the search. Furthermore, the database can also generate maps, which in their turn are linked to the database, so that the latter can also be explored through direct hot-links on the maps. This makes it possible to produce diversified representations. A new interpretation of standard UNI 7310/74 has been developed, on the basis of a study conducted in connection with a survey of the historical centre of the City of Carignano, a small town characterised by significant medieval and baroque elements. This new interpretation serves as the basis of new analyses, focusing on the structuring of the urban texture, the detection of degnerative aspects, a diachronic perception of the way the city centre has evolved over the centuries.

INTRODUCTION

Conjectural philological survey is a technique that came into being through the studies promoted by a research team headed by Prof. Augusto Cavallari Murat, of the Turin Politecnico, in the late Sixties: the city is perceived not merely as a set of building containers, but rather as an unbroken cultural fabric, which can be rendered and interpreted through a special form of cartographic representation.
The possibility of expressing the urban configuration through a series of thematic maps had been the initial objective of the research project: the primary aim being the possibility of transferring the knowledge of the elements which had characterised the urban transformation process and which still determined its perceivable aspects.
To meet this need, it proved necessary to develop ad hoc notions and to define a series of graphic conventions.
A number of application experiences, such as the survey of baroque Turin and a study of the transformation of Piazzetta Ruccellai, in Florence, led to the coding of standard UNI 7310/74, i.e., a set of graphic symbols that would make it possible to "read" the city as it is at present and at the same time highlight the presence of the past.
Later one, whilst still working on the fine-tuning of symbols for the representation of urban transformations, the research team proposed a new interpretation of standard UNI 7310/74.
In the preparation of a map for the analysis of the urban context of the City of Carignano (see fig. 1), in the late Seventies, Prof. Cavallari Murat’s research team applied the previously coded standard, supplemented by the use of a two-colour scheme for thematic rendering. This map was prepared by working on a 1:250 scale and was printed with a reduction of 1/4, i.e., on a 1:1000 scale).

Figure 1
The map of the historical city centre of Carignano prepared by the research team of Prof. Cavallari Murat

As a support for the historical information, they relied on an abstract from a napoleonic map of the 19th century, and added a colour (red) to highlight the stylistic incongruence of subsequent transformations; through the use of this two-colour scheme the map offered a single, partly subjective, interpretation of conditions deemed pathological for the urban texture: the introduction of the red colouring was justified by the need to have a single map embody a survey of a "dynamic" nature.
The choice of graphic symbols and a 1:250 representation scale ware prompted by a desire to achieve a stereometric reading of the territory.
The map prepared in this manner made it possible to read the routes, the accesses, the horizontal and vertical distribution structures.

GIS-BASED TOOLS IN TERRITORIAL ANALYSIS

Starting from this wealth of studies and experiences, our investigation focused on the use of GIS tools for the management of data reflecting the transformations of the urban texture.
The multi-purpose and multi-user nature of the GIS system permits a critical approach to the territory: data of a great variety of types can be filed for the same surface, and several query keys can be used over the same quantity of elements. The geo-cross-referencing of the data and immediate data visualisation make for a total fruition of the results, so as to meet the requirements of investigations of all sorts concerning the urban texture.
Taking the lead from a critical re-reading of the conjectural philological survey of the City of Carignano, while we wait to be able to take advantage of a new numerical map-generating technique for the acquisition, storage, control, integration, processing and spatial representation of data, for the time being we have concentrated our efforts on the definition of a set of symbols suitable for use with GIS tools.
Logically, enough, in shifting from paper to EDP media, the problems to be addressed in connection with choice of symbols are altogether different.
The problem of the coding of the descriptive and symbolic elements for a paper map arises both from the need to ensure that the potential metric-descriptive information is always readable, and from the requirement of a univocal interpretation of the map itself regardless of type of user.
Studies concerning the search for a universal graphic convention for map drafting have long since demonstrated that some elements belong univocally to a given scale of representation rather to another.
Since its creation, the Italian Geodetic Commission has worked on the representation of the territory on a medium-to-large scale, by emphasising the need to define a specific graphic convention for each representation scale through detailed analyses and investigations.
This Commission also proposed the compilation of questionnaires for the determination "what a map should/should not contain" to be submitted to people belonging to different user basins. In connection with the creation of a database, having identified 10 different sets of homogenous contents, questionnaires were compiled in relation to the specific character of the map and the scale of representation.
Having selected three scales (1:2000, 1:1000, 1:500), a basic goal of the investigation was to identify positively essential as opposed to definitely superfluous elements.
The need to ensure the simultaneous presence on a single map of all signs relating to a given scale of representation prompted the definition of thematic maps enabling the quantity of data associated with specific topics to be reduced.
The use of GIS tools now makes it possible to produce geometric entity databases to file all data of interest, while making sure that the legibility of the data is not undermined by their very density. The possibility of interrogating the geometric entities - and hence the possibility of assuming that there is no need to render all the entity-related features in graphic form - enables level "0" to serve as a matrix for an infinite number of thematic maps and at the same requires that the utmost attention be paid to the choice of scale of representation and the ensuing graphic conventions.
The objective underlying the approach adopted in our investigation was to define a territorial data filing system which might lend itself to a multiplicity of uses, in addition to the customary uses of information systems as are typical of municipal administration practice (town planning applications, management of technological networks, aspects to do with the management of local taxation, etc.); we tried to fine-tune a system through which the geometric entity database might become significant for historians as well as people studying architectural themes in the urban context. Ultimately, the possibility of handling different thematic strains and formulating queries with different search keys makes for a diversified exploitation of the geometric supports: the double "valence" of the database, i.e, its ability to manage the material at data level to answer a query and to present the data in graphic form, makes for a greatly improved fruition potential.
Querying capability was a decisive consideration in the choice of the information support for the management of the data: a database, in fact, makes it possible to increase the quantity of data to be filed and sets no limits on the queries that can be submitted; a hypertext, instead, would impose a pre-determined path, designed by the author while creating the hypertext.

A CASE STUDY: THE CITY OF CARIGNANO

For the transposition of the philological survey data concerning the historical city centre of Carignano onto magnetic media, it was decided to work on a 1:1000 scale to be able to file simultaneously all the elements contained in the available thematic map, from which most of the information had been drawn.
To the contents already present on the paper support taken into consideration, other elements were added, concerning the uses of building volumes, their reading from the outside and the periods of construction.
The release from a fixed theme - a typical feature of paper maps - makes it possible to use colours and fields to obtain an infinite number of combinations to be associated with as many possible querying keys; the introduction of the toponymic data and the relative street numbers for the characterisation of accesses is meant for a wider spectrum of possible users.
If in the original Cavallari Murat survey the red lines were used to reflect and highlight the relationship with pre-existing structures, with the GIS tools the purpose of using colours for the fields depicting the areas defined in the data filing support is to identify the different historical phases and hence to make the evolution of the urban structure perceivable.
As is done on the paper support, the geometric entities are rendered with fields recognisable thanks to different line styles, and are labelled with area, linear and punctual symbols; the difference lies in the fact that the database makes it possible to manage a considerable amount of data, which are available for simultaneous consultation at different interrogation levels.
By taking into account that the support used was produced through the conversion into digital form of the 1:250 map (reduced to 1:1000), a basic step in the process of data filing was the decision as to which data should be maintained as basic geometric elements. Some of the symbols present in the 1:250 scale map, in fact, would cause a cognitive overloading phenomena if we tried to introduce them into a 1:1000 scale map, whilst they are important as data to be made available for consultation by the users interrogating the database. The digital conversion process was extended to:

1. all the buildings, areas under cover, yards;
2. green areas;
3. covered paths, such as porticoes, tunnels and driveways at ground level;
4. interior building areas taken up by vertical distribution elements.

Following this process, topology features were created and the relative data were entered. The correction of imperfections in the digitisation process, the creation of the topology features and the structuring of the data were achieved by means of the ARCAD software.
In the conjectural-philological surveying method created by Prof. Cavallari Murat’s research team, area fields were used to reflect the different heights of building; the numbers within the field, enclosed in a circle, or in a triangle for storeys originating from subsequent super-elevation works, indicated the number of storeys above ground of the individual buildings. In our information system, the area is associated with this type of information at attribute level, i.e., as data which can be obtained by submitting a query to this effect. Different types of field are adopted instead to show the destination of a building or area. Field colours are used to reflect the recognition of the different stages of evolution of the urban texture and structural type transformations.
As soon as the fields have been defined and have become a fixed presence in the basic support, it proves necessary to decide what other signs may coexist within a geometric entity without undermining the legibility of the data.
Inside the perimeters we still have the information concerning the presence of covered passages, such as entrance halls or porticoes, but vaulted covers are no longer defined, this feature having been introduced as a specific property of the area, which can be rendered in graphic form (through suitable symbols) by submitting a query.
In terms of linear symbols, different choices had to be made compared to the reference paper support where the double line defining the boundaries of a building - characterising the roofing/elevation structural node - hampered the perception of full/empty relations. The same information is given as a property of the line itself and can be accessed only by querying; this applies to many other characterising elements of the front views, such as the distribution of the openings.
The presence of the axes of the windows in a typical storey, occasionally enriched with the total number of the openings over the entire elevation (expressed in the reference graphic documents as a number enclosed in a rectangle), did not provide sufficient indications to figure out the actual distribution the openings; therefore it has been replaced by considerations such as the hierarchical, rhythmic, symmetrical or mirror-like arrangements of the openings, all of them expressed as data that can be looked up by submitting a query to this effect.
The accesses to the buildings, graphically represented in the reference maps by means of different types of arrows, have been replaced with the indication of the street numbers, to serve as an added tool for administrative purposes.
The introduction of toponymic data (street names and numbers) makes it possible to read the urban values as a function of specific research areas, bringing out the various transformations that have taken place in the urban texture.
This aspect is exemplified by three significant investigations.

Fig. 2 - a query by address concerning area coverage makes it possible to grasp the existence of a hierarchy governing the road axes, and to identify the one that can be rated as the matrix route as opposed to the other main axes. From this query, it proves possible to perceive at once the role played by the "roads" in the different successive expansions of the city texture. Whether to assign an area to one road rather than any of the adjacent roads is determined solely by the access to the area itself; when an area has several access points, it was decided to take into account only the most important. Even the evolution of the housing cell, from the construction of new wings of a building in the area of belonging, to the break down and/or re-aggregation of several buildings according to the requirements arising from progressive socio-economic transformations, can be looked up by interrogating the database.

Fig. 2 Interrogating the area coverage by address.

Fig. 3 - a query by use and street number concerning area coverage (and/or by address/access concerning linear coverage) makes it possible to correlate different aspects in the evolution of historical city centres. A typical instance of this is the arrangement of church buildings and the creation of spaces in front of them, for their complete perception and fruition. It was decided to take into account the following uses:

• housing areas
• public use area
• area for worship
• military use area

• portico
• yard
• vertical distribution (stairs)

• square
• road
• crossing

It was also decided to interrogate the database by "street number" and to have this field appear through a punctual indication, to be extended to all the internal volumes leading to the same access; the road labels, which have been left out in order not to undermine map readability, can be readily looked up and expressed in graphic form as shown in Fig. 4. As for the information of an administrative nature, the data concerning direct or internal road accesses have been connected with the lines; this might give rise to inconsistencies in the representation when some of the lines, clearly opening on a given road, have the property of belonging to a different one. From a query concerning the lines it is therefore possible to bring out only either the external envelope or the entire internal redistribution of the areas by turning on or off the elevations relating to "int." roads.

Fig. 3 - Interrogating the area coverage by use and street number; and/or the linear coverage by address/access.

Fig. 4 - zooming in on the query concerning area coverage by use and street number to have the address labels appear; detail of the area in front of the Cathedral. The choice of not providing a double interrogation key to display both the street numbers and the addresses simultaneously on the map was motivated by considerations to do with the legibility of the data at the different scales. In Fig. 2 road labels have not been activated, but this can be done at any time, as borne out by the detailed view depicted in Fig. 4. Each individual geometric entity included in the graphic support (areas, lines, points) can be described in greater detail through the application of ad hoc labels, which can be applied or removed instantaneously.

Fig. 4 - magnified view of the query concerning area coverage by use and street number, with the address labels applied.

Fig. 5 - a query by number of storeys concerning area coverage offers an immediate perception of volumetric values in historical city centres. Full and empty spaces take on an identity which calls for an interpretation: for instance, the yards and squares should both have the same value ("zero") in terms of number of storeys. In actual fact, the yards, which are generally crowded with roofings and sheds, have been assigned the corresponding numerical value, whist squares (as well as roads and cross-roads) have not been assigned any numerical value, so that they cannot be queried at all. The definition of a field for the "number of storeys above ground" poses a problem when dealing with church buildings.
These structures, in fact, have an outside volume which can be likened to an appreciable number of storeys above ground, but from the inside this correlation is not as straightforward. it was decided to diversify the different interior areas of a church building, by leaving their volumetric properties unaltered: for instance, the stairwell is assigned a given number of storeys above ground and the naves are assigned a different number.

Fig. 5- Interrogating the area coverage by number of storeys.

CONCLUSIONS

The study will be carried on to devise better ways to integrate the data of administrative interest. In addition to continuing work on the digitisation of existing documents, obviously of a two-dimensional sort, further tests will be extended to maps obtained through direct numerical photogrammetric restitution, i.e., to documents that retain their three-dimensional content in its entirety.

REFERENCES

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