Assessment of the visual impact made on the landscape by new buildings: a methodology for site selection

Landscape and Urban Planning 68 (2004) 15–28

Assessment of the visual impact made on the landscape by new buildings: a methodology for site selection Julio Hernández a,∗ , Lorenzo Garc´ıa b , Francisco Ayuga c a

Centro Universitario de Plasencia, Universidad de Extremadura, Virgen del Puerto 2, 10600 Plasencia, Spain Centro Universitario de Mérida, Universidad de Extremadura, Santa Teresa de Jornet 38, 06800 Mérida, Spain Departamento de Construcción y V´ıas Rurales, Universidad Politécnica de Madrid, E.T.S.I. Agronómos, 28040 Madrid, Spain b

c

Received 7 May 2003; accepted 13 May 2003

Abstract Geographical information systems (GIS) are excellent tools for landscape modelling and three-dimensional analysis. They allow easy digitalisation of geographical information and coverage structure, as well as facilitating graphical representation. The aim of this research, using a new computer programme which makes use of GIS, was to analyse the location of rural constructions in order to improve their integration into the countryside. Logical routines were employed to study planning variables (physical, social, economic and legal) and the visual impact of buildings on the landscape. These routines can be used for analysis and decision-making in environmental administration, and are useful to planners and designers attempting to choose locations where new rural buildings will best integrate into the landscape. Finally, a public survey was conducted on visual impact of rural buildings, in order to endorse the proposed methodology. © 2003 Elsevier B.V. All rights reserved. Keywords: Landscape planning; Visual impact; Rural buildings; Site selection; GIS

1. Introduction In recent years, a great deal of effort has been made to study the landscape not only from the artistic point of view but also from scientific and technical standpoints. Much work has been undertaken to analyse the landscape and to develop methods for evaluating the different impacts it suffers (Mayall and Brent Hall, 1994; Smardon and Karp, 1995; Peccol et al., 1996). A common characteristic of these approaches is the attempt to measure the impact of human inter∗ Corresponding author. Tel.: +34-927-427000; fax: +34-927-425204. E-mail addresses: [email protected] (J. Hern´andez), [email protected] (L. Garc´ıa), [email protected] (F. Ayuga).

0169-2046/02/$20.00 © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-2046(03)00116-6

ventions in the territory. Different methodologies have been developed, many based on assessing the landscape’s physical, aesthetic and psychological attributes (Litton, 1972; Zube et al., 1982; Lynch and Gimblett, 1992). Over the last few decades, ways have been sought to design a landscape assessment system based on methodologies seeking to reduce the visual contrast of the project in relation to its setting. Visual contrast can be analysed according to the visual elements of the landscape (Smardon, 1979; Jakle, 1987). Variation in a single element or variable that determines visual preference by means of computer simulations is a way to assess visual impact (Orland, 1988; Bishop and Leahy, 1989). Over the past few years, geographic information systems provide the means to implement routines and

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macros making feasible complex analyses based on territorial data. The use of this information technology allows measurements and calculations to be made with greater accuracy and objectivity than do methods which use human operators alone (Bishop and Hulse, 1994; Pullar and Tidey, 2001). Several problems must be considered when developing such programmes. A series of variables and attributes must be selected and then quantified in order to evaluate landscape impact (not all the elements necessary for making landscape assessments are measurable). Furthermore, these variables must be easily treated. This work is the continuation of previous research and aims to provide planners and designers with a tool covering the site selection aspect of the building. Though engineers and architects often have the opportunity to modify the spatial location of a building (the option of defining the surroundings of a construction is frequently a variable in a project definition), this is not always the case with design criteria, which are often imposed by the promoter.

2. Materials and methods The proposed methodology—which makes use of a computer programme—for the location study of new rural buildings can be divided into two processes with different aims: • Analysis of the territorial system: The territorial system represents a complex aggregate of interrelationships and different variables. A complete physical, socio-economic and legal analysis based on geographical information system (GIS) will be carried out with the aim of determining optimal locations according to the traditional planning criteria of the area. • Visual impact assessment: The evaluation of a building’s spatial location by GIS from the point of view of visual integration, using scenic composition and scenic background criteria. Spatial location is one of several visual landscape elements. The aim of this process is to evaluate the visual impact a construction will have on the landscape, and to select the points where this impact will be least.

These two methodological procedures are consecutive (Fig. 1). Optimal locations can then be determined from among the ones selected (according to planning criteria) from the point of view of visual impact. The computer programme GISCAD 2.0, developed by the authors, is based on GIS ARC/INFO, uses AML language (ARC/INFO Macro Language), consists of nine routines executed from a pull-down menu, and has 13 graphics windows for introducing required data. It employs this methodology using a series of logical sequences to select the locations that fulfil all planning and visual integration requirements. 2.1. Analysis of the territorial system The first phase of the proposed methodology is the study of the territorial system. This aims to make an initial selection of the possible building sites. In the analytical and diagnostic sequences required to process the planning directives of a territory, the study area can be characterised in terms of its physical/natural, socio-economic and legal/institutional subsystems together with human establishment (Gómez Orea, 1994). (a) The physical/natural subsystem refers to the natural features of the terrain: climate, topography, water, soils, vegetation, fauna, natural processes, etc. (b) The socio-economic subsystem includes the population and its economic and social activities. (c) The human establishment subsystem refers to urban nuclei, transport and communication networks, and basic infrastructures and their equipment (Fig. 2). (d) The institutional and legal subsystem comprises governmental bodies and the regulations governing the territory. Each of these subsystems is composed of a multitude of variables linked to others by complex relationships of varying intensity. Furthermore, these subsystems are not independent but interrelated. To diagnose a territorial system properly, a correctly-made inventory, which should include all the information needed for later analysis, is required. This information will be spatial, documentary and statistical, in the form of maps, graphical images and files, or the layers and coverages of a GIS.

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Possible sites for new rural buildings

PLANNING ANALYSIS ACCORDING PHISICAL, SOCIOECONOMIC AND LEGAL CRITERIA

ANALYSIS OF THE TERRITORIAL SYSTEM PHASE

Selected cells

VISIBILITY ANALYSIS FOR CHOOSING LEAST VISIBLE CELLS

Selected cells

VISUAL IMPACT ASSESSMENT PHASE PLANNING ANALYSIS ACCORDING SCENIC COMPOSITION AND SCENIC BACKGROUND CRITERIA

Selected sites

Fig. 1. Methodological procedure followed by GISCAD 2.0 running in ARC/INFO.

Once a complete diagnosis of the different subsystems composing a territorial system has been made, the information is synthesised for processing. To integrate this information, all the above results must have a compatible and comparable format. This is provided by using raster coverages, enabling operation in the GRID module of ARC/INFO.

To integrate the information obtained in the diagnostic, stored in the form of raster coverages, it must be homogenised and reclassified so that algebraic operations can be performed. This reclassification is undertaken using conditional expressions of ARC/INFO within the GRID operative module. Once reclassified, the resulting coverages lead to new raster

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Fig. 2. Window programmed in AML to enter auxiliary coverages and data to analyse human establishment subsystem. It works by running an AML routine to make buffer operations between data coverages. The result is a raster coverage of planning values.

coverages with the value of each cell presented as a linear combination of the values of the different starting coverages. The matrix algebra tools of GIS are used in this procedure. The final result is the coverage of the territorial units presenting optimal locations for new buildings. However, to date, these locations have only been optimised from the point of view of territorial planning, and do no take visual impact into account (Fig. 3).

2.2. Visual impact assessment 2.2.1. Spatial model of the territory To undertake a three-dimensional analysis of a given area, spatial modelling is necessary. The spatial modelling of a territory means producing a geo-referenced 3D model by computer. It then becomes possible to perform calculations and undertake analyses, the results of which can be extrapolated to the terri-

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Fig. 3. Methodological diagram for territorial system analysis.

tory represented. This can be done in two different ways:

Although these are alternatives, the result is the same: a three-dimensional model of the territory.

• The creation of a digital elevation model (DEM) based on regular cells and stored as raster coverages. • The generation of a network of irregular triangles (TIN) stored as vector coverages.

2.2.2. Spatial model of buildings For the analysis of spatial visibility, a three-dimensional model must be made of the building to be studied (see Lange, 1994). To this end, a three-dimensional

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vector field is created showing the most significant parts of the building in their spatial locations. These key points are obtained by breaking the building down into basic masses defined by their vertices. A maximum of 16 key points can be defined due to an ARC/INFO software limitation. The programming that generates this coverage is included in the CONSTRUC.AML routine. This automatically creates a coverage of points from the characteristic dimensions of the construction: width, length, height, presence of singular elements such as towers, masts and chimneys, etc. 2.2.3. Concept of scenic composition Scenic composition can be defined as the way of perceiving the environment where the project is to be executed (Garc´ıa Moruno and Hernández Blanco, 2001). This involves the three-dimensional layout of the different units that conform the landscape, and their interrelationships. This produces different visibility conditions for these units with respect to one another, and for the whole (or part of it) with respect to an external observer. Direct visual relationship is established between the construction and its surroundings by means of visual lines defining the direction and extent of view. This is calculated and represented graphically by using a GIS with three-dimensional analysis capacity. 2.2.4. Calculation of scenic composition using a GIS Previous to the scenic composition calculation process, the computer makes an estimate of the cells of the territory that are least visible from buildings and roads. This is the traditional analysis to determine the visibility of an area from one or more points. The command used is VISIBILITY with FREQUENCY option. After this, the least visible cells are selected in a new coverage. The building model is only located on the selected cells of the study area for the scenic composition calculation process. The final aim of this procedure is to reduce the number of the cells where the scenic composition of the building model will be analysed. It decreases the number of calculation cycles and the time taken for the next stage. The scenic composition of a building’s location in a given landscape can be calculated from the spatial analysis of these visual relationships (Hernández Blanco and Garc´ıa Moruno, 2001). This is a special

type of visibility analysis. These calculations are made by the computer tracing vision lines from each single cell of the DEM to each of the vertices representative of the building. The number of visible vertices is a value stored in each cell of the new visibility coverage generated. The command ARC/INFO used is VISIBILITY with OBSERVERS option. The visibility coverage obtained is referred to as visual values (VV). In order to calculate scenic composition, one must start with the information stored in the visibility coverage expressed as visual values. This information is not analysed in its totality. Instead, a circular area of visibility is defined, at the centre of which lies the possible location for the building. Scenic composition is calculated by the computer using the visual information contained within this analysis area. Several authors have estimated human vision to be capable of perceiving objects clearly at up to 3500 m (Steinitz and Merlyn, 1974; Ramos et al., 1976; De Veer and Burrough, 1978; Steinitz, 1979; Español Echániz, 1996). The diameter of the circle of the analysis area was therefore set at 7000 m, although this distance might vary in other investigations since it refers strictly to the conditions necessary to identify with clarity the object of observation. Scenic composition is then calculated by the COMPOSITION.AML routine. This performs a statistical study of the visual values of each of the cells in the visibility area. A similar statistical study is also made for the cells in the DEM coverage. The computer then makes a logical analysis of the results obtained. In the visibility coverage (VV), the average is calculated of the visual data of all cells in the visibility area. The value found is called the visual average (VA):
n VV VA = 1 n where VA is visual average and VV the visual values and n the number of cells in the analysis area. For DEM coverage, the average of the height values for the area is also calculated:
n HV HA = 1 n where HA is the height average, HV the height values and n the number of cells in the analysis area. Once these two figures have been obtained, thresholds for the processing of five cases corresponding to

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five types of scenic composition are defined. For the topographical values, a critical height interval of 20 m is established, so that the analysis describes three possible situations: 1. HA < z−20 (z is the absolute height of the building site in DEM coverage); 2. HA > z + 20; 3. z − 20 < HA < z + 20.

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2.2.5. Types of scenic composition Five characteristics of scenic composition can therefore be defined. These are not mutually exclusive nor do they have hard boundaries. Although they are defined with mathematical accuracy, so that the computer receives no ambiguities or errors when processing the information, reality is rather more complex.

In this case the scenic composition is said to be FILTERING.

1. Filtering: This is a way of gradually directing the observer’s attention so that during approximation the building is gradually revealed in its entirety. This can be achieved using an open screen of trees, through which one can see. This was implemented in a GIS using the vegetation coverage of the area to determine if there are vegetation areas close to the building model. If this is the case, and the result of scenic composition logical analysis is ‘open’, then the final value is ‘filtering’ (Fig. 4). 2. Open or panoramic: There are no apparent limits to the field of vision. Most elements are horizontal and the sky dominates the scene. In these spaces it is impossible to refer to any physical limit because the horizon is the limit. The building is perceived and the composite limits are not closed. 3. Closed: This is defined by the presence of visual barriers establishing marked boundaries. When the element under study has a composition of this type it is quickly perceived. There are well-defined spatial limits that attract attention due to the limited information available. 4. Singularity: A specific element that dominates the scene (e.g. an isolated tree). This is frequently found in open compositions, in which the distinctive object becomes the dominant feature. 5. Focused: This is marked by the existence of parallel lines or lined objects. Focal spaces are created due to strong linearity (originated by linear infrastructures) appearing to converge on a focal point that dominates the scene. Thus, elements of the landscape can be used to place a building in an outstanding position (Fig. 5).

This process does not aim to calculate the absolute visibility of the building, but rather the scenic composition of the building model located on the cells pre-selected according to traditional planning criteria.The following section explains the meaning of these logical calculations in greater depth.

2.2.6. Concept of scenic background In visual resource management, this is termed ‘visual absorption capacity’ (Smardon et al., 1986). This refers to the relative topographical location of formal elements with their own identity. Scenic background is defined in relation to the altitude and characteristics

The different cases are: Case 1. ⇒ VA ≥ 0.5 ⇒ z − 20 < HA < z + 20 In this case the scenic composition is OPEN. Case 2. ⇒ VA < 0.5 ⇒ z − 20 < HA < z + 20 In this case the scenic composition is CLOSED. Case 3. ⇒ HA < z − 20 In this case the scenic composition is a SINGULARITY. Case 4. ⇒ HA > z + 20 In this case the scenic composition is FOCUSED. Case 5. ⇒ VA < 0.5 ⇒ There is vegetation within the visibility area.

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Fig. 4. Filtering scenic composition (Esla River Valley, Spain). This raster coverage is the result of a calculation process using visibility coverage and DEM. Contour line coverage is overlapped. The building model is located at the centre of the circle. Many scenic composition coverages are obtained from the calculation process, one per building model site. In light colours show partial and full visibility areas and black makes reference to no visibility areas. The presence of vegetation elements close to the building model, in open scenic compositions, can be detected by the computer and processed.

of the composition. Consequently, it is a specific study of the siting of a building basically determined by its topographical position. The background is the backdrop to a view. In open compositions it may be the sky, in seashore scenes it may be the water, and in closed spaces the land itself. This is of great importance because it is a determining factor in the basic range of contrasts; it establishes the sharpness of the outlines and the continuity and size of the space. An object seen against a background of sky or water generally stands out far more than one seen against land. The qualitative values that can be adopted by the scenic background are: 1. Sky: This value can be adopted when there is an interruption of the horizon. The visual impact of a building varies enormously depending on its position with respect to the horizon. Buildings that break the horizon are seen from many different an-

gles and also from a great distance. This is a way of highlighting a building, but it can also break one of the most important lines defining the landscape. 2. Ground: A building located on level land can be easily absorbed by the landscape, therefore granting clearer visual continuity to the composition. In this case, there is no interruption of the horizon. The scenic background has the same value as the land that surrounds the building. This effect is obtained when the building is located in topographic depressions or on level land surrounded by mountains. 2.2.7. Calculation of scenic background For scenic background calculation, the concept of visual lines is used. Visual lines are produced by the command SURFACESIGHTING, working with the ARCPLOT module of ARC/INFO. This allows the calculation of an objective point located at some

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Fig. 5. Focused scenic composition (Esla River Valley, Spain). The building can be a focus of the observer’s attention.

distance from the observer and which lies on the straight visual line from the eye of the observer to the construction. Based on GIS, the programme allows one to work out whether the objective point is visible from the observer’s position, i.e. if the background of the calculated vision line is the ground (the objective point would not be visible to the observer) or the sky (the objective point would be visible to the observer). Scenic background depends, among other variables, on the relative position of each observer. It is associated with the relative position of the observed building. Therefore, for the determination of scenic background as a characteristic of the visual element space, multiple visual lines must be generated from the points where the greatest number of observers will be. These points are along roads and in buildings. One of the ARC/INFO commands is used to turn the linear coverages corresponding to roads and buildings into point coverages. The BACKGROUND.AML routine draws multiple visual lines from these points to the highest point of the building (building point). Once these lines of vision are drawn and the scenic background of each point is calculated, the number of

times that the scenic background is either the ground or the sky is obtained. Whatever happens to be the most frequent scenic background, either ground or sky, is considered the scenic background for the possible site of the building. This process is repeated for each location of the building model, consequently, for each cell selected according to traditional planning criteria.

3. Public survey on the visual impact of rural buildings A public survey was undertaken in order to endorse the proposed methodology with respect to visual impact, i.e. to ensure that the computer-made, objective measurements performed on parameters normally considered subjective, matched human appreciations. The idea of the survey was to convert a series of questions based on qualitative attributes into quantitative values (Bishop et al., 2000). The values are the number of times each attribute was mentioned. This survey was performed by showing 30 photographs of buildings to 150 people of different ages,

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Fig. 6. First page of the public survey on the visual impact of rural buildings. Photo realistic simulations (computer-generated) have been used in pictures #1, #3, #4, #5 and #6.

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Table 1 Questions of the survey How would you rate the integration of the building(s) in the scene that appears in the photograph? Very bad Bad Acceptable Good

Very good

What characteristic(s) of the group of buildings or their constructional components should be modified to improve their integration into the scene? Colour Texture of the materials Lines and forms Scale Spatial location

educational backgrounds and places of origin (Fig. 6). Two different questions and five answers were drawn up (Table 1). For the first question, only one answer was allowed, but for the second it was possible to choose one or more answers. The results were processed and compared to the spatial classification calculated. The aim was to achieve as heterogeneous a sample as possible.

4. Results and discussion In the survey, space was one of the visual elements most often indicated as requiring improvement (Table 2). This is logical since it is one of the high-priority factors that influence the integration of rural buildings into the landscape, and the first perceived by the observer.

Table 2 Survey results respect spatial location No. of picture

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Scenic composition

Scenic background

Valuation in the survey (%) Very bad + bad

Acceptable

Good + very Good

Closed Closed Closed Closed Singularity Singularity Open Closed Closed Filtering Open Open Closed Open Open Closed Filtering Filtering Singularity Focused Closed Open Open Filtering Closed Closed Closed Closed Open Open

Ground Ground Ground Ground Sky Ground Ground Ground Sky Ground Sky Sky Ground Sky Sky Ground Sky Ground Sky Ground Ground Sky Sky Sky Ground Ground Sky Ground Sky Sky

20 25 45 77 29 32 48 67 39 7 9 20 67 43 40 26 55 19 40 41 71 33 20 40 73 48 13 15 37 49

39 42 23 15 32 35 31 17 26 25 20 45 22 36 41 32 29 40 29 30 21 49 28 34 17 31 21 29 29 23

41 33 32 8 39 33 21 16 35 68 71 35 11 21 19 42 16 41 31 29 8 18 52 26 10 21 66 56 34 28

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The only pictures evaluated for the analysis of the survey results were those where subjects indicated spatial location as the main element to improve. This is because, in the proposed methodology, scenic background is studied from the visibility lines drawn from thousands of points for each possible location of a building. Evaluation of a single photograph alone could introduce a weakness into the results unless the visual element emphasised over all others was spatial location. This means that subjects believed that before acting on the building itself, it was important to act on its location, either varying the scenic composition or the scenic background. In the case of scenic composition, this procedure was not followed, since this spatial location attribute is less susceptible to variation in the results, according to the contrasts induced on other visual elements. Although results for singular and focused scenic compositions were recorded, these were not taken into consideration, since the number of pictures that represented such cases was low and since the results corresponding to these factors are influenced enormously by the type of building selected. The observer tends to pay attention to the focal point of the scene. If the building is attractive, and has cultural or aesthetic value, the scene’s value will be understood as positive. If the building introduces incompatible contrast, the score for the scene is negative. One of the assumptions in the present work is that interaction between the construction and the surroundings produces an incompatible contrast (Tandy, 1979; Di Facio, 1989; Garc´ıa Moruno and Hernández Blanco, 2001) with respect to visual integration. Mitigation of such contrasts can only be performed with respect to the spatial location of the building. This is true of most projects currently undertaken in rural areas. There are very few that contemplate an evaluation of the visual impact caused by the design of the building in terms of objective and measurable parameters. This work hypothesis has been established to progress the research about visual impact of buildings. We are also working on design variables of buildings to improve the visual landscape integration. In addition to the scenic background, the scenic composition is a characteristic of space as a visual element. Visual perception of the project by the observer depends partly on this (Hernández Blanco and

Fig. 7. Comparison of scenic composition and perception values obtained from the survey. New buildings (not landscape integration designed) in closed scenic composition are perceived as “bad and very bad”.

Garc´ıa Moruno, 2001). It is directly related to the visual impact beheld by the observer. Therefore, if visual perception of a construction is diminished, its visual impact is diminished too. Scenic composition is one of the variables that can be acted on, so as to reduce visual impact. Efforts should be taken to make the scenic composition ‘filtered’ (selecting sites close to vegetation areas) or ‘open’, since these options are related to lower visual perception levels (Fig. 7). Like scenic composition, scenic background is a composite characteristic of the visual element space. In this proposal, only two possible scenic backgrounds are considered in order to simplify the calculation: the sky and the ground. When the sky forms the scenic background of a building, the horizon is broken and the forms and lines of the building are perceived with greater intensity by the observer (Español Echániz, 1995). Assuming that the visual integration of modern constructions is negative and produces incompatible contrasts, a sky background originates high visual impact values. However, buildings whose scenic background is the ground do not break the horizon and their shapes and lines are ‘absorbed’ by the surrounding landscape. In this case, perception is lessened and the visual impact lower (Fig. 8).

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Fig. 8. Comparison of scenic background and perception values obtained from the survey. New buildings are valued as “good and very good” for scenic background ground and as “bad and very bad” for scenic background sky.

A possible conclusion, with respect to the survey results of the visual impact study, is that new rural buildings should preferably be located in areas where there is little rough topography, at low altitude, and in open rather than limited spaces where the building will not form a point of preferred attention in the landscape.

Acknowledgements This work received backing from Research Project P98-0215022 funded by Fundación Alfonso Mart´ın Escudero. The authors would like to thank Adrian Burton and Stephen Jenkins for the English supervision.

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