managing visual impacts through a gis: viewshed analysis for designing ...€¦ · managing visual...

Managing visual impacts through a GIS: viewshed analysis for designing a sanitary landfill site

A. Tsouchlaraki1 & G. Achilleos2 1Hellenic Open University, Greece 2National Technical University of Athens, Greece

Abstract

The idea of a location where waste is disposed causes prejudice to the residents and the visitors of the surrounding area, no matter how strictly the sanitary specifications are observed. Everyone realizes that most of these projects are necessary for regional planning and development, however no one wants these projects to be constructed near him/ her, especially when these projects are visible and cause negative visual impacts that are permanent or present for a long period. The selection of possible locations for an S.L.S. is based on technical and financial criteria. GIS technology provides many useful tools to the planners of such projects, which help them to take into consideration all the necessary criteria and parameters in order to take their final allocation decision. This paper presents the development of a process which was implemented in the design phase of an S.L.S. in Polygyros in the area of Chalkidiki, aiming to determine the viewshed before and after the activity is arranged, and to compare the viewshed to the prevailing land use of the area, in order to draw conclusions on the significance of the potential visual impacts. Viewshed analysis maps are created, presenting the spatial distribution of the visual impacts and therefore playing the role of a decision support tool. Keywords: visual impacts, viewshed analysis, GIS, Sanitary Landfill Site.

1 Introduction

When designing a Sanitary Landfill Site (SLS) we take for granted, right from the beginning, the fact that it will have a negative impact on the area of deployment. The refusal from the part of the future neighbours with regard to the

Waste Management and the Environment II, V. Popov, H. Itoh, C.A. Brebbia & S. Kungolos (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-738-8

spatial arrangement of an SLS, is called ΝΙΜΒΥ syndrome (Not In My Back Yard) [1]. The idea of a location where waste is disposed causes prejudice to the residents and the visitors of the surrounding area, no matter how strictly the sanitary specifications are observed. The degree of refusal of a ΝΙΜΒΥ syndrome may be high or low, depending on the facility's hazardousness and the neighbours' characteristics. The selection of possible locations for an SLS is based on technical and financial criteria. Two basic criteria are: a)the ground's characteristics and the geological profile of the wider area where the site would be arranged, so as to avoid the pollution of the watertable; and b)the land acquisition cost. However, another criterion, which should be taken into consideration, is the visibility from the site, where the activity is to be arranged, so as to prevent and minimize visual effects caused by the intervention. The more visible the intervention is, the more noticeable and important its visual and aesthetic impacts. Therefore, knowing in advance the visibility conditions, which will prevail after the intervention is realized, gives the designer the opportunity to assess the impact and to select the most suitable restoration measures, or even to modify it or find an alternative one [2, 3, 4].

The calculation of the viewshed from a certain observation post is nowadays a basic process which can be accomplished by Geographical Information Systems (G.I.S.) through their ability to manipulate hypsometric information [5]. This process has been extensively examined in the literature by Fisher [5, 6, 7, 8, 9, 10, 11]; Travis et al [12], Lee [13]; De Floriani et al [14, 15]; Erdas Inc. [16]; ESRI [17]; Burrough [18]; Pavlidis [19].

This paper presents the development of a process which was implemented in the design phase of an SLS in Polygyros in the area of Chalkidiki, aiming to determine the viewshed before and after the activity is arranged, and to compare the viewshed to the prevailing land uses of the area, in order to draw conclusions on the significance of the potential visual impact. The visibility analysis was performed using the “area seen viewshed map” algorithm, which is further described.

2 Visibility analysis – “area seen viewshed map” algorithm

The determination of the viewshed has been extensively examined with the development of DTMs and GIS, which resolved many problems regarding difficult and repetitive calculations.

A definition of visibility is given by Fisher [5, 6, 7, 8, 9, 10, 11]:

“Two points in a DTM (A, B) are intervisible, only when there is a straight line which connects point A to point B without intersecting the DTM at a point between points A, B.”

With regard to the viewshed, V table is formulated, whose elements are a)Vij=1 when Pij is visible from P; and b)Vij=0 in the opposite case. The viewshed contains the areas of table V, where Vij=1. It should be noted that if there is visibility from point A to B, the reverse is not necessarily true, i.e.

440 Waste Management and the Environment II


visibility from B to A (Figure 1). This is due to the observer's elevation, which changes position, and therefore alters hypsometrically the two points and their connecting lines.

The general visibility calculation algorithm is given in formula (1):

H = G(x,y) (1) where: H: the post elevation, x=f1(t), y=f2(t), z=f3(t) where [ ]Tt ,0∈

and

T: the length of the line of sight

If for a given t T∈ 0, the following formula is true:

G(f1(t),f2(t)) > f3(t) (2) target point B is defined as “not visible”. In the opposite case, it is defined as “visible”.

Figure 1: The case of a visible and not visible target.

The viewshed calculation algorithms vary as to the process and the techniques used with the aim to decrease the calculation time and optimize the quality of the results [20].

Many types of viewsheds have been devised and developed, in order to meet various needs. Therefore, the typical viewshed, where each point / pixel / triangle is determined as visible or not visible, is a binary representation of the problem. In other viewsheds, each point is codified in proportion to the number of times it is seen from the observation posts, or in proportion to the surface area of the wider region where it has visibility within a particular radius. One could easily identify from such maps the observation posts that provide a long range of visibility and towards a particular direction. New types of viewsheds are the probabilistic viewsheds [9] and the fuzzy viewsheds [11] which do not determine their viewpoints in the conventional manner, as “Visible” or “Not Visible”, but provide either the possibility for a viewpoint to be probably seen, or the value of the membership function in the viewshed (fuzzy).

Waste Management and the Environment II 441


The viewshed determination algorithm, which has been selected for this paper, is the "area seen viewshed map" or the "times seen" algorithm [Randolph]. Each point in the study area is examined as to the viewshed it provides within a specified radius and it is codified according to the number of the posts of interest from which it is seen. This algorithm has been selected because it provided us with the opportunity to define certain observation posts of interest in the area of the SLS, which determine the sensitivity of the landscape in question, and therefore their viewshed, and to determine the significance of the visual impact to be caused by the spatial arrangement of the activity.

Figure 2: “TIMES seen” viewshed (A, B, C are the view points).

A variant of this algorithm [21] is the codification of each point of the area of interest on the basis of the direction on which lies the major part of its viewshed. In this way, one could easily identify particular directions of interest. In our case, this variant hasn’t been selected, because we wanted to examine all the directions around the SLS area. It is a further possibility, which in other cases may be worth applying.

3 Landscape sensitivity in the broader area of the SLS

The sensitivity of a landscape depends on various environmental, historical, cultural and tourist factors that form the public opinion. The higher the interest of the public is, the higher the sensitivity of the landscape to new spatial arrangements [3, 22]. The distance from which the various visual elements of the environment are observed constitutes an important variable, since it defines the perceived accuracy of the elements. Landscape scientists have defined the distance range of 5km as a crucial zone, which receives the most significant visual effects by the spatial arrangement of a project [4, 22]. At a range of 5km around the Polygyros SLS project in question, the area does not present any particular historical and tourist interest, with the exception of the community of Polygyros. The prevailing land uses are tree cultures and shrubby areas, the road network is limited to a provincial road and some rural roads, while outside the Polygyros community there is the Petalidas settlement and some scattered country churches.



As to the morphology of the relief, which also constitutes a crucial variable in landscape analysis, there is a variety of relief forms, in which flat to average slopes prevail, and intense slopes are rare, often alternating. The elevation values range within 50-900m, with mean value 410m and the slopes lie within the range of 0 - 74% with an average slope of 14%. Table 1, shows the slopes classified in four categories, along with their percentages of appearance. The mean elevation of the SLS is 390m and of Polygyros 535m.

Table 1: Slope appearance percentage.

Category Slopes Percentage of appearance 1 0-8% 29% 2 8-15% 35% 3 15-40% 29% 4 >40% 7%

All the above elements have been determined with the use of GIS

applications, while the hypsometric information was collected from a topographic map of a 1:50.000 scale.

4 SLS visibility analysis

The SLS's viewshed was initially determined by using the typical general algorithm presented in section 2. A radius of 5km around the SLS area was used, with observation height being 1,5m above ground level, and the hypsometric information of a 1:50.000 scale. This scale was used in the beginning of the process, in order to avoid further analyses, in case visibility appeared to be limited and without particular interest.

Based on the results, it seems that the viewshed of the SLS is large enough. A large part of the town of Polygyros and certain parts of the road network are placed within it (Map 1). The settlement of Petalidas is not visible.

Polygyros is the main zone of interest for the assessment of visual impact and the probability that it could be visible calls for a further visibility analysis. The SLS contains four cells, three of which are not however designed during the first project phase and, therefore, there are no geometrical elements for their final configuration.

Furthermore, the analysis contains only Cell 1 of the SLS. It is possible that Cell 1 has a smaller viewshed from the entire SLS; but if a particular analysis is not carried out for this Cell, we cannot draw any result as to whether Polygyros is visible.

4.1 Visibility analysis for Cell 1

4.1.1 Definition of points of reference In the area of Cell 1, certain reference points are defined, densely and equally distributed throughout the area (Map 2), for which the viewsheds will be



calculated. Instead of examining all the points / pixels of Cell 1, some points are sampled. The sampling of the points is made for time saving in the processing required for the viewshed determination, and if made carefully, it does not affect the results [22].

SANITARY LANDFILL SITEVISIBILITY MAP

LEGENDROAD NETWORK

URBAN AREA

SANITARY LANDFILL SITE

VISIBLE AREA

POLYGYROS

PETALIDAS

NEW S.L.S.

POLYGYROS

PETALIDAS

NEW S.L.S.

Map 1: Visibility map for the entire SLS.

Map 2: Reference points for Cell 1 of the SLS.

Following this, the viewsheds of the points that have been sampled as binary data information levels, are cumulatively superimposed, in order to calculate the total surface area that is visible from Cell 1. In other words, this is the determination algorithm "area seen viewshed map", described in section 2. This



analysis takes place, by using hypsometric data that existed prior to and after the beginning of the works, in order to give us the opportunity to compare the alterations that will occur due to the spatial arrangement of the project.

4.1.2 Visibility analysis prior to the beginning of the works The viewshed of Cell 1 was determined only for the rectangular part, which includes marginally Cell 1 and Polygyros (Image 1), since Polygyros constitutes the main zone of interest.

Image 1: Relief shading map (green: Cell 1, red: Polygyros, blue: road network).

The result is presented in Map 3, where it is obvious that the viewshed of Cell 1 is small and is limited to a radius of approximately 1.5 km around it. Polygyros is not visible at all. Only certain parts of the road network are visible, however of minor importance (Map 3).

4.1.3 Visibility analysis after the completion of the works in Cell 1 After the completion of Cell 1, the elevation will have increased by 1m to 20m approximately. With regard to the new relief form, the viewshed of Cell 1 was determined, as above. The results are presented in Map 4 (Map 4) in three categories depicted in different colors. The first category shows the areas that are visible from the 1-30% of the reference points defined in the area of Cell 1, the second category from the 30-60% of the reference points, and the third one from 60% or more of the reference points.

It is obvious from the results that the viewshed of Cell 1 was increased after its completion. Besides, this effect was expected, since the area's elevation was increased. But Polygyros still remains not visible. Only the parts of the road network already visible before were increased (Map 4). Therefore, it can be concluded that the spatial arrangement of Cell 1 does not cause significant visual

CELL 1

POLYGYROS



impact in the broader area, since it is not seen from areas of particular cultural and tourist interest.

5 Discussion

To summarize, the process pursued included the following main stages: 1. The viewshed of the entire SLS was determined, using the relevant

general algorithm. 2. The SLS's viewshed was evaluated in comparison to the land uses

prevailing in the area, in order to determine the visible points of interest and the sensitivity of the landscape (towns, parts of the road network, etc.).

3. The viewshed of Cell 1 was determined, with hypsometric data prior to and after the spatial arrangement of the activity, and with the use of the “area seen viewshed map” algorithm.

4. The viewsheds of Cell 1 were compared again to the land uses prevailing in the area, and the results were evaluated as to the significance of the visual impact caused by the spatial arrangement of the project.

POLYGYROS

CELL 1

POLYGYROS

CELL 1

VISIBILITY MAP OFCELL1

(BEFORE CONSTRUCTION BEGINS)

LEGENDROAD NETWORK

URBAN AREA

CELL 1

VISIBLE AREA

POLYGYROS

CELL 1

VISIBILITY MAP OFCELL1

(AFTER CONSTRUCTION ENDS)

LEGENDROAD NETWORKURBAN AREACELL 1VISIBILITY CAT. 1VISIBILITY CAT. 2VISIBILITY CAT. 3

POLYGYROS

CELL 1

POLYGYROS

CELL 1

Map 3: SLS visibility map, Cell 1 prior to the beginning of the project. Map 4: SLS Visibility Map, Cell 1 after the completion of the works.

In the previous stages the viewshed of the SLS viewpoints has been determined with regard to the broader area. In this case, the viewsheds of Cell 1



appeared to be small. But if the viewsheds were large in terms of surface area and included important zones of interest, in a further stage of analysis particular points of interest could have been defined in the wider area, from which the viewshed towards the SLS could have been determined. And this with the aim to show which SLS points are visible from certain points of interest in the wider area, so as to take particular landscape restoration measures in these parts of the SLS. The visibility analysis, as proposed in this paper, may be used preventively for estimating the visual impact and its significance, in projects such as SLS, open mines, inert material mining sites, etc. It could be also of particular use, when somebody wishes to spatially arrange observation stations at locations that maximize the viewshed, such as tourist kiosks in unique landscapes for visitors, and guardhouses in forests for the timely identification of fires.

References

[1] Stamou A., “NIMBY syndrome of SLS and views for addressing it, Information Bulletin of the Technical Chamber of Greece, Vol. 2253, 2003 (in Greek).

[2] Felleman P.J., "Landscape Visibility", Foundations for Visual Project Analysis, John Willey & Sons, N.Y. 1986.

[3] USDA Forest Service, National Forest Landscape Management, Government Printing Office, Ag. Handbook 434, Washington 1974.

[4] USDA Forest Service, The Visual Management System, Government Printing Office, Ag. Handbook 462, Washington 1973.

[5] Fisher P. F., "Reconsideration of the Viewshed Function in Terrain Modelling.", Geographical Systems, Vol. 3, 1996, pp. 33-58.

[6] Fisher P. F., "First Experiments in Viewshed Uncertainty: The Accuracy of the Viewshed Area.", Photogrammetric Engineering & Remote Sensing, Vol. 57, No. 10, October 1991, pp. 1321-1327.

[7] Fisher P. F., "First Experiments in Viewshed Uncertainty: Simulating Fuzzy Viewsheds.", Photogrammetric Engineering & Remote Sensing, Vol. 58, No. 3, March 1992, pp. 345-352.

[8] Fisher P. F., "Algorithm and Implementation Uncertainty in Viewshed Analysis.", International Journal of Geographical Information Systems, Vol. 7, No. 4, 1993, pp. 331-347.

[9] Fisher P. F., "Probable and Fuzzy Models of the Viewshed Operation.", Innovations in GIS 1, (editor: Worboys M.), Taylor & Francis, London 1994, Chapter 12, pp. 161-175.

[10] Fisher P. F., "Stretching the Viewshed.", 6th International Symposium on Spatial Data Handling, Edinburgh, UK, September 1994, pp. 725-738.

[11] Fisher P. F., "An Exploration of Probable Viewsheds in Landscape Planning.", Environment and Planning B: Planning and Design, Vol. 22, 1995, pp. 527-546.

[12] Travis M. R., Elsner G. H., Iverson W. D., Johnson C. G., "VIEWIT: Computation of Seen Areas, Slope and Aspect for Land-Use Planning.",



General Technical Report PSW - 11 / 1975 (#Excerpt), Pacific Southwest Forest and Range Experiment Station, USDA, Forest Service, USA, 1975.

[13] Lee J., "Digital Analysis of Viewshed Inclusion and Topographic Features on Digital Elevation Models.", Photogrammetric Engineering & Remote Sensing, Vol. 60, No. 4, April 1994, pp. 451-456.

[14] De floriani L., Magillo P., "Computing Visibility Maps on a Digital Terrain Model.", Proceedings of European Conference, COSIT '93, Spatial Information Theory - A Theoretical Basis for GIS (editors: Frank A., Campari I.), Italy, September 1993, pp. 248-269.

[15] Magillo P., De Floriani L., Bruzzone E., "Updating Visibility Information on Multiresolution Terrain Models", Proceedings of European Conference, COSIT '93, Spatial Information Theory - A Theoretical Basis for GIS (editors: Frank A., Campari I.), Italy, September 1993, pp. 279-296.

[16] Erdas Inc., ERDAS: Field Guide, Second Edition, Atlanta, USA, 1991, Chapter 8, pp. 169-189.

[17] ESRI Inc., ARC/INFO User's Guide: Surface Modelling with TIN - Surface Analysis and Display, 2ond Edition, USA, March 1992, Chapter 6-11.

[18] Burrough P.A., Principles of Geographical Information Systems for Land Resources Assessment, Oxford University Press, 1986, Great Britain.

[19] Pavlidis T., Algorithms for Graphics and Image Processings, Computer Science Press, USA, 1982.

[20] Kim J. J., "High Target Visibility Analysis.", ASPRS / ACSM, Annual Convention & Exposition Technical Papers, Nevada, April 25-28, 1994, pp. 301-306.

[21] Randolph Wm., Ray C. K., "Higher Isn't Necessarily Better: Visibility Algorithms and Experiments", pp. 751-770.

[22] Tsouchlaraki, A., Assessment methodology for Physical Relief Optical Value, PhD dissertation, Dept. of Rural & Surv. Eng., NTUA, Athens, 1997 (in Greek).



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