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Visual Impact Assessment
This section describes visual impact assessment, firstly by examining what
we mean by visual and environmental impact assessment (VIA/EIA). The historical
background to VIA is discussed together with some of the politics involved and
some issues concerning VIA. The second half of the section describes the VIA
process - looking at methods of visibility analysis and examining the
methodologies of zones of visual influence (ZVI) and viewpoint analysis.
There is no precise definition for Visual Impact Assessment (or VIA) as a
single concept. But the term describes a systematic analysis of the possible
impacts of environment resulting from a proposed development and the
investigation of the means available to mitigate the effects of such proposals
prior to implementation. Visual impact is defined as a change in the appearance
of the landscape as a result of development which can be positive (improvement)
or negative (detraction) (IEA and the Landscape Institute,
Visual and Environmental Impact Assessment
VIA forms part of the Environmental Impact Assessment. EIA was introduced
in the USA in 1969, Europe in 1985 and formally in Britain in 1988. It also
covers other environmental issues such as the effects of development on flora
and fauna; noise, air and water pollution. Whilst a large proportion of these
issues can be assessed in quantitative terms, visual impact is assessed largely
by qualitative judgements, as it is concerned with the human appreciation of,
and interaction with, the landscape.
In EIA reports the visual landscape is often described, based on its
geomorphological features. Sometimes the visual impacts of the proposed
development are evaluated but rarely is there a good visualisation of the
project. There are three major problems in visual landscape evaluation: the
technical problem of how to visualise possible changes in the landscape; the
theoretical problem of how to evaluate scenic beauty; and the administrative
problem of how to integrate visual aspects in the planning process
In comparison with the other environmental issues, visual impact is perhaps
one of the best publicised and most contentious issues, usually because of the
adverse effect that a new development can have on natural or
"unspoilt" landscape and the intense feelings of the public towards
this subject (Fortlage, 1990).
At present, VIA studies are required to be carried out, as part of the
application for planning approval, only for major developments such as power
stations, wind farms, transmission lines in the countryside (Commission of the European Communities, 1985, cited by Flynn and
Pratt, 1993). VIA studies may also be required for other types of
development, but this is at the discretion of the local planning authority or
the Secretary of State. While small-scale developments (such as houses and farm
buildings) can also have dramatic effects on landscape quality, it is currently
too costly and time consuming to conduct visual impact studies on such
Historical background of VIA
There appears to be a misconception in the literature that the management
of visual resource originated in the US with the legislation of National
Environment Policy Act (NEPA) in 1969. In fact, visual resource management
already had a long established tradition in the UK. It started with the
publication of the Town and Country Act (1947), and the National Parks and
Access to the Countryside Act (1949).
The Countryside (Scotland) Act (1967) and Countryside Act (1968) required
every, "Minister, government department and public body to have regard to
the desirability of conserving the natural beauty and amenity of the
countryside in all their functions related to land" (Cullingworth, 1976 cited in Gillespie and Clark, 1979). These
legislations empowered the local planning authorities to implement their policy
on the preservation of visual amenity.
All developments must be approved by the local planning authority. Under
the Development Control Process (DCP) system the authority had the discretion
to insist on what documentation are required before planning approval will be
granted. Britain persisted with the DCP system, despite arguments by many
experts (Thorburn, 1978) that it can only be improved by
using a more formal or systematic impact assessment process such as the VIA,
which had already been introduced in the USA in 1969. These experts were
concerned about the effects of new large scale developments on landscape
quality and the ability of the design control process to take these effects
into consideration. Unlike the DCP, there is a statutory requirement for VIA to
be carried out on all large scale developments. Increased pressure for
development in the countryside, finally prompted the central government to
incorporate the formal methods and techniques of VIA into the development
control process in 1988.
Laws and politics in scenic beauty
In Great Britain, political and economic policies are still the major
influences on decisions about locations for new industrial developments.
Already, much of its lowland landscape has been intruded upon by extractive
industry and urbanisation, and its highland landscape by reservoirs for both
water supply and hydroelectricity (Aylward and Turnbull,
Although local government has at its disposal sophisticated planning
legislation, the community is still concerned that fundamental changes may
occur in the physical and visual quality of their environment and often
suspects that planning and consent may be given to a development without the
full disclosure of effects on the community. Thus, local government in a rural
area is often motivated by pressure groups and individuals to impose stringent
planning conditions which ensure that both the developers and the community are
aware of the effects of the development and the alternatives available. The
presentation of the evidence must be in a form that can be clearly understood
and assessed by all parties (Aylward and Turnbull, 1977).
In some countries the scenery is well protected. For example in the Swiss
constitution it is stated that the scenery has to be taken care of and in the
case of a great interest of the general public, it has to be preserved
undiminished (Lange, 1994). In the state of Wisconsin, in
the USA, scenic beauty has assumed an importance in the law that currently
serves as a major consideration in many of the state's regulatory functions. In
1952, the State Supreme Court ruled that the "right of the citizens of the
state to enjoy our navigable streams includes the enjoyment of scenic
beauty". The court held that "the occupancy (by the public) is
visual" and that indeed the enjoyment of the beauty of the land
constitutes a legitimate public use of land whether or not the public is
allowed to set foot on it (Bishop and Hull, 1991).
Issues in VIA
Regional and local issues
The visualization models must be reliable and generalisable, and must be
able to convey their meaning to ordinary citizens. The systems should be able
to accommodate detailed, fine-grained, data representations within
coarse-grained data sets to support both regional and local aspects of
modelling (Orland, 1992b).
Spatial, quantitative and qualitative issues
The visual impact assessment of a proposed development addresses three
types of issues: spatial, quantitative and qualitative. Spatial issues include
where the development is visible from or, more specifically, what or whom it is
visible to; quantitative issues include how much of the development is visible,
how much of the surrounding area is affected, and to what degree; and
qualitative issues include the visual character of the development and its
compatibility with its surroundings (Fels, 1992).
Basic functions of a VIA
Five basic functions are important in the VIA: clear identification of the
various types of impacts; organisation of spatially and temporally dispersed
inventory data; prediction of impacts based upon potential land use decisions;
a usable interface between these functions and the planner/manager; and
effective communication of potential impacts to the public and decision-makers
(Bishop and Hull, 1991).
The Visual Impact Assessment Process
Types of Visibility Analysis
The principle of intervisibility states that visibility is determined in
two ways either from the site or to the site, that is, if point A can be seen
from point B then the reverse is true. Thus although a site's visibility is
normally thought of in terms of it being viewed from outside its boundaries,
the outward view from the site to adjacent areas can be adopted to simplify
analysis (Aylward and Turnbull, 1977).
Different types of visibility analysis include intervisibility analysis to
produce levels of visual impact, dead ground analysis, identification of the
portions of the landscape forming a backcloth for the design object, situations
where the design object appears above the landscape horizon and the
identification of optimal location for vegetation screen placement
(Kennie and McLaren, 1988). The ability to map all of the
viewpoints may prompt detailed interactive investigations in portions of the
landscape, and suggest which portions of a planned development are most
problematic and which portions might be altered to reduce visual impact
(Fels, 1992). Quantifying the area visible from any location
is an objective measure of the extent to which a change in land cover will be
visible to an observer (Miller, 1995). Critical portions of
the development can be identified, critical viewpoints can be located
Traditional manual techniques of visibility analysis have emphasised the
zone of visual intrusion associated with site development. The most important
variable determining this is the terrain or landforms surrounding the site.
Frequently, it is the size of the area within which the installation can be
seen that is important, in other cases the critical consideration is site
visibility from individual locations, such as scenic drives or picnic sites
(Selman et al, 1991).
Of course determining a site's visibility does not produce the design but
it can go a long way to help the speedy evaluation of alternative ideas and
solutions, something that does not normally happen with more laborious and
graphic simulation methods (Aylward and Turnbull, 1977).
GIS-visualisation goes beyond the simple ability to discuss single anticipated
outcomes via traditional graphic tools or simple visibility analyses. It offers
the opportunity to visualise relationships across time and space, and to
explore more comprehensive ranges of possibility (Orland,
Local, wide area, analysis for viewpoints
An approach which uses GIS to assess resources from the perspective of
recreation through evaluating scenery and visual impact has been described by
Miller et al (1994). Three types of analysis are used: local
analysis of scenery; wide area analysis; and analysis for viewpoints.
Local analysis of scenery uses visual impacts at particular locations and
for particular scenes as well as three further factors: distance depth cueing
and size perspective; angle of intersection of the view with the terrain; and
land cover. The use of this method to visualise and measure the impact of
forest land use change in an example scene shows that the visual impact of
afforestation is generally low, being below the horizon and blending with the
dark texture background of the mountains.
Wide area analysis uses two approaches. Viewsheds for selected visitor
viewpoints are calculated to identify priority zones of visual importance to
tourists. A census of the total area visible from all locations is also
calculated. Analysis for viewpoints uses viewpoints to measure scenic potential
by determining the number of viewpoints from which any terrain location is
At present these GIS tools for the analysis of scenery are used for either
historical evaluation of land use change (Gauld et al, 1991)
or prediction of the impacts of future changes (Aspinall,
1990). The approach can also be linked to more traditional methods of
landscape analysis, such as the use of questionnaires to assess perception of
place (Miller et al, 1994).
Distant, close, panoramic, and corridor views
Four types of view can be analysed using using digital data: distant,
close, panoramic and corridor (Miller, 1995).
Distant and close are terms describing a concept. They have been coined to
enable scoring of the landscape with respect to observer value judgements and
predict scenic value for units in which only the data on the landscape elements
are known. They are views characterised by the distance of the horizon and the
land immediately below the horizon, from the observer. A working definition of
close may be one in which the observer can discriminate between features of
interest; for example, distinguish between coniferous and deciduous trees or
between woodland and heather moorland. The implications of this are that a view
from the same locality, in a particular direction, may be categorised
differently if the atmospheric conditions are dramatically different, and
therefore accompanying a categorisation of view must be a statement of
prevailing modelled conditions.
Corridors are characterised by the existence of lateral terrain features
such as valley sides or woodland either side of the observer, constraining
their view in a narrow field. Photographic panoramas are usually made up of a
number of laterally overlapping photographs taken in a sequence around 360
degrees with the disjoins evident. In a digitally produced panorama the number
of views generated can be significantly larger than those taken on the ground.
Intervisibility analysis is accomplished through many applications of
viewshed mapping procedures: projective and reflective, individual and
composite. The effects of temporary alterations to the topographic model can be
seen and evaluated (Fels, 1992).
Projective and reflective mappings
Projective mappings are initiated from viewpoints within the development
(inside looking out) while reflective mappings are initiated from viewpoints in
the surrounding landscape (outside looking in). The objective of projective
mapping is to reveal the extent of visibility of the development to its
surroundings. The objective of reflective mapping is to determine whether, and
to what extent, the development is visible from its surroundings. Reflective
mapping is more similar to conventional viewshed mapping procedures than is
projective mapping (Fels, 1992).
Single and cumulative mappings
Single viewpoints are useful in evaluating the effects of a specific
component of the development. The use of multiple viewpoints produces composite
intervisibility maps; true cumulative mapping is more useful than mapping from
a predetermined set of points.
Cumulative maps portray the visibility of every point in the development
with respect to every point in the landscape. If the object of study is a form
of development, such as a landfill or an electrical transmission line, the
study must include every model point within that landfill or along that
transmission line. If the object of study is a visual resource, such as scenic
river, the study must include every model point associated with that river.
These maps are the most effective means for producing comprehensive appraisals
of spatial and quantitative impact issues (Fels, 1992).
Colour coded impact maps
One version of such an evaluation can yield a qualitative impact, ranging,
for example, from beneficial and benign to harmful and disastrous. You can
produce an impact map using an evocative colour coding scheme, e.g. red for
bad, green for good, yellow for neutral. (This colour scheme is by no means
universal: no colour scheme is. But, as Ervin (1993)
observes, within a culture of viewers used to traffic lights, it is easily
Patterns of impacts that have an element of geographic or spatial
correlation (such as along drainage courses or ridge tops) became obvious in
this approach and seem much more immediate when seen in pseudo-3D than when
simply presented as a coloured impact map.
Zones of Visual Influence
Zones of visual influence (ZVI) and Viewpoint Analysis are two of the most
common techniques or visual tools used by experts for assessing visual impact.
The two techniques provide factual data about the visual appearance of the
development in the context of the existing landscape. ZVI is a map-based method
used to define the area of visibility (also known as the view envelope) of a
proposed development within the landscape. Originally developed by
Tandy (1971), it can be carried out manually.
The manual method has now been largely superseded by view area analysis
software such as VIEW suite (Aylward and Turnbull, 1977;
Turnbull Jeffrey Partnership, 1983), VIEWIT (Tlusty,
1979), GROUSE (Evans, 1984) and MAP - Map Analysis
Package (Tomlin and Tomlin, 1981 cited by Hadrian et al.,
1988). Some researchers have incorporated the digital terrain mapping
software into GIS (Geographic Information System) for analysing landscape
modification and mapping predicted visual impact levels of proposed development
from surrounding areas of visibility.
As the visibility of the development and natural landscape features are
illustrated on plan (i.e. map), sections and wire-line perspectives, the
ZVI technique only allows the user to develop a limited understanding of the
visual properties of the landscape and man-made structure. It is difficult for
the expert to judge the impact of the development on actual existing landscape
quality, when wire-frame perspectives only indicate whether the development is
seen or not.
In the viewpoint analysis technique, two or three dimensional
representations of the landscape before and after proposed developments, are
often produced. The common simulation tools used by experts for VIA are models,
perspectives and photomontages as viewed from specific points in the landscape.
The key viewpoints are identified or must be agreed between experts and the
local planning authorities.
There are limitations to the use of these simulation tools in viewpoint
analysis (MacAulay, 1988; Hershberger and Cass in Nasar,
1988). Scale models of proposed developments and surrounding landscapes,
are not only expensive and time-consuming to build, but are also limited by the
area they can cover, and how well they represent the actual settings. There is
also the additional problem of simulating the human viewpoint, even with the
use of a modelscope. Given that details of the landscape and structure are
drawn by artists, perspectives may not be totally realistic in representing the
actual environment. Therefore, even the best perspectives can suffer from a
degree of bias.
Photomontages, on the other hand, can provide a much higher degree of
realism provided the proposed development can be simulated and superimposed
accurately onto photographs or slides of the existing landscape. This technique
can, however, be slow and tedious when done manually. Fortunately, the
limitations of manual photomontage techniques have largely been resolved, using
computer simulation techniques. As a result, digital photomontages are now
commonly used in viewpoint analysis in the VIA process. The conventional
approach adopted by experts in VIA process typically consist of:
- a factual description of the existing site, based on surveys conducted on
site and desk studies;
- a description of the proposed development;
- an analysis of the impacts of the development using zone of visual
influence study (ZVI) and/or Viewpoint Analysis; and
- predicting the level of impacts. Impacts are classified as having
"major", "some", "minor" or "no"
- suggestions on the scope of mitigative measures (if necessary) to
ameliorate the potential impact.
Example of a VIA study using ZVI and viewpoint analysis
A typical example of a comprehensive VIA study is the Nicholas Pearson Associates' (1993) assessment of the Bryn
Titli windfarm for National Wind Power in Wales. The VIA statement contained a
description of the physical features (such as topography and landuse),
landscape characteristics, and visual policies (i.e. statutory and
non-statutory landscape designations as described in Task Two) of the existing
landscape in and surrounding the development site. The written description was
supported with a visual presentation which included maps and photographs.
The mitigative measures involved configuring the layout and spacing of the
wind turbines, to reduce the number of visible units; and the use of recessive
colours against the skyline. The impacts of the proposed windfarm development
were considered, in terms of its construction period and operational life.
These were analysed using ZVI and Viewpoint Analysis.
In their ZVI study, a digital terrain mapping computer software was used to
produce two sets of visibility maps of the development site. The first map
defined the area of visibility of turbines up to rotor height; and the second,
up to tip of the blade at its highest point. The areas of visibility were
divided into 4 classes, depending on the number of turbines seen.
In viewpoint analysis, the impact of the wind farm development from key
viewpoints were analysed using wire-line perspectives and photomontage
(i.e. computer simulated model of turbines superimposed onto digitised
images of the actual site). The expert's judgement (that the impact of the
development was of minor significance) was based largely on the consideration
of the number of visible turbines, their distance from key viewpoints, and the
complexity, diversity and variety of the surrounding landscape.
The conventional method of assessing visual impact, exemplified by Nicholas
Pearson Associates' VIA study is referred to as the "description and
analysis approach" (Landscape Research Group, 1988).
The prevalence of this approach in the VIA process can be attributed to the
fact that these studies are carried out by members of the design profession,
who have been trained traditionally and are, therefore, familiar with this
Limitations of the Conventional VIA Process
There are several potential limitations of the conventional VIA process.
The implementation of VIA using the descriptive and analytical approach has
provided a systematic and structured process towards the assessment of visual
impact. Whether its implementation has succeeded in improving, or preserving
the visual quality, or ameliorating the potential intrusion of new developments
in the countryside, remains questionable. Some critics have argued that, in
reality, VIA studies have been inconsistent with prevailing public perceptions
and reactions, to the impacts of development in the countryside (Brun, 1995).
The Decision Maker's View
It is interesting to look at some of the comments made by those who make
the decisions on planning applications about VIAs. Some admit that the final
decision "must be subjective even if informed by other material" and
that their conclusions "are based principally on my site visits" and
not on any evidence from the imapct assessment; another notes that "even
with the aid of various techniques in presenting the evidence, it is very much
a matter of judgement as to how much of the proposed development is likely to
be seen". Others question how to interpret the data they receive -
"(there) remains the question of calculating the significance of such
information". Not all inspectors fully trust the VIA findings, such as
"the photomontages tended to exaggerate the visual prominence" of the
windfarms. However, the majority do agree that "surveys are a reasonable
starting point from which to calculate the effects of the proposal on the
landscape" (Jones, 1996).
Landscape Simulation Methods
A wide range of landscape simulation methods have been developed and
implemented as tools for impact assessment. These include: plans, diagrams,
elevations, perspective sketches, renderings, modified photographs (photo
renderings and photomontages), slide projections, scale models, movies,
videotapes and computer graphics (Oh, 1994). Photomontages
are often used to get a subjective impression. However, it is a relatively
expensive method and is restricted to fixed observer locations (Zewe and Koglin, 1995). Simulation techniques are further
discussed in the section on Visualisation Tools.
Computer-based simulation methods include: two-dimensional drafting and
painting; three-dimensional wire frame models; surface and solid modelling;
image processing; and animation techniques. Several reports have shown
successful applications of advanced computer technologies such as
three-dimensional solid modelling, coloured and dynamic functions
Another possibility is the calculation of computer graphic images by
projecting aerial photographs as a texture onto landscape polygons. This method
is currently being developed in France by the EDF (Électricité de
France). It delivers very realistic images particularly for distant areas, but
not nearby the observer (Zewe and Koglin, 1995).
Criteria for good simulations
Sheppard (1989) states the following as criteria which
good simulations should fulfil:
- Representativeness - a simulation should represent important and typical
views of a project.
- Accuracy - the similarity between a simulation and the reality after the
project has been realised. Of course, judging this is a little difficult
- Visual clarity - detail, parts and overall contents have to be clearly
- Interest - a simulation should hold the attention of the viewer.
- Legitimacy - a simulation is defensible if it can be shown how it was
produced and to what degree it is accurate.
Visual simulation is only descriptive; it does not release the planner from
the difficult task of evaluation nor does it provide an evaluation in itself in
a publicly based evaluation approach. However, visual simulation is, or at
least should be, the prerequisite to predict and to evaluate the visual
consequences of planned alterations (Lange, 1994).
Changes due to the seasons
There is a need to visualise change over time, using colour cues and
animation. To create different simulations for the three different seasons
symbols can be substituted where appropriate (bare branched deciduous trees in
the winter, sparsely foliated in the spring and autumn). Fields can appear tan
in the autumn and white in the winter, and bright green/yellow/blue (depending
on crop) in the spring. Other dimensions of the landscape - such as visibility
distances, sun and shade pockets, or "sense of enclosure" or
"view of water" also change with the seasons (Ervin,
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