Sky view factor method as an analytical tool in the analysis of compactness of an urban interior

Bartosz Kaźmierczak

doi:10.37190/arc240309

Abstract

This article is aimed to present the results of the study on urban canyon compactness. The topic seems to be vital as physiognomic research on urban canyons has so far relied on visual assessment made by an observer intuitively. These types of studies comply with relevant standards which define elements of a visually sensed space and prescribe methods of recording such a sensual experience. Therefore, such studies are classified within descriptive research, and are sometimes charged with such drawbacks as the subjectivity and lack of comparativeness. To attain higher objectivity, the number of respondents is typically increased and results are calculated and presented in the statistical form. This article presents an analytical methods based on a normative approach, the application of which will render measurable and comparable results. The purpose of the studies consisted in testing one of the normative methods, i.e., the sky view factor (SVF) method, in the objective calculation of compactness of an urban interior.

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Bartosz Kaźmierczak*

Sky view factor method as an analytical tool

in the analysis of compactness of an urban interior

Introduction

Urban spatial transformation process can be controlled

and shaped by legible and harmonious compositional ar-

rangements. This, however, requires a denition of clear-

cut compositional boundaries of an entire spatial ar ran-

gement, which further form composite urban structures.

Depending on the urban tissue type – the built environ-

ment, greenery, infrastructure or a road – the outline and

purpose of the boundary will vary and will have a dif-

ferent spatial representation. In terms of good composi-

tion, legi bility of a spatial arrangement is very important

because it can successfully pose a barrier to penetration

of other, similar structures inside its interior. Clarity and

compactness of delimited elements facilitates legibility

of space. Christopher Alexander (2008) observed that

boundaries of urban canyons needed to be visually de-

ned. Aleksander Böhm, in his works (1981, 2004, 2006,

2007, 2012), highlighted the importance of ordered enti-

DOI: 10.37190/arc240309

Published in open access. CC BY NC ND license

ties that protected urban landscape against spatial dishar-

mony of uncontrolled and unlimited sprawl and ensured

an aesthetic experience of a harmonious image of space

According to Kevin Lynch (2011), legibility of rural and

urban landscape derived from easy identication of its

constituents, coherently organised into a pattern that

is unique for the place. The feeling of inclusion inside

an urban canyon depends on the degree of delimitation

of space by urban walls and on the ratio of view obstruc-

tions to openings. The author hereof calls it “compact-

ness”

. Andrzej Niezabitowski (1979, 43) called basic

features of each spatial object “constitutive features”

(i.e., features that are gradable). According to him such

features included:

Referring to perception of space, these researchers dened con-

stituent parts of a delimited landscape unit, which according to them in-

cluded: walls, urban oors and ceilings and elements of t-out, i.e., street

furniture.

The term compactness in urban planning typically does not refer

to individual urban canyons but to entire urban structures such as par-

ticular complexes of buildings or permanent settlement units (villages

and towns).

* ORCID: 0000-0001-8436-6963. Faculty of Architecture, Poznan

University of Technology, Poland, e-mail: bartosz.kazmierczak@put.

poznan.pl

Abstract

This article is aimed to present the results of the study on urban canyon compactness. The topic seems to be vital as physiognomic research on

urban canyons has so far relied on visual assessment made by an observer intuitively. These types of studies comply with relevant standards which

dene elements of a visually sensed space and prescribe methods of recording such a sensual experience. Therefore, such studies are classied within

descriptive research, and are sometimes charged with such drawbacks as the subjectivity and lack of comparativeness. To attain higher objectivity, the

number of respondents is typically increased and results are calculated and presented in the statistical form. This article presents an analytical methods

based on a normative approach, the application of which will render measurable and comparable results. The purpose of the studies consisted in testing

one of the normative methods, i.e., the sky view factor (SVF) method, in the objective calculation of compactness of an urban interior.

Key words: sky view factor, analytical tool, normative approach, urban compactness

86 Bartosz Kaźmierczak

– complexity,

– dierentiation,

– regularity and coordination,

– compactness or in other words cohesion.

Marshall McLuhan (1964) noted that perception was

of dynamic nature and vision created a moving image of

the surroundings, underlying the feeling of continuity and

coordination. Similarly, John Ormsbee Simonds (1961)

stressed that the observer felt discomfort if space did not

feel inclusive

. Simonds also found out that a well-dened

space evoked many positive feelings (curiosity, concentra-

tion, relaxation, etc.). He was of the opinion that images

could be controlled and sequenced to ensure their best vi-

sual presentation and sensual perception of the surround-

ing landscape. Panoramic image series and sequences

which formed systems of urban canyons could render the

highest aesthetic and compositional values, and thus, were

best suited for observation of a sensual experience that

they underlay (Szolginia 1981). Christian Norberg-Schulz

(2000) analysed urban space in view of three categories

of constituents: streets, squares and districts characterised

with a dierent purpose and importance of spatial, com-

positional and functional arrangements. According to him,

streets and squares represented basic components crystal-

lising the entire urban plan and created a system of inter-

related public spaces. These ndings initiated research on

transport routes and perception of moving observers, they

further underlay spatial separation of pedestrian zones and

vehicle lanes (Appleyard, Jacobs 1987).

Theoretical bases

Urban analyses most often focus on such aspects as

spatial and functional resources, terrain shape and ar-

It’s worth stressing that research in the eld of neuroscience and

cognitive science (Zeki 1999) conrmed that our brain rated visual stimuli

(Ramachandran, Hirstein 2006) according to the beautiful-ugly dimension

(Kawabata, Zeki 2004). Certain patterns of brain activity have been iden-

tied that correlate to declared intensity of highly aesthetic stimuli (Ishizu,

Zeki 2014). Therefore, it may be stated that perception of beautiful images,

egardless of their source of origin, may be dened through neuronal ac-

tivity (Zeki 2019), thus, without doubt, it can be concluded objectively

that human brain reacts to stimuli assessed as beautiful, though subjective

perception of beauty may dier between individuals.

chitectural forms, including issues of physical structure

(Bradecki 2021, 56) and visual appearance; landscape

is aesthetically assessed through physiognomy as visual

land scape (Wycichowska 2008). There are two dierent

approaches adopted in the study of urban form, namely

normative and descriptive analyses (van Nes, Yamu 2021,

4). Descriptive approach most often consists in subjective

analyses of aesthetic quality of an arrangement of land-

scape and urban canyons. Survey is here a basic assess-

ment tool, provided that an adequate representative group

of respondents has agreed to participate in the study. Sub-

jectiveness of their opinions shall be linked to a unique

mental and emotional experience of physical space sensed

by an individual. Though the assessments and ratings of

individuals are subjective, the researchers have observed

that – within the entire sample of respondents – there is

a group of observers reacting similarly to selected images.

(Mordwa 2009). Normative analyses are based on mea-

surable and comparable parameters that dene perception

in the context of an adopted model. Development of new

information technologies has brought about a number of

ways of visual representation of landscape, such as photo

collage, computer models, photorealistic 2D and 3D visu-

alizations and virtual reality environment. They represent

now basic tools of 3D presentation of the existing space.

Among those tools we can name quantitative mapping

methods that use algorithms and indices to get data on

spatial and visual features of landscape, e.g. analyses that

use GIS (Geographical Information System), Space Syn-

tax or ISOVIST (Weitkamp 2010; Kocki, Kwiatkowski

2016; Wang, Huang 2019). These methods render quite

objective results but they are time consuming and access

to data or identication of proper delimitaton criteria may

prove dicult.

Geometric bases of the Sky View Factor SVF method

(Fig. 1) were dened by Douw G. Steyn in his report of

1980. In the report he saw an opportunity for the method

to be applied in the calculation of horizon visibility or in

visibility analyses of urban canyons (Steyn 1980; John-

son, Watson 1984). Léa C.L. Souza, Daniel S. Rodrigues

i José F.G. and Mendes presented – in 2003 – a method for

the application of GIS to generate hemispheric images of

urban canyons and to calculate SVF (Souza et al. 2003).

Unfortunately, in terms of technology at the time, this

Fig. 1. Space representation

principle via the sky view

factor method

(elaborated by B. Kaźmierczak

on the basis of Park et al. 2017, 7)

Il. 1. Zasada odwzorowania

przestrzeni metodą

sky view factor

(oprac. B. Kaźmierczak

na podstawie Park et al. 2017, 7)

Sky view factor method as an analytical tool in the analysis of compactness of an urban interior 87

Fig. 2. Delimitation of view series and sampling points characteristic for respective panoramic image sequences in Gniezno

(elaborated by B. Kaźmierczak)

Il. 2. Wyznaczenie ciągów oraz punktów charakterystycznych dla poszczególnych sekwencji widokowych dla Gniezna

(oprac. B. Kaźmierczak)

method could not have been fully used as greenery was

not taken into account in the measurements which largely

aected the results. Owing to development of digital tech-

nologies, this method is now commonly used in research

on urban heat islands (UHIs) (Bärring et al. 1985), and

from that data we can now derive insolation degree and

heat absorption of urban canyons with account for green-

ery (White, Langenheim 2014). Bin Cong et al. (2018)

describe in detail the origin and theoretical bases of the

method, what’s more, they use it in comparative studies

of selected virtual development models generated based

on dierent data. The SFV method has facilitated the im-

provement of tools necessary for more accurate and easier

assessment of space. It has, furthermore, laid the founda-

tions for a number of innovative applications.

Purpose and method of research

This research paper intends to nd out how one of the

methods of analysis, i.e., the SVF

method based on hemi-

The analysis was originally planned to be made with the ISOVIST

method. Digital area models containing terrain shape as well as built

environment and land cover (trees and bushes), available in Geoportal

(a central node of the Spatial Information Infrastructure with access to

spatial data and related services) were planned to be used for that pur-

pose. However, for the selected case study there were no accurate green-

ery models that could be used in detailed view analyses. A point cloud

generated based on a LIDAR laser scan is relatively accurate, but only as

regards the upper part of tree crowns. The tree bottom part is practically

omitted. Models of greenery that reect geometric forms of plants oer

better options, yet they are too simplied for the analyses in question

88 Bartosz Kaźmierczak

spherical images, can be used to assess geometry varia-

tion derived from researched urban canyons. The author

has dened compactness of an urban canyon as a ratio

and would not render the expected results. 3D scans made with drone

photogrammetry could oer a solution to the problem but would be too

expensive and time-consuming.

Fig. 3. Set of equipment used in the research to measure visibility (photo by B. Kaźmierczak)

Il. 3. Aparatura badawcza użyta do pomiarów widoczności (fot. B. Kaźmierczak)

Fig. 4. Result of graphics editing intended for conversion of the image to the black and white representation

(elaborated by B. Kaźmierczak)

Il. 4. Efekt obróbki graficznej mającej na celu doprowadzenie obrazu do formy czarno-białej (oprac. B. Kaźmierczak)

of view obstructions to openings in selected sampling

points in a given panoramic image sequence. Application

of the SVF method allows us to dene the elds of vi-

sion and also the degree of visual obstacles spotted from

characteristic viewing (sampling) points within identied

panoramic image sequences. The paper explains impor-

tance of proper identication of sampling points for the

purpose of taking valid measurements. The available lit-

Sky view factor method as an analytical tool in the analysis of compactness of an urban interior 89

Fig. 5. Graphic representation of sequence distribution in the analysed series in view

of assessment of compactness of urban canyons based on Gniezno example

(elaborated by B. Kaźmierczak)

Il. 5. Graficzne przedstawienie rozmieszczenia sekwencji badanego ciągu

w aspekcie oceny zwartości wnętrz urbanistycznych na przykładzie Gniezna

(oprac. B. Kaźmierczak)

erary sources on methods of selecting relevant sampling

points most often refer to a method that assumes equal

distance between the points. However, the author hereof

has come to the conclusion that such an approach is inad-

equate for panoramic image sequences. It has been, thus,

decided that points – to take relevant photographs from –

shall be identied in accordance with the following rule:

each measurement point is to be located in the geometric

centre of a canyon and is to represent one panoramic im-

age sequence (wall-wall, wall-opening, opening-opening)

Two series (marked with a and b symbols) of urban

canyons that link the Old Market Square in Gniezno and

the railway station served as our research sample. The two

series of urban canyons converged in the rst ve sam-

pling points. Additionally, there were n+1 sampling points

in the b-marked urban canyon series as compared to the

a-marked canyon series. A photo was taken in each of the

marked sampling point (Fig. 2) with the dedicated set of

equipment. The set of equipment used for the studies con

tained Canon EOS 6D camera with Canon EF 8-15mm f/4L

Fisheye USM Lens xed on INDURO 75MM HI-HAT

(LFB75) tripod set with Triopo NB ball-head (Fig. 3).

The photos taken were processed to obtain black and

white images (Fig. 4). A relevant raster graphics editor

was used for the purpose, the edited images were correct-

ed up to maximum settings of light and shadow. An appro-

priate raster image processor, with an option of calculat-

ing white pixel percentage, was further used.

The SVF analysis method describes quantity of open-

ings in an urban canyon concerned, whereas the assumed

90 Bartosz Kaźmierczak

Table 1. Breakdown of results, with sequences lengths where compact-

ness is below the average value in the samples under our research

(elaborated by B. Kaźmierczak)

Tabela 1. Zestawienie wyników z wyliczonymi długościami sekwencji

o zwartości poniżej średniej dla badanych przykładów

(oprac. B. Kaźmierczak)

Point

symbol

Sum

Average value

SVF

View obstructions

in percentage

(+/–) average

compactness parameter refers to the number of view ob-

structions. Black pixels were computed and their number

was compared to the total number of all pixels. Thus, the

SVF result (range from 0 to 1) was subtracted from 1 and

multiplied by 100. This way, a percentage of view obstruc-

tions in the entire recorded image could be calculated.

Results

On the basis of computations of a percentage share of

view obstructions in respective sampling points, lengths

of sequences have been dened in reference to the number

of view obstructions as “over” or “under” a mean value

calculated for the entire selected series of the urban can-

yon. As a result, it was possible to nd out which sequenc-

es of series scored the lowest value of view obstructions

and at which length (Fig. 5). In the row titled

“total”, all

percentage shares of view obstructions were totalled in

order to calculate an arithmetic mean of the ratio and to

nd out the maximum length of a sequence characterised

with the value of view obstructions below the arithmetic

mean in reference to the entire length of the series (a, b).

Table 1 presents the graphic representation of the analysis

results.

Conclusions

Compactness of urban canyons understood as a ratio of

visual obstacles to openings is one of the most important

parameters dening physionomic quality of urban space.

Owing to the application of the SVF method, the compact-

ness factor can be easily calculated with objectively good

results. The presented herein method of carrying out re-

search does not require high nancial outlay or know ledge

of advanced tools. This method can be automated with

the use of mobile 360° cameras and ready to use photo-

processing algorithms that calculate pixels of the same

colour. The author has applied the said method for the as-

sessment of a component of space physiognomy, which

– when compared to the methods typically used in de-

scriptive analysis based on a subjective sensual experi-

ence of the observer – was of innovative nature. In the

light of the presented studies, it seems vital to search for

similar normative methods of calculating physionom-

ic features of urban canyons, such as for example those

listed by Niezabitowski: complexity, dierentiation or

regularity. Thus, the Sky view factor method may prove

a useful analytical tool in the work of an architect/urban

planner, which can, furthermore, successfully supplement

the descriptive methods of space valuation used so far.

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Sky view factor method as an analytical tool in the analysis of compactness of an urban interior 91

Streszczenie

Metoda sky view factor jako narzędzie analityczne

w badaniu zwartości wnętrza urbanistycznego

Tematem artykułu jest badanie dotyczące analizy zwartości wnętrza urbanistycznego. Temat ten może być interesujący, gdyż badania zjono-

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