Modelling urban growth evolution and land-use changes using GIS based cellular automata and SLEUTH...
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ORIGINAL ARTICLE
Modelling urban growth evolution and land-use changes usingGIS based cellular automata and SLEUTH models: the caseof Sana’a metropolitan city, Yemen
Mohamed Al-shalabi • Lawal Billa •
Biswajeet Pradhan • Shattri Mansor •
Abubakr A. A. Al-Sharif
Received: 16 December 2011 / Accepted: 18 November 2012 / Published online: 1 December 2012
� Springer-Verlag Berlin Heidelberg 2012
Abstract An effective and efficient planning of an urban
growth and land use changes and its impact on the envi-
ronment requires information about growth trends and
patterns amongst other important information. Over the
years, many urban growth models have been developed and
used in the developed countries for forecasting growth
patterns. In the developing countries however, there exist a
very few studies showing the application of these models
and their performances. In this study two models such as
cellular automata (CA) and the SLEUTH models are
applied in a geographical information system (GIS) to
simulate and predict the urban growth and land use change
for the City of Sana’a (Yemen) for the period 2004–2020.
GIS based maps were generated for the urban growth
pattern of the city which was further analyzed using geo-
statistical techniques. During the models calibration pro-
cess, a total of 35 years of time series dataset such as
historical topographical maps, aerial photographs and
satellite imageries was used to identify the parameters that
influenced the urban growth. The validation result showed
an overall accuracy of 99.6 %; with the producer’s accu-
racy of 83.3 % and the user’s accuracy 83.6 %. The
SLEUTH model used the best fit growth rule parameters
during the calibration to forecasting future urban growth
pattern and generated various probability maps in which
the individual grid cells are urbanized assuming unique
‘‘urban growth signatures’’. The models generated future
urban growth pattern and land use changes from the period
2004–2020. Both models proved effective in forecasting
growth pattern that will be useful in planning and decision
making. In comparison, the CA model growth pattern
showed high density development, in which growth edges
were filled and clusters were merged together to form a
compact built-up area wherein less agricultural lands were
included. On the contrary, the SLEUTH model growth
pattern showed more urban sprawl and low-density
development that included substantial areas of agricultural
lands.
Keywords Urban growth � Land-use change � Remote
sensing � GIS � Cellular automata � SLEUTH � Sana’a
Introduction
Most of damages and harmful effects in environment are
caused by anthropogenic activities. The changes of land
use and land cover occur due to urbanization caused by
unplanned and uncontrolled urban sprawl which leads to
change nature, destroy green cover and pollute the water
resources. The present needs are analysis, understanding,
modelling urban growth evolution and land use changes to
save and provide suitable and safe environment for the
mankind. Urban growth modelling is essential for analyt-
ical and the prediction of the dynamics of urban growth.
GIS technique has been used to study, analyze and corre-
late urban activities and land use changes and its effects on
ground water and environment. Most of these studies
M. Al-shalabi
Department of Geography, University of Sana’a, Sana’a, Yemen
L. Billa
School of Geography, University of Nottingham,
Malaysia Campus, Semenyih, Malaysia
B. Pradhan (&) � S. Mansor � A. A. A. Al-Sharif
Department of Civil Engineering, Faculty of Engineering,
Geospatial Information Science Research Center (GISRC),
University Putra Malaysia, Serdang Selangor, Malaysia
e-mail: [email protected]; [email protected]
123
Environ Earth Sci (2013) 70:425–437
DOI 10.1007/s12665-012-2137-6
revealed that the urbanization and change in land use are
important factors affecting the water resources and envi-
ronment (Bathrellos et al. 2008). Urban models have been
used to forecast future changes or trends of development, to
describe and assess impacts of future development, and to
explore the potential impacts of different policies. The
geographical information system (GIS) offers a powerful
tool for the spatial analysis of a multi-dimensional phe-
nomenon (Youssef et al. 2011). The susceptibility map is a
practical tool in natural and urban planning. It reflects the
availability of lands for urbanization and other require-
ments. Many factors such as slope, aspect, road network
and land use are generally used during the model building
(Bathrellos et al. 2009; Rozos et al. 2011; Al-shalabi et al.
2012). In the literature, it is common to encounter meth-
odologies using spatial multiple criteria analysis in GIS
environment for urban growth management. The Analytic
Hierarchy Process (AHP) and GIS has been popularly used
to predict urban growth using many factors that affects
future growth and water demand (Panagopoulos et al.
2012). In the last two decades, a lot of research has been
done on urban spatial modelling due to increased com-
puting power, improved availability of spatial data, and the
need for innovative planning tools for decision support
(Geertman and Stillwell 2004; Brail and Klosterman 2001;
Bathrellos et al. 2012; Biro et al. 2011; De Abreu and Filho
2011; Jabbar and Zhou 2011; Lv et al. 2011; Peng et al.
2011; Pradhan 2011). Urban system models have shown
potential in simulating the complexity of dynamic urban
processes and can provide an additional level of knowledge
and understanding of spatial and temporal change (Wu and
Silva 2009; Kaya and Curran 2006; Pettit et al. 2002).
These aforementioned models include both macro and
micro integrated cellular automata (CA) models (RIKS
model) (Engelen et al. 1997), fuzzy CA model (Wu 1996),
ANN CA model, (Li and Yeh 2002), multi-CA model
(Cecchini and Rinaldi 1999) and the SLEUTH urban
growth model that has been widely applied in the United
States and many other parts of the world (Jantz et al. 2004;
Leao et al. 2004; Yang and Lo 2003; Esnard and Yang
2002; Silva and Clarke 2002; Clarke et al. 1997; Zeug et al.
2006).
These models have been useful in planning and decision
support for urban environmental management. They are
applied together with other models, such as; the logistic
regression model to identify the significant variables and
rules that differentiate urban or city from rural and forest
environments; the relative probability model which uses
spatial interactions of neighbourhood, distance, patch size
(parcels), and site-specific characteristics; and the focus
group involvement to create a human input layer, set the
growth scenarios, evaluate predictions and disseminate the
information (Allen and Lu 2003).
In a data deficient country like Yemen, urban environ-
ment planning and management have many challenges and
most particularly the lack of a mechanism to forecast and
predict urban growth trends and patterns. In Yemen,
development in uncontrolled and planning is haphazard,
lacking any clear vision about the future. For that reason,
cities have many socio-economic, infrastructural and
management problems. In this study, two urban growth
models, i.e. the CA and the SLEUTH are applied in
modelling the urban growth trend of Sana’a the biggest city
in Yemen and to predict and forecast the growth pattern
from the year 2004–2020. These advanced urban growth
models have been developed using the factors by consid-
ering certain assumptions which have been popularly
applied in many cities around the world (ESCWA 2007;
Pedro and Zamyatin 2006). This study tests the applica-
bility of these models in a city where growth condition may
be different; it also compares the output growth pattern of
the two models.
Urban growth modelling using cellular automata (CA)
and the SLEUTH models
Cellular automata (CA) models were developed for many
modelling purposes but have been popularly applied in the
area of modelling urban studies and growth processes
depending upon their transition rules, and calibration
methods (Couclelis 1985, 1997). Many forms of urban CA
have been used to simulate the spatial pattern of an actual
city or a synthetic city (Batty 1997; Yeh and Li 2001).
Wagner (1997) explored the potential use of CA in urban
planning and discussed theoretical obstacles in incorpo-
rating CA models in geographical context. These models
have been used to study the evolution of urban land use,
modelling urban forms; urban models and many other
urban studies (White and Engelen 1993, 1994; Clarke et al.
1997, Clarke and Gaydos 1998a, b; Batty and Xie 1997;
Batty 2000). In recent years, new CA models, such as
URBANSIM, UPLAN, SLEUTH have been used to fore-
cast future changes, trends of development, describe and
assess impacts of future development, and to explore the
potential impacts of different policies (Herold et al. 2001).
CA models have many advantages in the modelling of
urban processes that include the abil-ity to perform spatial
dynamics, and time explicitly. After successfully analysing
the similarities and capabilities of CA, Wagner (1997)
proposed that CA can be considered as analytical engine of
GIS. Raster GIS with map algebra can be integrated with
enhanced capabilities as discussed by Takeyama and
Couclelis (1997). CA are considered to have a ‘‘natural
affinity’’ with raster data. It has similarities with GIS, such
as the representation of attribute information in a layered
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fashion, and the manipulation of information with opera-
tors (Overlay in GIS, Transitional rules in CA). The focal
sum or focal mean functions of GIS has direct analogous
with neighbourhood functions. Because of its natural
affinity with GIS, it was obviously adopted by geographers
as a tool for modelling spatial dynamics (Singh 2003).
Clarke et al. (1997) introduced the Clarke urban growth
model, a precursor to the SLEUTH model for simulating
historic change. The model aids the illustration and expla-
nation of growth processes at a regional scale and predicts
future urban growth trends. The model was successful in
simulating urban change between 1900 and 1990 for the San
Francisco area, and was later applied to the Baltimore
Washington corridor (Clarke and Gaydos 1998a, b), where
calibrations and long term predictions for both San Francisco
and Baltimore Washington were presented, allowing for an
effective comparison to be made between the growth pat-
terns and processes of the two urban systems. The SLEUTH
model is now in a public domain C-language source code,
available for download with documentation online from
USGS website at http://www.ncgia.ucsb.edu/projects/gig.
The model simulates land use change as a consequence of
urban growth by working on a grid space of pixels, with a
neighbourhood of eight cells of two cell states (urban/non-
urban), and five transition rules that act in a sequential time
steps. The states are acted upon by behaviour rules, and these
rules can self-modify to adapt a place and simulate change
according to what have been historically the most important
characteristics.
SLEUTH requires five GIS-based inputs: urbanization,
land use, transportation, areas excluded from urbanization,
slopes, and hill-shading as a background. The input layers
must have the same number of rows and columns, and
should be correctly geo-referenced. For statistical calibra-
tion of the model, urban extent must be available for at
least four time periods. Urbanization results from a ‘‘seed’’
urban file with the first urban year used, with at least two
road maps that interact with a slope layer is used in order to
allow the generation of new nuclei for outward growth.
Besides the topographic slope, a constraint map represents
water bodies, natural and agricultural reserves. After
reading the input layers, initializing random numbers and
controlling parameters, a predefined number of interactions
take place that correspond to the passage of time. A model
outer loop executes each growth history and retains sta-
tistical data, while an inner loop executes the growth rules
for a single year.
Study area
The city of Sana’a, Yemen is located within the latitudes
15� 100 0000 and 15� 300 0000 North and longitudes 44� 050
0000 and 44� 200 0000 East (Fig. 1). The total area was
15,284.84 km2 and the built up area was around
138.66 km2 in 2003 (Al Shaibi et al. 2006). The city has a
moderate climate all year round, due to its location at about
2,200 m above sea level. Sana’a city is located to the
Northern-central part of Yemen in a high valley extending
from south to north. The old city has a unique urban and
architectural design that dates back to over 2,500 years. In
2004 the total population of Sana’a was estimated at
1,747,834 (Al Shaibi et al. 2006; Alderwish and Almatary
2011).
Data and methodology
Various data used in this study includes Quick bird satellite
imagery (0.60 m) acquired in 2003, aerial photography
(4 m) acquired in 1994, digital contour line map (10 m
interval), hydrological map 1:50,000, road network and the
city master plans maps from the Ministry of Public Works
and urban planning and the statistical information for dif-
ferent years from Central Statistical Organization in
Yemen. In developing countries the availability of data is a
major concern for this kind of studies. In this study, the
available data were collected between the years
1978–2003, this made it possible for the calibration process
to register the progression of urban growth signature over
the time period. Data for growth after 2003 were not
available so could not be included in the calibration pro-
cess. Data generally comprised maps of different types,
dates, scales and time, those were pre-processed into a
uniform geo-reference to create a profile of urban extent of
Sana’a city over space and time and organized into spatio-
temporal GIS database. Other types of data such as statis-
tical information were collected from various reports
published by different ministries and departments in
Yemen in 1999. These data was further classified and
clipped to the map extent (Hurskainen and Pellikka 2004)
and transformed to the same cell size raster grids at 45
meter resolution. This resolution was used for all data
layers. The grid dimensions were 527 columns by 811
rows. These datasets were also used as an input for the CA
and SLEUTH model calibration process.
Prediction of urban growth and land use change using
CA transition rules
A suitability map was used to estimate the annual demand
of land for urban development depending on the historical
growth of Sana’a city. The evolution of a cell was deter-
mined by the suitability value and the number of cell that
were developed. The cells were then calculated to establish
how many cells had attained a particular state of a time
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123
transition rule (Xi et al. 2009; Watkiss 2008). Cells are
allocated to a particular state by selecting them from the set
of available (the cells which can undergo transition) cells
that are spread over the city. Subsequently, the cell can-
didatures were evaluated using multi criteria analysis
(MCA) technique to determine their possible inclusion in
the model. Equations 1 and 2 ere used for formulation of
the model (Samat 2005).
utþ1i;j ¼ f ut
i;j;Xti;j; S
ti;j
� �ð1Þ
where, utþ1i;j = the state of the cell at row i and column j at
time t ? 1; uti;j = the state of the cell at row i and column j
at time t; Xti;j = the development of cells within the
neighborhood of the cell at row i and column j; and
Sti;j = the suitability score for the cell at row i and column j
for urban development.
In the above equation, the function f is formulated using
IF, THEN and ELSE statements as shown in Eq. 1. The
repeated application of this rule produced a complex spatial
pattern.
IF ðSti;j � Xt
i;j � threshold value); ð2Þ
Then utþ1i;j = urban; Else utþ1
i;j = non-urban.
The urban growth rules involve the selection of a
location by investigating the spatial properties of the
neighboring cells, and urbanizing the cell under consider-
ation based on a set of weighted probabilities using land-
use and suitability map. High priority and suitable areas
such as the master plan areas and proximity to existing
developed areas are given a score factor of 5. The unsuit-
able and development restricted lands are given a score of
0. The details of these score factors are developed for
transition rule processes (Table 1). After cells have been
assigned score, the top scoring cells (cell available for
transition) are allocated to particular states (land use type)
using Eqs. 1 and 2 and afterwards each cell is assigned to
an integer value based on land use classes.
Prediction of urban growth using the SLEUTH model
SLEUTH is a moniker for the input data required to use the
model: slope, land use, exclusion, urban, transportation and
hill shade. It employs a raster data (8-bit GIF) as input
format but it does not use GRIDs directly. For the mod-
elling and prediction of urban growth, the model supports
three different modes: test, calibration, and prediction.
The model is computation-intensive especially in the
Fig. 1 Location map of the Sana’a city and its urban extent
428 Environ Earth Sci (2013) 70:425–437
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calibration mode (Xi et al. 2009; Candau 2002). The
computation time varies from hours to days depending on
the spatial resolution and the image size. According to
Jantz et al. (2004), it requires a several days to calibrate the
urban growth for a medium-size city. The outputs include
a series of GIF images, representing the yearly urban
scenarios, which can be compiled into time series for
animation that illustrates the urban growth pattern and
change over time. Figure 2 shows the structure of the
SLEUTH model as illustrated by Yang and Lo (2003).
Model calibration
Urban areas throughout the world grow at different rates
and in different ways due to varying economic conditions
and environmental constraints (Kaya and Curran 2006;
Daniels 1999). Urban growth modelling (UGM) of Sana’a
city will thus require predictions that are consistent with
the factors and constraint with the city. Sana’a city model’s
coefficient values were derived from historical dataset that
was ‘‘fit’’ to area through the UGM’s calibration phase
using the brute force method (Goldstein et al. 2004). The
model was first run in a calibration phase to obtain a
suitable set of parameters which was further refined in
the sequential calibration steps. The calibration allows the
Table 1 The factors and scores effect of the transition rule
Factors Score
Proximity to existing developed areas 5
Proximity to prioritized land (master plan proposed
areas)
5
Suitability factors
High suitable 5
Suitable 4
Moderate suitable 3
Less suitable 1
Unsuitable Restricted
Land use factors
Agricultural land 5
Industry land Restricted
Mountains 1
Green areas Restricted
Other land use Restricted
Fig. 2 General structure of the SLEUTH model (after Yang and Lo
2003)
Table 2 The parameters of the coarse, fine, final calibration (45
meter pixel size)
Dispersion Breed Spread Slope Road
gravity
Parameters of coarse calibration
Start 1 1 1 1 1
Step 25 25 25 25 25
Stop 100 100 100 100 100
Possible units 5 5 5 5 5
Monte Carlo
iterations
4
Possible
combinations
3,125
Elapsed time 4 Days, 8 h, 7 min and 3 s.
Parameters of fine calibration
Start 80 70 20 45 45
Step 5 5 5 5 7
Stop 100 85 35 65 100
Possible units 5 4 4 5 12
Monte Carlo
iterations
6
Possible
combinations
4,800
Elapsed time 7 days, 3ours, 49 min and 13 s
Parameters of final calibration
Start 88 78 17 40 94
Step 1 1 1 2 1
Stop 93 84 23 56 98
Possible units 6 7 7 9 5
Monte carlo
iterations
8
Possible
combinations
1,3,230
Elapsed time 9 days, 4 h, 7 min and 33 s
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model to simulate urban growth from the past into the
present with a very high degree of fit between the simulated
years and the control years. The coarse calibration, took 25
unit steps for each entire coefficient space, for all coeffi-
cients. The fine calibration, took 5 unit steps and the final
calibration took either 1–2 unit steps through the coeffi-
cient space. Table 2 shows a summary of the parameters
and processing outputs obtained in the calibration.
Results and discussion
The prediction of urban growth change was performed by
using GIS-based CA transition rule. The annual demand of
land was estimated based on the models calculation of rate
of change and urban growth for the calibration period/
historical data (1978–2003). The results presented in
Table 3 shows that urban growth/extent of 138.6 km2 in
2003 will increase to 149.63 km2 in 2005, then to
183.62 km2 in 2010 and 272.23 km2 in 2020 respectively,
with an average annual growth rate of 4.05 %. The pre-
diction of the urban growth extent was paced with a dif-
ference of five years (2005, 2010, 2015 and 2020) so as to
allow enough significant growth during the period to reg-
ister a change in the prediction. The graphical presentation
of predicted urban growth is shown in the sampled results
for the year 2010 (Fig. 3). The figure shows the growth
pattern using a GIS-based CA model are compact in the
suitability lands, i.e. the growth has filled spaces and has
merged many clusters together around existing built-up
areas and occupies the prioritized land in the master plan.
In this study, land-use was defined as state of cells at
time ‘t’, represented by five major categories: built-up area,
agricultural land, industry land, green areas, and moun-
tains. The suitability land was represented by five catego-
ries: high suitable, suitable, moderate suitable, less
suitable, and unsuitable. The sampled results obtained from
the prediction modelling for the year 2010 are presented
here for better discussion and explanation of the growth
and change of the land-use. Table 4 and Fig. 4 show the
spatial growth and land-use changes respectively. Agri-
cultural land is reduced from 444,742 km2 in 2003 to
339 km2 in the year 2010, while the green areas was
increased from 6.62 km2 in 2003 to 24.9 km2 in 2010. The
industrial land does not show any changes because; it has
Table 3 Prediction of spatial urban growth in Sana’a City
2004–2020
Years Area (pixel) Area (km2) Annually developed cells
2004 71,052 143.88 2,578
2005 73,891 149.63 2,839
2006 76,912 155.75 3,021
2007 80,130 162.26 3,218
2008 83,529 169.15 3,399
2009 87,061 176.30 3,532
2010 90,676 183.62 3,615
2011 94,407 191.17 3,731
2012 98,342 199.14 3,935
2013 102,372 207.30 4,030
2014 106,550 215.76 4,178
2015 110,917 224.60 4,367
2016 115,408 233.70 4,491
2017 119,986 242.97 4,578
2018 124,647 252.41 4,661
2019 129,461 262.16 4,814
2020 134,436 272.23 4,975
Fig. 3 Predicted growth areas for 2010 using the CA model
Table 4 Land-use change for the year 2010
Land use 2003 2010 Value
Built up area 138.66 184.75 45.74
Agricultural land 444,742.36 399.04 -43.43
Mountains 229.48 227.17 -2.31
Green area 6.62 24.90 18.28
Industry 10.12 10.12 None
Other land use 11.30 11.30 None
430 Environ Earth Sci (2013) 70:425–437
123
already been included in the western part of the city as
future industrial area as documented in the master plan
1999.
Validation of the CA model
The validation of the modelling results of CA was carried
out by predicting the spatial urban growth from 1994 to
2003 and comparing it with the actual growth pattern of the
city in 2003. The validation result (Table 5) shows an
overall accuracy of 99.6 %. This high level of accuracy
was achieved because very high spatial data was used in
the processing. The producer accuracy however was
83.3 % while the user’s accuracy was 83.6 %. Considering
the high level of accuracy achieved by the prediction, the
dataset (1994–2003) was further used in the application of
the model. The graphical maps of the actual urban area of
2003 and predicted were overlaid for visual comparison
(Fig. 5).
Fig. 4 The change in land-use from 2003 to 2010 using GIS-based CA model
Table 5 Validation result of the GIS-based CA prediction for the
year 2003
Actual
pixels
Predicted
pixels
Correct
pixels
Overall
accuracy
%
Procedure’s
accuracy
%
User’s
accuracy
%
68,451 68,688 57,204 68,451/
68,688
57,204/
68,688
57,204/
68,451
99.6 83.3 83.6
Fig. 5 Actual and predicted urban extent for the year 2003
Environ Earth Sci (2013) 70:425–437 431
123
Results of the SLEUTH urban growth model
The five coefficient values that were obtained in the cali-
bration process were used to predict the urban growth from
2003 to 2020 for the city of Sana’a. Table 6 shows the
results of the predicted urban growth. The result indicated
that urban area will increase dramatically from 138.6 km2
in 2003 to 143.75 km2 in 2005,159.18 km2 in 2010 and
199.71 km2 in 2020. The growth patterns indicate a very
high value of diffusion and breed and also showed very
high road gravity coefficient and low slope coefficients.
Growth clearly occurred at the urban fringe resulting
organic growth and increase in suburbs. Very high value of
diffusion can be attributed to the fact that the city has no
zoning plan which has allowed unrestricted outward
growth in all directions. Table 7 shows the effect of the
urban growth on land-use change. The annual probabilities
transition from agricultural land to urban land is about
11.92 % and the annual transition for the mountain area is
0.03 % with 0.065 km2 per year. The graphical results of
the SLEUTH model prediction of urban growth for the year
2010 is shown in Fig. 6. In this map it can be seen that the
outward growth is scattered all over the map. The predic-
tion of land-use change for the same year is shown in
Fig. 7.
Comparison between the CA and SLEUTH model
A comparison was made between the two sampled mod-
elling results for the year 2010 for both CA model and
SLEUTH model to understand the dynamics of transitional
probability change and also to estimate the variance
between the two modelling approaches. Figure 8 shows an
Table 6 Model prediction of growth extent of Sana’a urban area
(2004–2020)
Years Area (pixel) Area (km2) Annually developed
cells
2004 69,730 141.20 1,281
2,005 70,990 143.75 1,260
2006 72,413 146.65 1,423
2007 73,858 149.56 1,445
2008 75,415 152.72 1,557
2009 76,993 155.91 1,578
2010 78,609 159.18 1,616
2011 80,312 162.63 1,703
2012 82,028 166.11 1,716
2013 83,744 169.58 1,716
2014 85,722 173.59 1,978
2015 87,667 177.53 1,945
2016 89,789 181.82 2,122
2017 91,885 186.17 2,096
2018 93,988 190.33 2,103
2019 96,257 194.92 2,269
2020 98,622 199.71 2,365
Table 7 Annual land-use
transition probabilitiesUnclass Urban Agricultural Mountains Park Industry Airport Others
Unclass 99.57 0.02 0.31 0.11 0 0 0 0
Urban 0 98.89 1.08 0 0 0 0.02 0
Agricultural 0 11.92 86.87 0.69 0.01 0 0.5 0
Mountains 0.03 0.03 0.08 99.73 0 0 0.13 0
Park 0 0 0 0 100 0 0 0
Industry 0 0 0 0 0 100 0 0
Airport 0 0.05 0 0 0 0 99.95 0
Others 0 0 0 0 0 0 0 0
Fig. 6 Predicted growth areas for 2010 using the SLEUTH model
432 Environ Earth Sci (2013) 70:425–437
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overlay comparison of the two modelling results. The
SLEUTH model results showed a wider spread of the
growth patterns and smaller urban clusters, while the GIS-
based CA model showed a compact growth pattern where
small clusters are merged together. The CA grow pattern
reflected the proposal made in the Sana’a master plans
where it is suggested to incorporate adjacent lands into the
city’s residential development. In Fig. 9 the comparison
results are presented based on the five land-use classes. It
showed that the predicted growth for 2010 using the CA
model is about 184.75 km2 when compared to 159.18 km2
Fig. 7 Predicted land-use change for 2010 using the SLEUTH model
Fig. 8 Comparison of prediction results of 2010 for the CA and
SLEUTH models
24.911.3
24.911.310.12
159.18
424.45
228.33
10.12
184.75
399.04
228.17
0
50
100
150
200
250
300
350
400
450
landBuilt up Agricultural Mountains Industry Green land others
Km
2
area
SLEUTH Model GIS-based CA Model
Fig. 9 Statistical comparison of the CA and SLEUTH models in the
land-use change prediction
Environ Earth Sci (2013) 70:425–437 433
123
using the SLEUTH model. In general the results from the
SLUETH model showed extensive urban sprawl and spatial
distribution when compared with the CA model.
The verification of the predicted urban growth for
Sana’s city by field measurement was not possible due to
logistical constraints. Moreover, due to scarcity of high
spatial resolution satellite images for prediction time zone,
the verification of the models application was conducted
using the Google images (Digital Globe image 0.6 m) in
Google Earth free software, that provide the capability of
time tuning through 2003–2010. The predicted results for
the year 2010 was possible by overlaying the images on the
google images. For the validation purpose, we chose the
Google image of 2010. A comparison was thus made by
visual validation process that showed significant similarity
between the existence and predicted urban growth for the
Sana’a. By logical inference and visual inspection, the
study concluded that where the predicted urban growth
shows a good correlation over the Google images. Fig-
ures 10 and 11 show the validation results of 2003 and
2010 that inevitably support the predicted spatial urban
growth in Sana’a.
The actual urban growth and land use changes in Sana’a
city that occurred from 2003 to 2010 are shown in Figs. 10
and 11 respectively. These figures give a clear assessment
and comparison of the results of urban sprawl and land use
changes that we have predicted using CA and SLUETH
models used in this study. By comparing the results of both
models with the real urban growth from 2010 (Figs. 10 and
11), it is evident that that models employed in this study is
sufficient enough for prediction of urban growth.
The advantage of GIS-based CA is that it generated
results with geographical reference and also handles large
amount spatial data which allows for the creation of
Fig. 10 Validation of Urban growth predicted using CA-model for the year 2010
434 Environ Earth Sci (2013) 70:425–437
123
constraints and other criteria by assigning the weight that
helped in determining whether sites were suitable or
unsuitable to the growth modelling. The prediction by GIS-
based CA model reflected the projections of the master
plan; thus can be used to develop master plan projection to
support planning and decision making in the future. The
prediction also shows a growth trend towards increased
urban expansion due to increasing population growth. The
pattern of urban growth would be greatly influenced by the
controlled and protected lands and the physical character-
istic of the terrain such as mountains ranges in northeast
part of the city. The limitations of the model are highly
dependent on the data quality, i.e. data of higher quality
results in more accurate outputs. Other weaknesses
affecting the models are growth in multiple directions and
also their inability to recognize new urban development
that occurred away from existing urban areas.
The SLEUTH model was found to be useful in visual-
izing and quantifying spatial growth extent. It also showed
saving of about 25.41 km2 of growth encroachment on
agricultural land-use when compared with the CA model
because its growth trend has more sprawl and low-density
development patterns that leads to substantially less con-
sumption of agricultural lands. The limitation of the
SLEUTH model is that, it is very sensitive to the spatial
resolution of data. Consequently, a higher resolution data
improves the quality of data and the model output. The
SLEUTH model also require high end computational
capabilities such as the use of parallel computing envi-
ronment, making this model extremely difficult to apply in
Fig. 11 Validation of Urban growth predicted using SLEUTH model for the year 2010
Environ Earth Sci (2013) 70:425–437 435
123
a developing country with limited data inputs. It also
observed that the composite results of the optimum values
for the diffusion, spread, slope and road gravity parameters
show successive improvement in the parameters that con-
trol the behavior of the system.
Conclusions
Urban modelling is an important technique for forecasting
and studying urban dynamics to understand the potential
impact of growth and future development. This study
found that the major urban growth occurred in agriculture
areas and around unplanned (low price) areas outside
township boundaries where no infrastructure facilities
(such as drainage systems and disposal management sys-
tem) exist. This reflects the lack of clear policy that could
control and guides urban sprawl in the city. This situation
will results in serious socioeconomic and environmental
problems in the near future. In this study the growth is
mainly affected by topography and road networks. It is
necessary to accommodate the fast population growth by
following clear plan that take into consideration of the
socioeconomic conditions, and exploit the growth factors
to put clear policy and regulation by providing facilities
and infrastructures which encourage the urban growth in
controlled planned areas. The CA and SLEUTH are some
of the models that have been extensively used in growth
modelling. Although these techniques are useful for fore-
casting and prediction for future growth which support
planning and decision making, they have seen little used
in developing countries such as Sana’a, Yemen where
their impact could be greatly appreciated. In this study the
results of their application for prediction and simulation of
the urban growth and land use change in Sana’a city
showed a high overall accuracy of 99.6 %, producer
accuracy of 83.3 % and user accuracy of 83.6 %. The
growth pattern of CA model presented a compact and high
density development, while the SLEUTH model presented
an urban spread pattern with very low-density develop-
ment. The study concluded that both models are very
useful urban modelling tools and enable the prediction and
generation of different urban growth scenarios in support
of planning and decision making. By coupling the models
GIS, output results showed geo-referenced map that
helped to identify and demarcate specific locations for the
implementation of planning policies. Implementation of
the SLEUTH model however required many datasets and
high end computing power for processing; this will be a
major limitation in developing countries that have defi-
cient resources. The study demonstrated an efficient
implementation of the CA and SLEUTH models coupled
with GIS in urban growth and land-use modelling and
allowed the testing of different policy alternatives on
growth scenarios.
Acknowledgments The study benefited from the academic schol-
arship provided by the Ministry of Higher Education Yemen. Authors
are also very thankful for the support given by various ministries and
departments in Yemen with the provision of data and other infor-
mation used in the study. Thanks to two anonymous reviewers for
their useful comments on the earlier version of the manuscript.
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