As of 2018, 55 percent of the world's population lives in urban areas. This ratio is expected to be 2/3 of the total population in 2050 [1]. In Turkey, 92.3 percent of the total population lives in urban areas in the year 2018. This ratio was 64.9 percent in 2000. Approximately 50 percent of the urban population lives in small and medium-sized urban areas [2]. Although the concept of small and medium-sized cities is defined differently by various sources [3–6], it is possible to include cities with a population between 5,000 and 250,000 in this category [7,8]. Turkey has a total of 882 urban units in this category [9].
Opportunities such as jobs, education, health, culture offered by the urban area to the inhabitants constitute the main reasons for urban population growth in developing countries such as Turkey. The increasing population in urban space causes the city to grow and spread over time. The city, which has been positioned at the most appropriate place throughout the historical process, grows spatially over time, with various factors. However, the land that surrounds the urbanized area is not always suitable for new built-up. The fertile agricultural lands, streambeds, forests, and other ecologically vulnerable areas in the periphery of the city are destroyed and harmed sustainability in the urban growth process. Balanced development cannot occur in cases where spatial growth in urban areas cannot be accurately predicted and planned.
The literature emphasizes the importance of urban models [10]. The essentials of urban models are to anticipate the problems that urban areas may encounter and develop solutions before the occurrence [11]. In this context, there is an increase in the number of studies on the subject in the academic literature of urban science over time. Various methods and techniques are used for many different purposes, such as estimating the spatial growth of cities, predicting land-use changes, predicting the interaction between transportation and land use, determining the reflections of investments and spatial decisions to be made [12–15]. While some of these models are, micro-scale models produced for certain parts of the city; some also evaluate the city's relationship with its environment and evaluate the entire city as a whole. Also, some integrated models evaluate micro and macro scale together [16,17]. According to their predictive capacity, urban models can also be divided into short, medium, and long-term models. Besides, while many models were produced with spatial variables, some were tested by statistical data not dependent on the location. Models in which spatial and non-spatial data are evaluated together have also been developed [18–20]. According to the methods used in the model production process, many different models such as statistical models, geographical information systems-based models, cellular automata models, artificial neural networks models, agent-based models, and integrated models have been defined [16].
Undoubtedly, the models produced for urban areas do not reflect the actual situation exactly where very different systems coexist and different social, spatial, and economic actors interact continuously and mutually. In this context, it is possible to define urban models as a simplified simulation of reality without losing the essence[21]. Models can be constructed within the framework of many different criteria. However, there are primary factors that need to be considered in the construction of any model. In producing a model, the subject and scope of the study and the purpose of the model should be determined first. A pool of parameters directly related to the research subject should be created, and the relationship between these parameters and the degree of influence of each parameter on the result should be determined. The best version of the model should be selected in the next step, and the model should be tested under various conditions. In the last stage, the model's output should be compared with the actual situation, and the model should be revised if necessary.
In predicting urban growth, many criteria should be evaluated together in the natural environment, built environment, social and economic structure categories. Elevation, slope, aspect, geological condition, stream beds, seismicity, vegetation, soil capability are some of the natural environment criteria. In the built environment category, transportation networks, urban facilities, attraction points, existing urban boundaries, and infrastructure are the main drivers influencing urban development. It is possible to exemplify the socio-economic variables as the population, demographic structure, income status, vehicle ownership rate, and household size [22–25].
It is necessary to determine the population that the urban area will accommodate over the years by evaluating the demographic characteristics of the population, such as income status, household size, and vehicle ownership status, to predict urban spatial growth. After detecting the demand for the urban area, many elements should be evaluated together statistically and spatially in order to estimate the urbanization. At this point, multicriteria decision-making methods (MCDM) are used frequently.
MCDM refers to the determination of the relationship between variables and the severity and influence to reach the final output [26,27]. The method is functional for determining new urban development areas [28,29], as well as industrial facilities site selection [30], landfill site selection [31–34], energy facilities site selection [35–39], health facilities site selection [40,41], shopping mall site selection [42], public institutions site selection [43], parking lot site selection [44], urban greenways [45], vulnerability assessment [46–51] and sustainability assessments of urbanized areas [52,53].
The variables are determined by questionnaire, expert opinion, academic knowledge, literature research, or pre-acceptance. In the second stage, the weights of each variable are predicted or defined with expert-based scoring, statistical methods such as Analytical Hierarchy Process (AHP)[54–58], linear and logistic regression [59–61] or machine learning algorithms such as artificial neural networks, decision trees, support vectors, random forest, closest neighbor, deep learning [62–66]. In the last stage, the final product is obtained by evaluating the variable weights together.
In urban growth modeling, after determining the crucial variables directing urban growth, spatialization is vital to locate urban growth areas in the geographic space. Geographical Information Systems (GIS) technology is used as a tool for this purpose. The ability to analyze, visualize, and evaluate urban dynamics statistically and spatially is one of the main reasons GIS technologies are frequently used in urban growth modeling [31,67,68].
In urban growth modeling, MCDM, AHP, and GIS are used together to increase model prediction capacity. Akbulut et al. (2018) used AHP to determine the most suitable urban growth areas within the context of sustainable urban development. They evaluated six main criteria: slope, streambeds, natural protection areas, forest areas, agricultural areas, and water basin protection zones. They determined each criterion's weights by evaluating them with paired comparison matrices and scored the sub-criteria according to the degree of conformity. As a result of the study, they determined the most suitable areas for sustainable urban development [69]. Malmir et al. (2016) determined the most suitable urban development areas for the Ahwaz settlement of Iran, a small-medium-sized city, using the AHP method. Within the study's scope, 45 sub-criteria were selected using expert opinion, literature research, and legal documents. The relationship between these sub-criteria was analyzed with a matrix-shaped questionnaire applied to various experts. The results were visualized in five groups according to the degree of suitability for urban growth[70]. Zheng et al. (2017) aimed to find the most suitable areas for the expansion of the urban area by using various sub-criteria under three main groups as natural, socio-economic, and ecological. Altitude, slope, geological structure, proximity to rivers, lakes, and water reservoirs as natural; built-up area, transportation, port, population density as the socio-economic and coastal, agricultural areas, and ecological protection zones are evaluated as ecological criteria. The main criteria and sub-criteria were weighted by AHP[71]. Dong et al. (2008) weighted various sub-criteria such as elevation, slope, geological structure, average temperature, river density, land use type, highway density, railway density, population density with the help of AHP under three main headings: environmental, land resources and socio-economic factors to determine the most suitable urban expansion areas[72]. Aburas et al. (2017) determined the most suitable urban development areas for Seremban, Malaysia, by using various social, physical, economic, and environmental factors such as elevation, slope, soil type, population density, land cover, highways, railways, power transmission lines, and streambeds, residential, commercial and educational facilities[73]. Park et al. (2011) evaluated many criteria to detect the most suitable urban development areas by logistic regression and artificial neural networks[74].
In urban growth models, criteria such as slope, land use, land value, proximity to transportations, linearity, aspect, flood risk, groundwater level, geomorphology, the density of transportation infrastructure are weighted with the help of AHP in various studies and used in determining the most suitable urban development areas [75–77].
Low-resolution monotonous raster formatted data are used to determine the most suitable urban development areas as the smallest geographical unit [78–80]. Therefore, there is a lock of urban growth models with high spatial resolution. Exploring the possibilities, capabilities, and positive aspects of parcel-based urban growth modeling is the essential investigated topic in this study. Therefore, instead of pixel-based modeling, a high-resolution suitability analysis is performed using actual cadastral parcels, the smallest unit where urban growth occurs. Besides, a wide range of factors revealing the parcels' original characteristics is used to estimate construction pressure on each parcel. In this respect, it differs from the studies in the literature and expands the spatial capacity of urban modeling.
Urban growth occurs at the level of cadastral parcels on a micro-scale. Many of the unique conditions of the parcel, such as its location and proximity to various attraction points, its relationship with the natural structure, its interaction with the existing built environment, and spatial plans, define the degree of pressure and suitability for urban development. In this study, the 32 criteria affecting the urban growth potential of a parcel are compiled under four main groups, and the weight of each criterion is determined with the help of AHP based on expert opinions. The overall development pressure on the parcels is calculated by overlaying the weights of each criterion geographically by using GIS technics. Furthermore, the demand for the urban area, calculated by simulated population density for the projection year, is allocated to the urban space according to the overall development suitability analysis of the parcels.
The primary purpose of this study is to build an urban growth model that predicts urban growth with high resolution and high accuracy. The proposed model can be used by city managers and stakeholders as a decision support tool to have a more sustainable and livable urban form with proactive policies in urban planning processes.
In the introduction part of the study, the importance of the subject and the relevant literature are evaluated. In the second part, the methods and techniques used in the study are expressed. In the last part of the study, the results of the proposed model are evaluated, the original contributions to the literature are explained, and the powerful and open-to-improvement features of the model are discussed. In addition, suggestions are made for future studies.