Bangladesh has experienced healthy economic growth over the last several decades, largely driven by a transition from a rural agriculture-based economy to a more modern urban economy [1]. This has resulted in a surge of migration from the rural areas into the cities [2] and rapid expansions of urban footprints, particularly among the largest cities such as Dhaka and Chattogram [3]. However, due to the lack of adequate forward-looking planning, the city’s expansion has largely taken place in an unregulated and chaotic manner [4]. For example, in Chattogram (also known as Chittagong), one of the world's largest port cities and the second-largest metropolitan area in Bangladesh, large-scale housing projects and constructions have taken place without prior approval, and subdivision/sales of land lack provisions for basic amenities [4]. The encroachment of agricultural land and lowland areas is also very common to make space for housing or commercial developments. A large portion of this urban development has occurred in the form of slums and squatter settlements, which have little to no access to facilities and services [5]. This form of unregulated growth patterns has led to creating negative environmental issues such as pollution, sanitation, traffic congestion, and crime, putting severe adverse pressure on the overall ecosystem of the region [4]. There is an immediate need to revisit the regulatory and policy instruments to address these concerns.

2. Literature Review on the Urban Growth Boundary within Developing Countries

UGBs, in simple terms, can be defined as land regulations that have been put into place, in most cases, by the local government to prohibit urban growth and development beyond a defined boundary ^[6][7][8][9,10,11]. The UGBs are designed to protect non-urban land outside the boundary and to promote compact, contiguous, and sustainable urban development ^[7][10]. The UGB, as an urban growth policy tool, has been implemented in a wide variety of cities in both the developed and the developing world. In the United States, several states, including Oregon ^[9][12], Washington ^[10][13], and Tennessee ^[11][14], have implemented UGBs for various cities within the state. Outside of North America, developed countries such as New Zealand ^[12][15], Belgium ^[13][16], The Netherlands ^[14][15][17,18], and Spain ^[16][19], to name a few, have also effectively utilized UGB as a prominent urban growth-restriction strategy. The adoption of UGB in developing countries and transition economies ^[17][20] has been limited. To the best of ouresearchers' knowledge, there have only been five developing or transitioning countries where UGB has been explicitly implemented as an urban growth control mechanism: namely Albania ^[18][21], Chile ^[19][22], China ^[20][23], Saudi Arabia ^[21][24], and South Africa ^[22][25]. Although the concept of UGB remains the same in both developed and developing countries, it should be noted that the planning and implementation of the UGB for developing countries will require distinctly different approaches as compared to developed countries, particularly due to differences in the nature of urban expansion observed in these countries ^[23][26]. For developed countries, such as the US, urban sprawl has been largely signified by low-density, non-residential development, and UGBs have been used to restrict this expansion ^[24][27]. In comparison, for developing countries, urban sprawl, particularly around the periphery of megacities and emerging cities, is associated with the expansion of compact and high-density built-up areas, usually consisting of informal dwellings and slums ^[25][26][28,29]. Furthermore, the urban sprawl in developing countries is often linked to the rapid growth of a city, leading to the inability of the city to provide sufficient services to its citizens, and hence resulting in poor, unplanned neighborhoods that lack basic necessities such as sanitation, running water, electricity, and paved roads ^[25][27][28,30]. Thus, when planning for a potential UGB within a developing country, such as Bangladesh, these additional issues regarding urban growth would need to be addressed in the adoption and implementation process for the UGB to be successful. A number of studies, related to both developed and developing countries in which UGB has been implemented, have expressed concerns regarding the overall impact of UGB on urban expansion, its ability to address the sustainable growth of the city ^[28][29][30][31,32,33], and the containment of built-up urban areas within the designated boundary ^[31][32][33][34,35,36]. Specifically, amongst the developing countries, the primary reason for concern regarding the success of UGB can be attributed to the underlying issues related to urban governance and political/policy conflicts. Studies of UGBs in Chile and Albania have indicated the absence of clear regulations, inadequate supporting policies, and an unclear definition of jurisdiction as some of the leading factors related to issues with effective administration of the UGB ^[19][34][22,37]. These countries have also suffered from a lack of institutional capacities needed to support and enforce property rights ^[35][38] and housing policies ^[36][39] within the UGB that have led to large tracts of land being developed outside the designated boundary. In other countries, including China and Saudi Arabia, the external influences from the business elite in the urban policymaking process, pressures for economic development-driven private businesses and political interests, as well as the exploitation of loopholes and a general lack of oversight have resulted in urban buildup outside of the intended UGB in these countries ^{[19][20][37][38]}[22,23,40,41]. Furthermore, in countries such as South Africa, a lack of collaborative efforts between the municipal and provincial governments has created disputes over the UGB delineation, which has led to the eventual dissolution of the UGB itself ^[31][39][34,42]. Following these concerns regarding urban governance, particularly in the initial adoption of the UGB, a lack of effective delineation of the boundary has been seen as a major concern with regard to its success. For China, the limited success of the UGB implemented in the first planning period has been attributed to the ineffective delineation that was produced solely based on the planned-economy architecture that had been used for the past several decades ^[32][35]. The archaic methodology used for delineation vastly underestimated the 10-year growth trajectory of the city, thus leading to insufficient allocation of land within the UGB and an ineffective UGB ^[40][43]. Similarly, for Saudi Arabia, the inefficiencies in the UGB have also been linked to delineation issues that have mainly arisen from technical limitations and institutional deficiencies due to the absence of trained planners, surveyors, and architects in most of the municipalities ^[41][44]. These delineation issues were further exacerbated by the use of outdated base maps and census data, hence leading to an overall inefficient UGB ^[41][44]. For Albania and Chile, the delineation process of the UGB has been largely arbitrary, with limited use of spatially explicit data used in the delineation process leading to ineffective implementation of the UGB ^[42][43][44][45,46,47]. Despite these criticisms, there has been a growth in popularity of UGB as an urban growth-restriction tool, with China ^[45][46][48,49], Saudi Arabia ^[47][48][50,51], and South Africa ^[39][49][42,52] at the forefront. These countries have been in a continuous process of updating and upgrading the UGB based on the needs and requirements of the cities, with significant research being conducted on the delineation and implementation of UGB policies ^{[47][50][51][52]}[6,50,53,54]. Additionally, over the last few decades, there has been a resurgence in the literature examining the adoption and delineation of UGB within other developing countries where it has not been previously implemented. 

3. Delineation of UGB

3.1. Methodological Approaches to the Delineation of UGB

A review of the literature showed the utilization of a variety of methods for the UGB delineation process. However, there is no one consensus or a universally accepted model with regard to the delineation of the UGB ^[53][55]. Sinclair-Smith ^[22][25] divides the process in which UGBs are delineated into three approaches. For the first approach, little or no quantitative assessment was performed for boundary delineation, and it was particularly prevalent in the initial adoption of UGB. The UGB implemented in Saudi Arabia during its first iteration ^[37][40] and the UGB delineated in China during the first planning period ^[32][35] provide good examples of this approach. Due to the lack of an analytical framework supporting the design of the boundaries, spatial plans for the Chinese cities during the period have been compared to artwork by urban designers rather than a plan to establish growth boundaries ^[32][35]. In Albania, the UGB delineation was based on boundaries separating agricultural land from urban land for cities with a population greater than 10,000 ^[42][54][55][45,63,64]. Whereas in Chile, the delineation of the boundary changed numerous times based on the subsequent political principles guiding these policies ^[56][65]. The second approach to UGB delineation is defined as the conventional approach. This approach is seen as being governed through guidelines provided by planning agencies such as the American Planning Association (APA) ^[7][10] in the Growing Smart Legislative Guidebook “Model Statutes for Planning and the Management of Change” ^[7][10]. Additionally, the conventional approach has also relied on systems such as the inventory-based system, as proposed by Knaap and Hopkins ^[57][66], that applies the concept of event-driven inventory for urban growth management. A large proportion of UGBs in the United States is based on these approaches. The APA guidelines propose using a future forecast for land demand to reserve sufficient developable land for the UGB over a set time period, generally over the next 20 years ^[7][10]. It further recommends the integration of 110 to 125 percent of projected urban growth as a long-term land-use planning strategy. Sinclair-Smith ^[22][25] explains that the purpose of including additional land within the UGB, than what is required, is to prevent owners from monopolizing vacant land, thus allowing for effective and competitive real estate markets. As compared to the time-driven approach, where the expansion of an established UGB to accommodate future growth occurs at a set time interval, the inventory control system proposed by Knaap and Hopkins’ ^[57][66] is an event-driven approach, where an increase in the developable land for the UGB is triggered only after the available land within the UGB is diminished to a predetermined level. The inventory control system for the UGB extension was further revisited by Han and Lai ^[58][67]. They translated the inventory approach into a decision network framework, where rather than the extension of UGB being based on one single event occurring, the change in the delineation would be based on the analysis of the complex system of linked actors, problems, and solutions that have an impact on the expansion of urban areas within the city. In a comparison of the new Decision Network Framework for the time-driven approach with the event-driven inventory approach, results indicated the Decision Network Framework-supported system to be more efficient and cost-effective at UGB allocation ^[58][67]. The third category of the approach used in the delineation of UGB utilized growth simulation models that included scientific and quantitative techniques to predict future growth and, based on it, delineate the appropriate UGB. This included a variety of methodologies that made use of constrained cellular automata (CA) to support the establishment of UGBs. Compared to the traditional method, the CA-based system, as used by Long et al. ^[32][35], included containment conditions such as macroeconomic, locational, institutional, and neighborhood constraints, aiding in the simulation of urban growth within the model. This in turn resulted in a more effective spatiotemporal simulation of urban expansion and an overall improvement in UGB delineation ^[32][35]. Other CA-based urban land-use change modeling techniques such as SLEUTH have also been used for predicting urban growth and, based on it, the creation of urban containment policies ^[59][60][68,69]. Bhatta ^[53][55] utilized the Ideal Urban Radial Proximity (IURP)-based design to examine the implementation of UGB for Kolkata, India. However, IURP does not provide any simulation or modeling of urban growth processes and patterns, but rather is a theoretical construct that uses a radial distance from the city center to create a circular urban growth boundary beyond which urban growth would be restricted. The use of radial distances was also implemented for the UGB models by Tayyebi, Pijanowski, and Tayyebi ^[61][56] for the city of Tehran, Iran. The authors used artificial neural networks to predict the radial extension of urban areas in individual azimuth and, based on the growth of the urban area, an urban growth boundary for each azimuth was designated ^[62][70]. Tayyebi, Pijanowski, and Pekin ^[61][56] also used the information on the radial distance from the center of urbanized areas for individual azimuth to delineate UGB using two separate rule-based methods, the distance-dependent method (DDM) and the distance-independent method (DIM). DDM used the points on the initial urban boundary to estimate the growth of urbanized areas and predict the future UGB for subsequent time periods using percentage increments across individual azimuth. DIM, on the other hand, used the rate of change in distance from the center of the urbanized areas for two different time periods within each azimuth to predict the boundary for the next time period ^[61][56]. The use of radial distance in delineating UGBs was further investigated by Tayyebi et al. ^[63][71] by using it in conjunction with spatial logistic regression (SLR). The SLR-UGB model considered the impact of spatially explicit biophysical factors such as topography to derive its impact on urban growth and hence simulate future urban growth boundary changes across each individual azimuth. Another interesting approach that has been used to allocate UGB is the application of the ant colony optimization (ACO) technique. Considering UGB as a problem of spatial optimization for land use allocation, Ma et al. ^[46][49] used the ACO method for optimizing land use for UGB delineation with the purpose of creating a balance between urban growth, planning regulation, and characteristics of the landscape. While these models provide a dramatic improvement in the methodological approach over having no systematic approach or even over having a conventional approach for delineation of UGB, it should be noted that urban development in the future more than likely will not necessarily follow historical trends of urban development, and subsequently, the UGB delineation approaches might not necessarily produce the optimal UGB for the city ^[64][72]. Therefore, in addition to the delineation of the boundary, specific targets and constraints in line with the emerging trends along with supporting policies would be essential to optimize and support the UGB delineation ^[64][72].

3.2. Use of SVM in Aiding the Delineation of UGB

ReIn this searcherstudy, we investigate the use of support vector machines (SVM) to aid in UGB delineation through the simulation of future contiguous built-up urban footprint expansion for the city of Chattogram over the next 20 years. SVM is a well-established methodology that has previously been used in a wide variety of research related to monitoring machine conditions and faults ^[65][73], language and speech recognition ^[66][74], diagnosis of diseases ^[67][75], and recognizing human motion patterns ^[68][76]. Additionally, SVM has also been extensively used in examining and modeling urban growth ^{[69][70][71][72]}[77,78,79,80]. The primary reason for using SVM  in this study is due to the exceptional performance of this methodology as a classifier ^[73][81], and specifically in forecasting land-use change and urban growth as compared to other methodologies, such as the logistic regression approach ^[70][78] or the artificial neural network or decision tree ^[74][82]. Factors that have a significant impact on urban growth were used as the input parameters for the SVM model, and the past 20 years of data on these factors were used to train the model. This trained SVM model was then used to simulate the future expansion of the urban footprints for Chattogram. Unlike the UGB delineation methodologies discussed previously, the outcome of the simulation is not used as a delineation of the UGB itself, instead, it is used as a robust methodology that can be employed as a part of the overall UGB delineation process. The goal is to utilize the SVM approach to provide an insight into potential areas that are more likely to undergo urban expansion. This information would be valuable for planners and decision-makers to optimize the delineation of UGB. Results obtained from the simulation would act as a guide for planners by highlighting the type of current land cover that is likely to change into urban built-up areas, identifying areas that are at a higher environmental risk, restricting/regulating growth in these areas, and subsequently allowing for expansion in areas with lower environmental risk as well as access to essential services for a more effective delineation of UGB. Additionally, the urban built-up simulation would provide planners and decision-makers a roadmap to propose policies directed at addressing these concerns related to sustainable urban expansion of the city.