What makes a place urban?

What makes a place urban? Abstract We argue that “urbanity” is a function of population concentration. Empirically, this means urbanity can be measured along a spectrum, with increased population concentration in a place denoting increased urbanity. Phenomenologically, large concentrations of people in space—or mass corporeal co-presence—generates the essentially “urban” experience of living surrounded by the bodies and minds of strangers. We use a series of thought experiments to demonstrate the conceptual limitations of other historically common definitions of urbanity, such as the economic structure of a community, the presence of physical infrastructure, the political or administrative status of a geographic unit, or the degree of connectivity between people. These are not essential urban characteristics, but rather common epiphenomena associated with places that have large, spatially concentrated populations. A density-based definition does not require a settlement to be permanent, allowing for ephemeral urbanity (dense but temporary settlements). While density-based approaches to classifying human settlements based on gridded population data are conceptually robust, such as the methodology adopted by the United Nations in 2020 for cross-national comparison, we present an alternative measurement approach that is more closely aligned with our phenomenological understanding of urbanity. Keywords: urbanity, urbanism, urbanization, city, human settlement classification 1 Introduction The concept of an “urban place” is ancient and ambiguous. It is pervasive in popular culture and public discourse, it is a foundational concept and site of study in many academic subdisciplines, and it is one of the most enduring statistical categories deployed by researchers and policy makers. Yet there is no consensus definition of what makes a place “urban.” Every country in the world has a unique “urban” definition for the purposes of statistical classification (Cohen, 2004; Buettner, 2015). Moreover, the central concepts in these definitions can change over time: in 2021 the United States Census Bureau changed a long-standing urban definition from one emphasising population characteristics in census tracts to one emphasising residential structure density (USCB 2022). This lack of consensus and consistency powers a rich theoretical literature, but also complicates empirical research and has concrete policy implications. If we are to develop an accurate understanding of the ongoing global urban transition—one of the most profound geodemographic phenomena in human history—we need a conceptually robust and globally consistent definition of urbanity for measurement and comparison. This was a key motivation for the development of a new global classification system for human settlements, known as Degree of urbanisation (DEGURBA), which was formally adopted by the United Nations in 2020. Developed by the European Commission, the DEGURBA system uses gridded population data and simple population size and density criteria for human settlements classification. However, this approach is controversial, with some suggesting demographic criteria alone are insufficient to capture the nature of urban places (Angela et al 2018). We argue that debates about empirical definitions of “urbanity” arise from conceptual confusion about what place attributes are distinctly “urban.” Broadly speaking, existing definitions used for human settlements classification incorporate some combination of geodemographic attributes (such as population size and density), socioeconomic or political attributes (such as labour market structure or political institutions), or the physical characteristics of the built environment (such as presence of public infrastructure). These are related: geodemographic attributes affect the shape and structure of socioeconomic and political phenomena, which often unfold in (or reshape) built environments. Nevertheless, they are conceptually distinct types of attributes. Attending to this distinction allows us to differentiate between cities and urban places. While cities are the physical environments most commonly associated with urbanity, we argue that not all cities are urban, and some urban places do not have the physical or political characteristics traditionally associated with cities. From this perspective, empirical efforts to “delineate urban areas” using data on building footprints are better understood as efforts to identify cities rather than urban places (e.g. Taubenbock et al. 2012, Usui 2019, Montero et al. 2021, Bellefon et al. 2021, Arribas-Bel et al. 2021). In contrast, we argue that urbanity arises from bodies, not bricks. These distinctions are important, because the geodemographic, socioeconomic, and physical attributes of cities and urban places have often been conflated in urban research, undermining the analytic value of the geographic concepts of the city and urbanity. Louis Wirth, in his highly influential essay Urbanism as a Way of Life (1938), asserted that “a city is a relatively large, dense and permanent settlement,” which is a strictly geodemographic definition. But he went on to highlight the “complex of traits which makes up the characteristic mode of life in cities” (pg. 7), such as economic diversification and exchange, superficial human relations, social heterogeneity and segregation, and the emergence of complex and overlapping social group structures. Similarly, in his classic essay The Urban Revolution (1950), V. Gordon Childe proposed a list of 10 criteria for identifying ancient cities, including their demographic size and density, but also the presence of a wide range of non-agricultural specialists and “truly monumental public buildings.” More recently, Scott (2021) has argued that “’urban’ refers 2 both to the city as a concrete entity and to urbanisation as the process by which the city is generated and socially reproduced” (2) and that “the division of labour is fundamental to the genesis and durability of cities” (3). In each case ”the urban” and “the city” are conflated and used with reference to a host of social and physical characteristics. Both terms become conceptual vessels for describing complex cultural, socioeconomic, and political developments—assemblages of versions of modernity. To confuse matters further, scholars have used these fuzzy concepts to describe conditions in non-urban places. Wirth argued that cities foster modernity, but they do not confine it geographically: “the urban mode of life” (a socioeconomic phenomenon) “is not confined to cities” (a type of geographic locale). More recently, Brenner and Schmid (2014) have argued that there is no longer an outside to ‘the urban’ given the planetary-scale consequences of contemporary urban living. Urban places exert cultural, economic, environmental, and political influence far beyond their functional or administrative boundaries (cf. Jacobs, 1984; Sayer, 1984; Brenner and Schmid, 2014; Shin, 2017). While theoretically stimulating, conflating cities and urbanity with modernity or capitalist globalisation undermines the geographic specificity of these concepts and their application to empirical research. If ‘urban places’ have engendered a ‘way of life’ that can be found in non-urban places, this way of life cannot be used to empirically differentiate between urban and non-urban places. Yet most people would agree that such places exist and are different. We attempt to disentangle this conceptual morass by making clear distinctions between the geographic concepts of urbanity and ‘the city’, and between their geographic nature and their socioeconomic or political implications.1 Our focus, then, is on identifying the core defining characteristics of urbanity rather than on defining “the city.” Following key principles of concept formation, we aim to define urban places in a way that is (1) familiar, (2) offers both theoretical and practical utility, (3) clearly differentiates urban places from non-urban places, and (4) does so with reference to observable attributes (Gerring, 1999). Like Scott (Scott, 2017, p. 23), we seek a “disciplined minimal concept,” but in our case we seek to define urbanity rather than “the city.” We focus very narrowly on identifying the attributes that are both necessary and sufficient for a place to be “urban” rather than theorising about social processes that may arise there or reach far beyond a particular location. We intentionally avoid the word ‘urbanism’ and its sociological (and ideological) connotations. Approaching this challenge from a comparative international perspective, we recognise that there are many ways of life to be found in urban places—and many types of urban places (Parnell and Pieterse 2015). Consequently, a singular grand theory of “the urban” is problematic. Yet there is scope for a universal definition of urban as an adjective for the purposes of geographic classification, statistical measurement, and comparative research. Indeed, clear criteria for classifying human settlements can help support an appreciation and systematic analysis of this diversity. Building on a series of thought experiments motivated by diverse existing approaches to human settlement classification, we argue that urbanity is experienced wherever there is a large concentration of human beings in a geographic place for a sustained period of time. In practical terms this means that urbanity can be measured with reference to the demographic characteristics of places alone. Urban places are where the people are. This conceptualisation echoes Wirth’s geodemographic definition, but makes space for ephemerality: the urbanity of a place can wax and wane, on timescales shaped by social or physical factors that mediate urban ways of living. Our emphasis on physical interaction and co-presence reveals the shortcomings of those who see the city purely as a networked place of social interaction. We can see clearly now, in the 21st century, that social interaction and networks can be abstracted from places and exist in wholly disembodied forms. Physical copresence often yields distinct forms of social contact.2 3 Our conceptual approach resonates with the DEGURBA approach adopted by the UN Statistical Commission in 2020 to facilitate international comparison. However, while we make the case for the primacy of demographic factors in the definition of urbanity, we suggest that current approaches relying on strict population density thresholds are problematic. It is now technically possible to measure urbanity along a continuous spectrum and describe settlements and settlement systems in more nuanced ways than traditional measures such as rural/urban ratios or primacy measures. Improvements in both measurement and computation make far more sophisticated analysis possible (Hugo, Champion and Lattes, 2003; Schroeder and Pacas, 2021). To illustrate how we might better align our phenomenological understanding of urbanity with empirical measurement, we introduce a novel approach that tackles the perennial boundary problem in geographic research. Rather than classify places based on population density—which requires a boundary to be imposed “from above” – we introduce a “population proximity index” that requires no such imposition. Instead, it is a continuous measure of population concentration in a place that reflects the “catchment” area required to reach a minimum population threshold at any point on the planet. Places with small catchments have high degrees of urbanity; places with large catchments have low levels of urbanity. Broadly speaking, we call this concept “population proximity.” Some scholars resist discrete geographic classification of places, emphasising instead the socioeconomic, political, and ecological processes that link human settlements to each other and to uninhabited landscapes (e.g. Brenner and Schmid 2014). We contend that there are urban places and rural places, and these are observably different types of places—just as valleys and mountains are distinct features of the landscape, even if the precise dividing line between the two is subject to debate. The fates of hamlets and villages and towns and cities may be deeply intertwined, and the socioeconomic and ecological consequences of urban settlements may be planetary in nature, but these observations do not justify a rejection of human settlement classification altogether. If anything, these observations bolster the argument for more sophisticated and relevant approaches to human settlements analysis built on firm conceptual foundations. Decoupling geodemographics from social processes Wirth’s essay is a useful foil for thinking through the problems of eliding the classification of places with observations about the social phenomena within them. In his own words, Wirth’s goal was to “discover the forms of social action and organization that typically emerge in relatively permanent, compact settlements of large numbers of heterogeneous individuals” (pg. 9). It was an explicit attempt to generalise the social consequences of urban living. Wirth therefore understood urbanization to be the “development and extension” of the “complex of traits which makes up the characteristic mode of life in cities” to other types of settlements— not merely growth in the share of a region’s population living in “cities in the physical and demographic sense” (pg. 7). In this understanding, Wirth’s theory of urbanism is an historical theory of broad social change, building on the work of scholars like Durkheim and Weber. Wirth argued that classifying places based upon arbitrary population thresholds obscured analysis of important social processes that are not geographically confined.3 This is sensible: an arbitrary population threshold is unlikely to define when urban social processes start or stop. Further, there is no inherent logical problem with using Wirth’s theory of urbanism to understand the consequences of large numbers of people living in cities defined in the “physical and demographic sense.” However, it is problematic to use this theory of urbanism to classify places because the social consequences of urbanism can arise in very different circumstances than the ones Wirth analyses. Unfortunately, the power of Wirth’s critique has 4 led many scholars and statisticians to do exactly this, inverting Wirth’s historical theory to classify geographic places with peculiar results. We can see this in the diverse criteria that have been used by national statistical agencies to classify human settlements as either rural or urban. While many countries use demographic criteria consistent with Wirth’s foundational geodemographic definitions (population size and density), many also include criteria that reflect a holistic or historical understanding of urbanization and urbanity. Broadly speaking, these include administrative status (e.g. “Localities proclaimed as urban”), economic characteristics (e.g. labour market structure), or the presence of “urban characteristics” in the form of physical infrastructure or amenities (e.g. paved roads, electricity)(Buettner, 2015). 4 According to an analysis of the United Nations’ World Urbanization Prospects 2011 Revision, 121 countries use a single criterion to classify a settlement as rural or urban, with 64 using administrative criteria, 48 using demographic criteria and nine using ‘urban characteristics’ (ibid). A further 65 countries use two criteria, while a further 19 use three or four criteria (ibid). We consider the appropriateness of each of these classes of criteria in turn. The urban as a space of special administrative status As noted by Wirth, “the city, statistically speaking, is always an administrative concept” (pg.4). Indeed, administrative status is the most common criterion for urban settlement classification among the 121 countries that use a single criterion. This can yield counterintuitive results. For example, the city of St David’s in Wales registered a population of just 1,751 residents in the 2021 census—a population smaller than many settlements without “city” status in the UK. Its status as a city is due to the presence of a cathedral and its historical role as a site of Christian worship. The settlement was stripped of city status in 1886 but it was re-designated as a city in 1994. Yet it is doubtful that visitors to the bucolic St David’s would describe the settlement as urban. Conversely, a place may be identifiably urban but not a city. Clear examples can be found in India. There is a category of settlement referred to as Census Towns, which are settlements that have grown to a minimum population size of 5000 living at a minimum population density of 400 people per square kilometre and with at least 75 per cent or more of the male population engaged in non-agricultural activities (Mukhopadhyay et al., 2016). These are classified as ‘urban’ for census purposes but retain rural administrative status, which has important implications for resource allocation . More awkwardly, the ‘village’ of Rahri, which is classified as rural, registered a population of 36,569 in the 2011 Indian census—much larger than most ‘urban’ census towns. These cases clearly illustrate the ontological distinction between urbanity, which we argue has a universal character, and “city-ness,” which is highly contextual. Simply put, a place need not be urban to be a city, and an identifiably urban place may not necessarily be a city. Administrative status is too arbitrary and politically contingent to rely upon for a durable definition of urbanity. The urban as a labour market Many statistical and social scientific definitions of urbanity refer to the characteristics of local labour markets. As noted above, settlements in India must have at least 75 per cent of the male population working in non-agricultural jobs. Over 30 countries employ similar thresholds as part of a multi-criteria classification system (Buettner 2015) as do many scholars (Wirth 1938; Shin 2017; Angel et al. 2018; Scott 2021). This is problematic when it leads to a classification system that runs counter to the lived experience of place. 5 For example, workplace data is used by the OECD to build “functional urban areas,” linking the self-evidently densely populated urban “core” to “peripheral” places where people commute into the city to work (Dijkstra et al., 2013). This is reasonable when delineating the space of urban functions, but these places are not “urban” in a geodemographic sense. These places may be functionally integrated and one (or more) may be urban, but this does not mean that all places within a functional urban region are urban themselves. Indeed, it is precisely their less-than-urban character (i.e. suburban or exurban) that some find attractive. This confusion propagates when “function” is not clearly specified; the 2020 US Census’s revised definition of “urban” now merges places together where commuting is “50 percent in at least one direction,” (USCB 2022, p. 16712), bringing relatively small (2,500-person) commuter villages into the urban fold. This is no longer a “functional” area surrounding an urban core, but rather a definition of the “urban” place itself. Technological and social changes that have made such “commuter villages” possible will continue to heighten the contradictions between urban places and the economic characteristics of functional urban areas. Urban neighbourhoods and commuter villages feel different even when their labour market characteristics are very similar—a difference that we argue arises primarily from distinct geodemographic contexts. In these cases, we find the roots of urban economies reaching deep into rural landscapes—a phenomenon Wirth and many others before and since have highlighted. But it does not follow that these commuter communities are therefore urban unless we entirely discard demographic criteria and ignore human perception and experience. Some theorists may be comfortable abstracting urbanity from geodemographics; most people would likely find this unintuitive. Given that we seek a familiar understanding of urbanity, we argue that economic characteristics like employment sector or commuter balance are neither necessary nor sufficient to classify a place as urban or rural. These “functional” characteristics situate places in relation to geographically extensive economic systems, but they do not capture relative urban character of a settlement. The urban as a built environment This brings us to the final (non-demographic) class of criteria commonly used to classify places: the presence of physical infrastructure and amenities, such as piped water, paved roads and medical or educational facilities. Forty-four countries use such “urban characteristics” as part of their settlement classification system (Buettner 2015). In popular culture, urban places are frequently represented as dense built environments with large, multi-story towers and congested streets—places like New York, London, Tokyo, Shanghai, Lagos, Sao Paulo or Mumbai. We imagine buildings and infrastructure forming a larger-than-human built environment with a sense of monumentality. And again, these built environments often host urban places. But there are abundant examples of places with highly developed infrastructure that would not be classified as urban, as well as urban places that lack such infrastructure. Consider the ancient city of Tikal in Guatemala, which reached a population of 62,000 around AD 700 (Wright, 2012). Today the temples and squares and water infrastructure are a tourist attraction. But is it urban? With no inhabitants, Tikal is a physical artefact—perhaps a city but surely not an urban place. If one accepts this line of reasoning, the same could be said of socalled “ghost cities” in China (Sorace and Hurst, 2016) or any lightly-populated “urban laboratory” like Paolo Soleri’s Arcosanti (Evans et al. 2016). These are human modified landscapes, but they are essentially devoid of the complex and intensive human activity we recognize as urban. The presence of buildings and infrastructure to support human habitation may render them suitable for classification as “cities” but they have none of the social potential of a crowded place. And, while we suggest ephemeral urbanity to explain how these places 6 might easily become urban, their existence as bundles of infrastructure does not make them urban outright. In contrast, the sprawling “slums” or informal settlements of cities in many low- and middleincome countries often lack permanent buildings and the kinds of water and sanitation infrastructure that are associated with ancient and contemporary cities alike. Yet they have the ‘buzz’ that comes from the concentration of human interactions that arise from population density—the type of buzz characteristic of urban places (Storper and Venables, 2004). Similarly, refugee camps are often characterised by high population densities, impermanent structures, and the absence of infrastructure. Yet the concentration of people in large camps gives rise to many of the very phenomena that social scientists and statisticians associate with urban living, such as socioeconomic specialisation and exchange and the emergence of land and rental markets. For example, the Zaatari refugee camp in Jordan, which was established in 2012 to support refugees from the Syrian war, has become home to roughly 80,000 people (Dalal, 2015). It has a lively souk fuelled by both humanitarian aid and an informal labour market and “all kinds of business one expects to see in a city” (ibid, 271). The camp has evolved into something that feels distinctly like a de facto city despite the absence of major infrastructure or permanent structures. Urbanity beyond the legal, economic, and (infra)structural city People make places urban. Sometimes these places have monumental buildings and infrastructure ferrying workers to jobs regulated by special legal or administrative arrangements. But there are many urban places in the world that have none of these things yet are experienced as urban places by residents and visitors alike. Decoupling the geodemographic characteristics of a place from its political, socioeconomic, or technological characteristics throws into relief the essential characteristic of urbanity. The constant that unites New York City, a Mumbai slum and a Jordanian refugee camp is the geographic concentration of people in space. In all these places the geographic concentration of people— or mass corporeal copresence—has common consequences and stimulates diverse emergent social phenomena. Corporeality, strangers, and permanence Wirth highlighted the “coincidence of close physical contact and distant social relations” (1) as a defining aspect of urban living. Similarly, Iris Marion Young defined the essence of “city life” as “the being together of strangers” (Young 1990). Here we find the truly distinguishing attributes of urbanity. It is the sight and sound of strangers all around; the possibility of a physical encounter with an “other”, unfamiliar person. It is the geographic concentration of human bodies needing food and water, and excreting waste, that creates an urban context. It is a geographic concentration of people—not buildings—that make places urban. Disembodied forms of human congregation (e.g. on social media platforms) do not replicate physical co-presence. Non-corporeal copresence and contact is distinctive by virtue of its technological mediation. In some cases, digital interaction complements physical interaction instead of supplanting it (Craig et al. 2017; Kujath 2011; Sinai and Waldfoegel 2004). But there are systematic differences in the nature of interpersonal contact online versus in person. Research has shown that experiences are amplified when shared when others, and spatial proximity is an important mechanism of joint experience (Boothby et al 2016). Online interactions offer fewer nonverbal cues and greater anonymity between individuals (Lieberman and Schroeder 2020). And there is evidence that groups work differently online versus in person. Research at Microsoft during the COVID-19 pandemic showed that 7 increased online work reduced interaction between groups, resulting in “static and siloed” communications (Yang et al. 2022), while a recent study across five countries found that teams relying on videoconferencing produce fewer creative ideas than ones working in person (Brucks and Levav 2022). At a very fundamental level, disembodied interactions do not have the potential for immediate physical contact—for physical violence, intimacy, or disease transmission. Urban places facilitate social interaction (like the internet), but the internet cannot replicate the effects of physical proximity inherent in urban places. Thus, corporeal copresence and its environmental effects is distinct from copresence in general and is necessary for a uniquely urban experience. It isn’t just the enhanced possibility of contact with strangers, but the enhanced possibility of physical contact that makes urban places distinct from non-urban places. The presence of strangers is also a key feature of urbanity. Wherever people congregate in sufficient numbers there will be strangers. This fact is physically determined: it is not possible to maintain meaningful interpersonal relationships with an infinite number of people because meaningful relationships require time and attention to maintain. Empirical research suggests that people generally maintain between 100-300 “meaningful” relationships (Dunbar, 1992; Gonçalves, Perra and Vespignani, 2011). As a result, from the perspective of any individual, the share of strangers in a group increases with group size. For example, if we take the most common estimate of the number of people any individual can maintain meaningful relationships with (150), the share of strangers for any individual in a community of 1,500 is 90%. If the relationship threshold is doubled to 300, a “stranger share” of 90% is achieved in a group of 3000 individuals. Figure 1 further illustrates of how population size relates to the share of strangers in a place. In small populations, each additional member makes a sizeable contribution to the stranger share (over any given threshold). But the effect rapidly diminishes with scale: in large populations each additional stranger makes a negligible contribution. In a small town, every new resident may be noticed; in London, New York or Lagos new arrivals are unseen by the vast majority of residents. This thought experiment helps to provide an answer to the question “how large is large?” that follows naturally from Wirth’s geodemographic definition of an urban settlement as a “large, dense and permanent settlement.” If we accept that physical copresence with strangers is a defining characteristic of urbanity, we can calibrate the intensity of urbanity in relation to the share of strangers in one’s immediate vicinity. It is notable that minimum population thresholds have long been used by statistical agencies in classifying urban places (Johnston, 1980, p.13, see also Truesdell (1949) for the US from 1874-1949) and now range between 200 and 50,000 (Bandyopadhyay and Green 2018; Buettner 2015). The UN’s harmonized classification system (DEGURBA) identifies high density settlements of 50,000 people or more as urban centres. Even with a generous “relationship threshold” assumption, over 99% of people in such a settlement will be strangers to one another. Figure 1 | The share of strangers as a function of group size 8 It is not our contention that there is a “natural” threshold at which a group grows into an urban community, but rather that group size is a critical variable because it conditions social context in a universal fashion (i.e. through the relative presence of strangers).5 This generates demand for social institutions to manage relations between strangers (pace Wirth), but there is no automatic solution to this challenge at any density of habitation. What of permanence? Must a congregation of human beings be permanent to qualify as urban? There is something intuitive about this idea, but it quickly breaks down when we are asked to define permanence. For example, Black Rock City is “a temporary metropolis” that springs up once a year in the Nevada Desert as part of the Burning Man festival (burningman.org/allevents/ - accessed 03 July 2023). Putting aside whether there is enough infrastructure for it to be considered a city or “metropolis,” it is certainly a “notable urban and geographical experiment” (Rohrmeier and Starrs, 2014; 169). For one week every year, tens of thousands of people congregate to live together in a temporary settlement with a distinctly urban character (see Figure 2). It has even been held up as a potential model for managing rapid urban growth in low- or middle-income countries (Keil, 2021). We would argue that Black Rock City is an urban place when it exists, but that the quality of urbanity evaporates from the site when the revellers return home. It is an exemplar of ephemeral urbanity.6 Figure 2 | Black Rock City 9 Source: REUTERS/Jim Urquhart Measuring urbanity by inverting the boundary question Spiro Kostof argued that cities are “where a certain energized crowding of people takes place. This has nothing to do with absolute size or with absolute numbers; it has to do with settlement density” (1991, 37). As he observes, for most of our approximately 6000-year urban history very few towns or cities had populations greater than 10,000. Their identifiable urbanity was essentially a function of population density (ibid). But how dense is dense? Because density is a relationship between population sizes and geographical extents, any urban definition based on density requires us to deal with the fact that density is an “intensive” property, dependent on (a) how many people are (b) within a boundary of measurement. Taking each in turn, there is no obvious or natural threshold population size at which a human settlement or region becomes urban. A population of 50,000 living in an area 50,000 km2 is not intuitively urban; the same population living in 10 km2 likely is. Thus, it is often the measurement boundary (and not the size of a given population) that determines the “urbanity” of that population given that this ultimately influences measured density (pace Kostof). Traditionally, statisticians have used administrative or enumeration boundaries to make such calculations. But there is no global consensus on how to draw urban boundaries; the choice is generally pragmatic, driven by the application at hand and available data (Storper and Scott, 2016). For the purposes of global comparison, the DEGURBA classification system uses a simple and transparent approach to deal with the non-comparability of statistical definitions across countries. Drawing on gridded population data, geographic regions are constructed and classified as “urban centres” if there are one or more contiguous cells that contain a minimum of 50,000 people living at a density of 1500 inhabitants per square kilometre; a region is classified as an “urban cluster” if it contains a minimum of 5000 people living at a density of 300 inhabitants per square kilometre. Collections of regular 1km2 grid cells provide the necessary boundaries to calculate density: the outer edges of contiguous cells form the “natural” boundary of an urban centre or cluster, irrespective of local administrative geographies. However, this approach to imposing de facto urban boundaries is both arbitrary and rigid. As Angel et al. (2018) observe, the DEGURBA classification thresholds do not 10 necessarily match the lived realities of both high-density rural communities or those living on the fringes of cities. A more nuanced hierarchy of thresholds could be used to improve representations of the different ways of living that are condensed into “urban” classifications7, but the the resulting boundaries of the urban places themselves will always be an artificial imposition only loosely tethered to human experience.8 The arbitrariness of these boundary problems and their effect on aggregate estimates is wellknown in geographical research (Openshaw & Taylor, 1979)—the “mythical beast” of the urban boundary is “a little difficult to capture” (Hall, 2007), even with new data and stronger methods. This is because the boundary issue is a theoretical, not empirical issue: any boundary is not only fuzzy, it is local: the boundaries for a city are contingent on a wide variety of local physical factors, and the odd and uneven surface of our Earth can yield an extremely large diversity of unusual corner cases where places may feel urban, but do not meet arbitrary inclusion/exclusion thresholds at the urban edge. Inverting the boundary problem There is, however, an alternative way to use the concept of density to measure urbanity that does not require the urban geography of a place to be imposed from above. People do not experience districts or enumeration blocks or grid cells in a visceral way; we feel the presence of other people through sight, sound, smell, and touch. We can feel the energized crowding or the absence of it. Indeed, using data from the American Housing Survey, a study by the US Census Bureau found that the single best predictor of whether a respondent identified their area as rural, suburban, or urban was the population density of their area (Bucholtz, Molfino and Kolko, 2020). Thus, the perception of urbanity is intrinsically linked to individuals’ experiences of population concentration. Our suggestion is therefore to invert the boundary problem, borrowing from sociological and community psychology work on ‘egohoods’ (Hipp and Boessen, 2013). Instead of focusing on density, we can instead measure urbanity with an alternative indicator of population proximity: the distance (or time) one would have to travel until some specific threshold (say, 99%) of the people encountered in that generalised activity space (Spielman and Singleton, 2022) are strangers. Practically speaking, we can construct a “population proximity index” (PPI) as a proxy for urbanity as d, the distance you would have to travel (in any direction) from a given location to reach some catchment population, k.9 This distance is effectively the urbanity value of a location. If the area required to capture some minimal population is very large, that location is not very urban; if it is a small area, it is a very urban location. Instead of measuring population density within a defined set of boundaries, we are measuring the boundary required to meet a certain population threshold, thereby “inverting” the boundary problem. In other words, the geography of urbanity emerges “organically” from this approach, offering a more nuanced representation of urbanity as experienced. This can be seen in Figure 3, which visualises differences in population density and population proximity for three case study cities: Nairobi, Kenya; Sao Paulo, Brazil; Sydney, Australia. 11 Figure 3 | Comparing population density and population proximity in three cities. Population data are from WorldPop 2020 (constrained). Population Proximity was calculated with a medium population threshold of 3000 people. In the disagreement pane, red indicates strong disagreement, while yellow indicates weak disagreement. The differences between standard gridded population (left column) and population proximity are not dramatic, but significant at the margins—particular in per-urban areas and urban fringes. This is illustrated by the central column of panels, which shows disagreements between the two for each city (red is strong disagreement; yellow is weak). The greatest differences occur around the urban fringes. This makes sense: in areas with high population counts in cells, the population proximity index will be small (i.e. highly urban). In effect, the PPI is censored at the lowest values (i.e. highest degrees of urbanity). Consequently, it is areas 12 of moderate and variable population density that see the greatest disagreements. Among these cases, Sydney is the starkest example. The extensive suburbs to the north and southwest of the city center appear substantially less urban with the PPI than a measure of population density alone might suggest. This is in part due to the way in which the PPI embeds information about geodemographic context. In a standard population grid, every cell contains an independent measure of population, with no reference to ‘neighbourhood’ context. By contrast, the PPI reflects demographic information about the wider neighbourhood, offering a more nuanced measure of context at any given location.10 In this sense, our approach resonates with the settlement classification system recently developed in the US by Schroeder and Pacas (2021), which uses population size and concentration as key dimensions for measuring urbanity across a spectrum. In this system, one can find a distinctly rural-like (i.e. low density) settlement within a wider (urban) metropolitan region. However, our proposed approach does not require regionalised data and is therefore more globally applicable. Figure 4 illustrates the effects of changing the population threshold of the PPI, using the case of Sydney and thresholds of 1,500, 3,000 and 5,000. There is little change in the urban core, but as the threshold increases peripheral areas appear increasingly less urban, with a notable loss of urban “hotspots” in the outer areas. Increasing the PPI threshold throws into sharper relief the most “urban” areas of a large urban region. Figure 4 | Population proximity over three thresholds in Sydney, Australia. Finally, Figure 5 shows how the relationship between population density and PPI changes with settlement size (measured in terms of population size). Here, the cell-level correlation between density and PPI has been calculated for 49 urban areas. The correlation coefficients are then plotted against city size (log of city population). This “correlation of correlations” shows us that (a) the cell-level correlation is consistently negative, as we would expect (smaller PPI = greater urbanity), and (b) that the strength of the correlation increases with city size. This is consistent with the evidence in Figures 3 and 4, which show the areas of greatest disagreement and sensitivity to population threshold are those of moderate population density. 13 Figure 5 | Cell-level correlation between PPI & density in 49 cities. The Figure shows the rank correlation between population proximity and density at cell-level in a global 49 city sample across three population thresholds. Note the cell-level correlation is negative because dense places tend to have small PPI values (i.e. exhibit greater urbanity) and this negative correlation increases with city size. This approach to measuring urbanity has some practical and conceptual advantages over population density. Practically, it offers a high-resolution “organic” representation of urbanity within any given city or region, at any moment in time, or even at different times of day. As argued above, urbanity-as-experienced ebbs and flows with the demographic tides. Conceptually, the population proximity index re-centers the physical co-presence of strangers as the animating concept for urbanity and makes explicit what density makes implicit: the geography of urban experiences. Measures of population density must treat the geography of populations as a ‘known’ fixed container in which it makes sense to count people—invariably enmeshing density with intractable geographic questions about scale and the modifiable areal unit. Population proximity resolves this issue by treating scale (i.e. distance to the catchment population) as an outcome, rather than as an assumption required implicitly by the measure. The benefit of his approach is scalability and comparability, and further metrics could be developed from it to provide comparative statistics, such as average population proximity for whole cities or PPI variability across urban regions. But this approach doesn’t capture the nuance of actual human activity spaces. As Stülpnagel et al. (2019) demonstrate, simple circles rarely reflect the true geography of individual activity and hence the social context they may experience. Alessandretti et al (2020) show that individuals have multiple scales of mobility, which can be significantly impacted by gender, transport infrastructure and settlement type. People experience similar places differently, which can substantially alter their experience of urbanity—their regular exposure to large numbers of strangers. Our simple cartesian approach glosses over much more complex social dynamics. Nevertheless, it offers a pragmatic approach to evaluating urbanity in a relatively nuanced way across spatial scales with minimal data, and can easily incorporate richer approaches to modelling activity spaces when data is available. Conclusion The question posed in the title of this paper signals our intentional treatment of urban as a as an adjective. Rather than seek to define the urban or the urban condition (nouns) we have made a case for defining urban as a geodemographic attribute—one that can be found in a wide range of places where large numbers of people congregate for sustained periods of time. This perspective sets our case apart from those such as Wirth (1938) and Scott (2021). In an effort to define the urban way of life or the city, these scholars tend to over-specify the 14 consequences of urbanity, while many others make unnecessary assumptions about the causes of urbanity or conditions under which it arises. An urban place only emerges where there is sufficient surplus to support a dense concentration of people (Childe 1950), but this surplus can be from local production, trade, humanitarian aid or some other mechanism of acquisition. And while living in close proximity creates similar challenges wherever it occurs,11 there are many ways these challenges can be and are addressed. Diverse urbanisms emerge from the shared context of urbanity. There is nothing deterministic about the social and economic consequences of human congregation. To quote (Merrifield, 2013): The urban is nothing in itself, nothing outside dynamic social relations, a coming together of people. As long as human beings can come together, as long as separation can be resisted, there is always a possibility of encounters between people. (p. 916) It is the possibility of physical encounter—and frequent encounter with strangers in particular—that gives a place an urban character. There is no specific form or function to urbanity; it is merely a geodemographic context ripe with social potential. This means that urbanity is, at its core, ephemeral: it emerges as well as dissipates. This is experienced through the daily rhythm of concentration and de-concentration in cities with large commuter belts. As downtowns swell in population during working hours, they change in character—they become more urban. As they empty at night, they become less urban. This has significant implications for planners and policy makers who need to account for de facto urbanity – how population density is actually experienced in places – rather than residential density (Boeing 2018)and highlights the importance of recent innovations in estimating and analysing daytime populations (ibid; Moro et al. 2021; Xu, 2021). Residence remains an important aspect of urbanity as it is experienced but should be treated as one aspect among many. Similarly, we have argued that “permanence” should not be considered necessary for a place to be considered urban. Just as urbanity ebbs and flows in large cities, it can emerge and dissipate in places outside of cities, with festivals and refugee camps as prime examples. Our proposed approach to measuring of urbanity, inspired by the corporeal essence of urban places, inverts the traditional approach to geodemographic place classification by doing away with pre-defined boundaries from administrative geographies or grid cells. Instead, variable population catchment areas are used to encode information about the geodemographic contexts of locations, and this can be done at multiple spatial and temporal scales. This globally applicable approach offers a nuanced way of representing urbanity as experienced by people. 15 References Angel, S., Lamson-Hall, P., Guerra, B., Liu, Y., Galarza, N., & Blei, A. M. (2018). Our Not-SoUrban World. Working Paper No. 42, The Marron Institute of Urban Management, NYU. http://www.thisisplace.org/i/?id=0150beca-e3f5-47e0-bc74-9ccc5ef1db8a Batty, M. (2013) The n ew science of cities. Cambridge, MA: MIT Press. Bandyopadhyay, S., & Green, E. (2018). Urbanization and mortality decline. 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In some cases, co-presence is not accompanied by co-present social contact, generating a peculiar form of urbanity. For example, COVID-19 stay-at-home orders heavily constrained physical copresence in urban centres. We would argue that this does not mean we became less “urban” during COVID-19. Instead, we collectively experienced a peculiar form of urbanity characterised by proximity without contact—similar in many ways to modern prison complexes. Hence, it is important to recognize that it is the demographics, not permanence or socioeconomic relations, that is the defining feature of an “urban” place. 2 A similar critique underpins the works of Lefebvre, Harvey and Castells, which probe the entanglements of urban places with late capitalism. As with Wirth, this approach has a deeply spatial dimension, but obscures the concrete distinctions between different types of places. 3 4 Why different countries have adopted different definitions is a fascinating and understudied question. Research to understand the classification logics of national statistical agencies would likely reveal diverse origins and logics. In some cases, urban criteria might be legacies of colonial occupation designed to reflect or facilitate ‘development’ activities. Others may reflect logics of political patronage and control or administrative necessity. Different cultures may define family, kin and strangers in different ways. Yet in all cases, the maintenance of meaningful relationships requires time, which is limited. Therefore there is always an upper bound on the number of meaningful relationships that can be maintained by every individual. 5 One can debate precisely how long a large group of people must congregate to classify a location as urban, but there is no conceptual need to restrict our understanding and definition of urbanity to places of permanent settlement. 6 7 There is a Level 2 Degree of Urbanization classification system that has seven categories, including urban centres, dense urban clusters, semi-dense urban clusters, suburban cells, rural clusters, low-density rural cells and very low-density rural cells. This can be used to classify collections of cells into cities, towns, suburbs and villages. See (Dijkstra et al. 2021) Indeed, the long chain of post-processing steps (including smoothing and gap-filling) used in GHS-SMOD to construct boundaries from the process described here indicates how the arbitrary population size and cell-level density decisions do not capture the local aspects of the geographic structure of urban places. 8 Absent any specific information on the social networks in communities, the population of strangers grows as a function of k as shown in Figure 1. 9 The use of alternative experiential “distances,” such as travel time or perceived distance, also can be easily accommodated and the specific distance metric used to define urbanity is not intrinsic to this style of local contextual measure of urbanity. 10 Securing the basic provisions for human life and mitigating social conflicts are generally central. 11 20