Population density is a phenomenon related to the way in which the population is distributed on the land surface. The population is unevenly distributed across the land, reaching high concentrations in large urban areas while large areas of land remain uninhabited, because people naturally tend to concentrate in areas with desirable conditions, like those of urban areas. The differences in population distribution are high both across countries and between regions of the same country , and thus the concept of density of the built environment has relative connotations when trying to compare indices of different areas.
Starting from this fact, it becomes clear that the exact definition of the term high-density architecture can be difficult. The term “density” is itself a complex concept, involving some diversity in terms, and defining it is important before beginning a discussion on density in relation to architecture.
The Definition of Density
High density refers either to physical density, namely the density of a large number of people or the built density related to the land surface, either to subjectively perceived density in relation to the environment and to other participants into a certain space. Physical density is a numeric measure of the concentration of individuals or of physical structures in a certain geographic unit, being a spatial objective indicator, quantitative and neutral. In practice, it only makes sense when linked with a specific reference scale. On the other hand, perceived density represents the individual perception of an estimated number of people present in a given area, of the vacant space and its organization. The character of space itself is important for the perception of density, but the interaction between individuals and environment as a whole is more important. Also the individual cognitive attributes and socio-cultural norms are factors that contribute to this interaction. Perceived density doesn’t only refer to the relative relations between individual and space, but also to the relations between individuals located in the same space.
The definition of density in relation to built environment can have different meanings or different quantification according to the method of analysis and spatial indices taken into account. There is no universal standard formula, only some of the formulas are being used more as opposed to others. Usually, in architecture and town planning, two categories of measurement are used for physical density: population density (Fig. 1) and building density (Fig. 2). Population density is defined as the number of individuals or households per given area, and the building density is defined as the ratio of building structures related to plot surface. All those measures are used in urban planning policies.
Building density has a complex relation to urban morphology, playing an important role in determining the urban form. Different combinations between the plot ratio and site coverage will manifest into a variety of different built forms, and urban development of the same density can take very different urban forms .
The differences between the surface distribution of population density and that of building density are visible when comparing data of almost every urban tissue. The following example illustrates the difference between the two densities in the case of Timișoara, by graphically expressing the data for the year 2011.
Figure 3. The difference between the distribution of densities of people and buildings in Timișoara.
Source: illustration from the board Development Analysis I 13_Densities, General Urban Plan (PUG) of Timișoara, drawn by Vitamin Architects based on data provided by the City Hall of Timișoara and used with the consent of Vitamin Architects .
In reality, especially when the geographic reference unit is one at large scale, the distribution pattern of people or buildings can vary significantly. To solve the spatial variation of density, different measures have been introduced, such as density gradient or density profile. The density gradient is a composite measure of density and is defined as the rate in which density decreases in relation to the distance towards a reference location, usually calculated according to concentric circles (Fig. 4). By comparing the patterns of density over a period of time, the process of spatial evolution of the city can be depicted, showing either decentralization with a drop of population density in the center and increased density towards the outskirts, either centralization with a growth in population density in both center and outskirts, and the expansion of the borders (Fig. 5) .
Figure 4. Population density gradient for an abstract case.
Figure 5. Density gradients over time: (a) progressive decentralization; (b) centralization.
Source: Illustrations adapted and redrawn after Vicky Cheng (2009, p. 8).
From the perspective of rapid urbanization, the relationship between urban density and built form has always been an important focus of research, determining massive mathematical and geometrical investigation upon the spatial benefits of buildings with multiple levels. The presented definitions and measurements of density are the most relevant ones in relation with the subject of density from an architectural point of view, but they are not the only ones.
 Freedman, JL. Crowding and Behavior. San Francisco: W. H. Freeman and Company, 1975.
 Cheng V. Understanding Density and High Density. In: Ng, Edward (Ed.). Designing High-Density Cities For Social & Environmental Sustainability. London: Routledge; pp. 3-17, 2009.
 Public document downloaded from the City Hall of Timisoara web-site: http://www.primariatm.ro/uploads/files/PUG/URBANISM/parte%20desenata/03_ANALIZA%20DEZVOLTARE/13_ad_DENSITATI.pdf.