This section summaries definitions and classifications of space an portions of space presented in eight fields or disciplines, notably (i) navigation; (ii) positioning and localization; (iii) building micro-climate and thermal comfort; (iv) landscape, urban planning and design; (v) urban heat island; (vi) interior design and planning; (vii) transportation and (viii) intelligent space. All of them a closely related to urban applications.
2.1. Spaces in Navigation
Spaces in navigation have been classified as being located either indoors or outdoors. No strict definition for outdoor are found in the literature, except that people regard objects in open air as outdoor (assuming unbounded from above), such as streets, pavements, squares, rivers. In contrast, research dealing with indoor environments is more explicit with partitioning spaces into cells and attempts to provide formal definitions. For instance, References [3
] defined the indoor space as a place bounded by physical boundaries (e.g., walls, floor, doors) and intended to support human activities [66
]. Indoor space is often referred as to physically enclosed space. Underground enclosures, which offer platforms for human activities, are also referred to as indoor space [32
]. Reference [67
] presented an indoor space definition with the analogy to the human body. The body is a container bordered by the skin. Similarly, the wall, floor, roof, fence can be seen as the “skin". According to these definitions, semi-enclosed spaces such as a veranda or an inner court would be outdoor spaces. Indoor space in Reference [31
] refers to building environment rather than natural. According to these authors, underground cavities (caves, natural passages) would be classified as to outdoor.
Only a few papers discuss semi-enclosed spaces that cannot be clearly attributed to indoor or outdoor. Examples are covered footbridges, sheds, balconies or partially roofed courtyards. Reference [17
] defined these spaces as transitional spaces. Reference [68
] suggested introducing quasi-indoors and quasi-outdoors but no strict definition is provided. For example, a courtyard surrounded by buildings is quasi-indoors, because the yard is an inseparable part of the surrounding building and should be included as part of the building’s indoor map to ensure continuity in navigation.
The research presented above is based on examples or human notions to clarify space. Reference [19
] made the first attempt to develop a framework for formally distinguishing between spaces, depending on their construction characteristics and methodology to define cells. The authors defined space as “hollow (unoccupied)”, constrained by some partial (or complete) physical boundaries, where human activities can take place. In follow-up research [20
], the authors introduced three construct elements (Top
), which allows them to define strict rules for the classification. To quantify the space definitions, the authors proposed three more notions namely Closure
, Physical boundary
, Virtual boundary
. Then spaces can be formally defined using side closure (
) and top closure (
). Three thresholds (
and two (
allow to quantify the closure. Applying this mechanism, the authors classified the space for navigation as indoor (completely bounded), outdoor (unbounded) or semi-bounded (side-/top-bounded). Figure 2
illustrates the cases. Note, according to this framework, the courtyard from the above example [68
] would be classified as semi-outdoor, because it is side-bounded. Although compatible the concepts still can lead to contradicting classifications if no strict definitions are provided.
2.2. Spaces in Positioning and Localization
The term “space” is utilised in positioning and localization to partition space and allow to distinguish between indoor, outdoor and semi-bounded cells from the sensor (e.g., GNSS ) reception perspective.
A typical example is the space classifications provided by Reference [15
]. The authors used lightweight sensing service to analyse indoor/outdoor environments with respect to positioning options for mobile applications. They partitioned and classified space into indoor, semi-outdoor and outdoor (Figure 3
a). The outside of a building is defined as outdoor, while inside is indoor. Close to or semi-open building space is considered as semi-outdoor. Reference [18
] also provided a space classification according to the reception of the GPS signal as follows: open outdoors, semi-outdoors, light indoors and deep indoors. As shown in Figure 3
b, areas which have an open sky condition (i.e., unbounded from above) and provide enough satellites for positioning are open outdoors. Areas such as urban canyon or wooded area, are classified as semi-outdoors. Light indoor is similar to semi-outdoor but inside the building. These are areas around windows, which still have some satellites availability. Deep indoors refer to places without any satellite coverage. Reference [69
] developed a visual aid to the visually impaired person, in which they classified the navigation environments into three types, indoor, outdoor and semi-indoor. The criterion for this space partitioning is the type of signal received by the users, for instance, outdoor is the space which only can receive GPS signal, indoor is where ZigBee signal (communication protocol) is dominant and semi-indoor is the portion of space where both signals can be detected. Reference [70
] defined indoor and semi-indoor spaces as GPS-denied environments.
Comparing the space definitions in research on navigation and localization, we can conclude that space is regarded in a similar manner, that is, portions of spaces are defined with respect to the built structures.
2.3. Spaces in Building Micro-climate and Thermal Comfort
Research in building micro-climate and thermal comfort largely utilises the concept of space and much emphasis is given on the transition between indoor and outdoor. Such sections of space are referred differently (e.g., semi-enclosed, semi-indoor, semi-outdoor, transition) with respect to their function or purpose. Without explicitly aiming at completely enclosed cells, the authors discuss possible bounding constructions. Many architectural design papers operate with such types of spaces to improve residential and building micro-climate and reduce cooling or heating energy requirements [4
]. Typically, three main spaces are identified: semi-indoor, semi-outdoor and connection/transition/buffer and an attempt is made to provide definitions(Table 2
As can be observed, the definition of semi-indoor space is related to the roof, because it can have a significant influence on the climate of the space. Reference [73
] defined the covered space as a semi-indoor space, which is partially surrounded by indoor spaces. Reference [34
] presented that a semi-indoor space can be created by using a special roof (Vela Roof) as cover to passively avoid uncomfortable (coldest and overheated) conditions, thereby reducing the energy demand significantly. Meanwhile, they defined space that is not entirely enclosed by walls, windows, doors and so forth. as outdoor. The semi-indoor space defined in the research [35
] is a semi-indoor stadium, which has a roof that can be used to close the indoor volume to a relatively large extent. Thus, spectators and equipment are protected from wind, rain and snow. This space still has direct openings to the outside. In contrast, Reference [36
] took the stadium as a semi-outdoor space in the assessment research of thermal comfort. In the research of condensation in residential buildings [4
], semi-indoor spaces are created by installing external windows to balconies in Korean apartment units, which are used as environmental buffer spaces to improve comfort and reduce cooling and heating costs. In research on the impact of improved cook-stoves on indoor air quality in the Bundelkhand region in India, Reference [74
] mentioned the kitchen with 3 walls is semi-indoor compared with outdoor (open-air) kitchen with a makeshift thatched roof for summer. Reference [37
] showed the open space equipped with overhead shed are semi-indoor spaces, which can provide the citizens with sheltered space for public activities. In their research, they argued that illuminating such a huge semi-indoor space only by artificial lighting is against energy saving principle. Thus, they added lighting ducts to enable natural light to travel through the plate of the collector shed and reach the hall on the ground floor. In South Korea, traditional markets have been enclosed as semi-indoor by installing arcades along street edges to improve their physical environment [6
], for instance, to alleviate inconveniences caused by inclement weather. Reference [38
] considered semi-indoor and semi-outdoor spaces are two transitional spaces for thermal comfort. In their examples, a semi-outdoor space is covered by a fabric membrane while semi-indoor space is a studio of 8m high where its roof has 33% of zenital apertures for natural lightning.
In contrast, Reference [82
] took the space, which is partially open towards the outdoor environment as a semi-outdoor space. They even reinforced the concept that outdoor space partly enclosed by a semi-transparent pitched roof (e.g., glass roof) is a semi-outdoor space in a later research [77
]. Reference [83
] defined the semi-outdoor as locations that, “while still being exposed to the outdoor environment in most respects, include human-made structures that moderate the effects of the outdoor conditions.” Examples include roofs acting as radiation shields or walls acting as vertical windbreaks. Reference [33
] defined the semi-outdoors as exterior spaces that are sheltered and attached to the building. The authors also mentioned that an outdoor environment indicates a space without any covering to provide shelter and an indoor space refers to a naturally ventilated room, which is similar to the definitions in the positioning and navigation. The micro-climate of the semi-outdoor (partially enclosed space) usually has a lower effect of wind and is less hot than the outdoor [77
]. Semi-outdoor spaces in Reference [84
] are areas covered by large roofs, leaving a direct connection with the outdoor environment. Museums and cultural centre gardens, university campuses, shopping and leisure areas, hotels and resorts, are a few examples of building environments where covered semi-outdoor spaces are commonly integrated. In Reference [85
], semi-outdoor spaces are defined as the spaces which are partly open in the direction of the outdoor. Three categories are introduced: inside the buildings such as entry atrium; covered spaces; shaded spaces, situated in an outdoor environment entirely. Covered streets are regarded in this category. Furthermore, semi-enclosed space [5
] or semi-open space [79
] are used to name the space that is not enclosed entirely and has some settings including human-made structures that moderate the effects of the outdoor conditions.
Some research offered the definitions only by examples. For instance, bus shelter is a semi-outdoor in Reference [86
], because it can offer shelter in the form of a roof. The semi-outdoor space in Reference [87
] refers to the internal architectural space with maximum exposure to the lobbies, corridors, atrium, courtyards, passages and verandas. In the research of building microclimate and summer thermal comfort in free-running buildings with diverse spaces, Reference [71
] named the space between indoor and outdoor as semi-outdoor space with the example of the space combination of eave section and courtyard. Similarly, References [33
] defined semi-outdoor as “exterior spaces that are sheltered and attached to the building". Balconies are regarded as shaded semi-outdoor spaces to provide the much needed thermal relief to the occupants of flats during the hot seasons [76
2.4. Spaces in Landscape, Urban Planning and Design
The term of space is also employed in landscape, urban planning and design. Reference [44
] defined the vegetation space unit by introducing two concepts: positive (solid) volume and negative (void) volume. From the view of landscape space generation, the space can be defined with the analogy to the building spaces. All plant materials are positive volumes that can be likened to architectural elements such as buildings or wall, while void spaces are enclosed or semi-enclosed volumes enclosed or surrounded by plant materials (left figure of Figure 4
). On another hierarchical landscape cell, Reference [39
] showed landscape spaces are created (enclosed) by the surrounding vegetations (e.g., trees, bushes) and named the space as “exterior room". Similar to the space in building environment, vegetation are architecturally used as structural materials (e.g., floors, ceilings, walls). Based on the type of enclosure, five types of spaces are formed: open, partially open, vertical, canopied and enclosed canopied (right figure of Figure 4
Space in this field also can be interpreted as another important notion: unit
. Defining a unit means that such an entity is distinguished from the background. A landscape is considered a unit if it is possible to delimit borders and assign a distinct function inside either a matrix or a mosaic of distinct landscapes. In such a way, the spatial scale referred to should be large enough to distinguish the units from the surroundings. Units are characterized by autopoietic properties. In fact, units are closed systems with the capacity to self-regulate and auto-maintain themselves. There are several empirical evidences that are in action inside a landscape (when considered as a unit), including feedback and auto-catalytic mechanisms. For instance, the ecotope can be considered as the simplest landscape unit [41
]. The unit can differ by scale. The research on the basic unit types of landscape space can be divided into three types of scales, from small to large, respectively, vegetation space unit [39
], space organization unit [40
] and ecosystem unit [41
]. Units are organized in the hierarchy of scale, that is small-scale space unit sets are contained by large-scale units [41
In urban planning, public space has historically been described as “Public Open Space (POS)”, meaning the streets, parks and recreation areas, plazas and other publicly owned and managed outdoor spaces, as opposed to the private domain of housing and work [42
]. In the process of landscape space design, four steps are needed [40
]. In particular, (i) establish the composition space units; (ii) propose and confirm function demands; (iii) enclose spaces by vegetation characteristics to realise spatial conception; and (iv) realize the spaces by selecting vegetation species.
The boundary enclosure gives the space a sense of domain. A landscape space is considered a unit if it is possible to delimit borders [41
]. Enclosure patterns are used to express features of the unit vertical boundary, on the view of vertical boundary enclosure degree [88
], the number of sides enclosed [90
] and enclosure morphology [89
]. The aggregation, partition and separation of the vertical boundary composition material create an open-closed boundary, which makes external space enclosure not as completely as indoor environment enclosed by building components, such as walls and roofs [20
]. Thus, another two classifications of spaces are proposed based on the boundary status and inner status of the space, including the number of sides [88
] and patterns [89
]. In particular, Reference [88
] used the degree of enclosure to classify space that is the length of the perimeter enclosed by vertical boundaries. Then concluded that different degrees of enclosure result in spaces that vary in character from introverted to extraverted and four types of spaces are raised: four sides-introverted (360 degree), three sides-protected (270 degree), two sides-extraverted (90 degree) and object-focus (0 degree), see the left figure of Figure 5
. Reference [89
] classified the spaces into four types by insulting space enclosure patterns (□, ⊔, L
and ‖) formed by vertical boundaries (right figure of Figure 5
). Indeed, the aggregation, partition and separation of the vertical boundary composition material create an open-closed boundary. The discontinuous feature of landscape boundary makes the enclosure different from architecture spaces. From partly enclosed to fully enclosed, all enclosure types represent for a certain existing status of landscape space.
2.5. Spaces in Urban Heat Island
Green space [91
] or urban green space [92
] are the notions related to space used in research of urban heat island (UHI). The UHI is an urban or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference is usually larger at night than during the day and is most apparent when winds are weak. It is one of the most obvious characteristics of urban climate, reflecting the impact of dense urbanization on urban environment. The formation of the city “heat island" is caused by a large number of artificial structures inside the city changing the thermal properties of the underlying surface and urban activities such as industrial production. Artificial structures such as asphalt and cement concrete have the characteristics of small heat capacity and fast heat absorption [91
Urban green spaces are able to provide multiple ecological benefits, that is, support sustainable production of ecosystem services and foster urban resilience [94
]. For instance, the reduction of rainfall run-off is important for sustainable urban development, particularly for cities experiencing severe flooding and water hazards [93
]. Urban green spaces also ameliorate the climate; filter the air, water and soil of many pollutants; and provide habitats for fauna and flora [92
]; and increases the biodiversity [99
]. Reference [91
] shows that green space can alleviate the heat island effect, due to the ecological benefits generated by plant physiological activities. Plants are able to absorb a large amount of heat and carbon dioxide from the environment and reduce the temperature of the ambient air by transpiration.
Therefore, spaces in UHI usually refer to (urban) vegetated/green spaces, which can be defined as areas partially or completely covered by grass, trees, shrubs and/or other vegetation in the form of parks, golf courses, forests, green roofs, streams, community gardens and yards. These spaces can effectively reduce temperature through shading and evapotranspiration [100
]. Similar to Localisation, space in UHI are often partitioned into cells enclosed by non-physical boundaries derived from sensor measurements or land cover characteristics (i.e., greenery).
2.6. Interior Design and Planning
Space is one of the most important elements of interior design since it acts as a foundation where the entire interior design and planning is implemented. Interior design is the art and science of enhancing the interior space of a building to achieve a healthier and more aesthetically pleasing environment for the people using the space. More straightforward, it is the practice of space planning and designing interior spaces in homes and buildings, in which, creating floor plans, furniture layouts and designing the look and feel of a space are involved. Interior design also includes the specification of furniture, fixtures and finishes, and coordinating their installation. Therefore, it is a multifaceted profession that includes conceptual development, space planning, site inspections, programming, research, communicating with the stakeholders of a project, construction management and execution of the design. Interior designer implies that there is more of an emphasis on planning, functional design and the effective use of space, such as furnishings, colours, lighting, textures and materials [48
Hence, it is essential that the designer is well aware of the space available, its dimensions and its utilities. The space planning is a process to find a balance between positive and negative spaces, in which a space that is essentially filled with “stuff” (e.g., furniture/decoration items) is a Positive Space and an “empty” space around and in between everything else is a Negative Space. The balance has to be maintained between the positive and negative spaces and either overcrowding or skimping on the stuff is going to affect it [105
Space in interior design and planning refers to indoor space, which is bounded by/inside the physical boundaries of a building. Except the artificial components (e.g., wall, roof, ceiling, floor, door), furnitures and indoor vegetation, even some markers can act as boundaries for space planning. The physical 3D objects offer physical boundaries for the spaces while the objects like markers bring in imaginary boundaries.
2.7. Spaces in Transportation
The concept of space appears in transportation, where road space [106
] is the most frequently mentioned. Transportation refers to movement of humans, animals and goods from one location to another and in which happen in the road space. A road space is designed to have traffic area and auxiliary traffic area (space) [108
]. The former includes pedestrian pavements and vehicle lanes, while the latter can be further classified into grass areas and tree areas (Figure 6
). Thus, specific spaces (areas) are planned and created to isolate people and vehicles although not all the spaces are bounded physically and almost always unbounded from above. The international standard CityGML provides an approach for digital representation of the transportation space and offers options to include other entities (CityFurniture, vegetation, waterbody, tunnel and bridge) as a part of the transportation spaces. Other standards such as Land and Infrastructure (LandInfra) or IFC have their own approach in handling digital representations of transportation space.
Road space is a general name of all spaces used for transporting. In addition to the engineering names (traffic and auxiliary traffic) mentioned above, it is further planned/subdivided based on diverse transportation options, typically including walking, cycling, public transit and auto-mobile and accounts for land use factors that affect accessibility [109
]. In other words, spaces in transportation include walking space, cycling space, public transit and auto-mobile space, as well as the isolation zones.
Another type of space considered in transportation is parking space, such as finding public parking spaces [110
] in parking lot [115
], on-street parking spaces [116
] or managing parking spaces [118
]. The space used here means the place where people can park their cars. As for the space classification, it entirely depends on the place itself, that is, it can be an indoor, outdoor, semi-indoor or semi-outdoor space.
2.8. Intelligent Space
In the last decades, the concept of intelligent spaces has been gaining much interest. Intelligent spaces are environments that can continuously monitor what is happening in them, communicate with their inhabitants and neighbourhoods, make related decisions and act on these decisions [51
]. Intelligent spaces can be built to augment human capability to sense and make sense of the physical world [58
]. In the research of cooperation between intelligent spaces and robots, Reference [119
] defined the intelligent spaces as rooms or areas that are equipped with sensors (e.g., microphones, cameras) that enable them to perceive what is happening in them. In such spaces that have an intelligence of their own a world model no longer is something the robot has alone but a service offered by the information infrastructure of the space. In this respect, the portion of space that belongs to the intelligent spaces is a completely artificial partition, in which certain communications and functions of interconnected systems are provided even without human interaction. Such portions of spaces might include or aggregate several of the above discussed spaces.
For example, intelligent transportation spaces (ITSp) is one of the most prominent contemporary examples. ITSp was developed to lead the effort toward a cyber-physical-social system to further improve the vehicles, traffic and transportation safety, efficiency and sustainability [53
]. Not only various intelligent transportation system (ITS) modules but also pedestrians, vehicles, roadside infrastructures, traffic management centres, sensors and satellites are integrated in it [54
]. The entire constellation would behave like intelligent agents travelling in portions of spaces. The argument behind partitioning space into intelligent transportation cells is driving safety, transport efficiency and comfort that accrue from increased traffic information, reduced driving loads, and improved route management [53
Intelligent spaces cannot be attributed to one discipline, rather they expected to be container of a broad range of applications (such as in homes, offices, factories, etc.). The concept of space partitioning is least specific and resamples the notion of space as physical universe. However, the notion of intelligent space is helpful in defining the scope of smart city [120
]. A typical characterising of intelligent space is that the digital representation cannot be isolated from the physical environment. It need agents in the physical world to carry out actions [121
]. Therefore, the intelligent space can be regarded as a container of digital and physical environments in which different things are connected by ad hoc networks (Figure 7
). Intelligence means that agents obtain intelligence from the system embedded in it, rather than the space controlled by a human [51