The “Reach” measure (Sevtsuk, 2010) captures how many surrounding buildings each building reaches within a given Search Radius on the network.
the Gravity measure assumes that accessibility at building i is proportional to the attractiveness (weight) of destinations j surrounding i,and inversely proportional to the distances between i and j
The Betweenness of a node (building) is defined as the fraction of shortest paths between pairs of other nodes in the network that pass by node i.
The Closeness of an Input Building is defined as the inverse of cumulative distance required to reach from that building to all other buildings in the system that fall within the Search Radius along the shortest paths.
The Straightness metric illustrates the extent to which the shortest paths from a node of interest to all other nodes in the system resemble straight Euclidian paths.
In studying the spatial configurations of cities, and their related social, economic, and environmental processes centrality is a key factor in shaping urban space and urban life (Porta, Crucitti, Latora, 2008). Understanding and capturing it is crucial to the functioning of our cities. Currently, centrality is a fundamental concept in network analysis which persuades the capturing the structural aspects of it as well as its multifold nature. UNA Toolbox is aimed at urban designers, architects, planners, geographers, and spatial analysts who are interested in studying the spatial configurations of cities, and their related social, economic, and environmental processes. The tool is a structured process aimed at the evaluation of the spatial distribution of centrality over geographic systems like systems of urban streets or spaces. Using the Toolbox will help to reveal hidden hierarchical order of urban spaces, to predict the spatial distribution of social-economic development, to help to identify urban factors for pedestrian flows, land use, way-finding, bottom-up opportunities, etc.
The tool incorporates three important features that make them particularly suited for spatial analysis on urban street networks and distinguishes it from other existing spatial network analysis techniques. First, they can account for both geometry and topology in the input networks, using either metric distance (e.g. Meters) or topological distance (e.g. Turns) as impedance factors in the analysis. Second, unlike previous software tools that operate with two network elements (nodes and edges), the UNA tools include a third network element - buildings - which are used as the spatial units (destination points) of analysis for all measures. Two neighboring buildings on the same street segments can therefore obtain different accessibility results. And third, the UNA tool optionally allows buildings to be weighted according to their particular characteristics - more voluminous, more populated, or otherwise more important buildings can be specified to have a proportionately stronger effect on the analysis outcomes, yielding more accurate and reliable results to any of the specified measures.
The toolbox (Sevtsuk, Mekonnen 2012) can be used to compute five types of network analysis measures on spatial networks: Reach; Gravity; Betweenness; Closeness; and Straightness. The toolbox builds upon previous efforts by the Martin Center for Built Form and Land Use Studies, the Space Group at the UCL, the Human Space Lab, and other research work in the area of spatial network analysis (Martin & March 1971; Harary 1969; Porta, Crucitti et al, 2005).
The user can choose which of the five network analysis measurements are to be computed by checking the boxes next to the five measures: Reach, Gravity, Betweenness, Closeness, and Straightness. These measures are described below.
The “Reach” measure (Sevtsuk, 2010) captures how many surrounding buildings each building reaches within a given Search Radius on the network. Reach can be calibrated to measure access to any destination (intersections, buildings or other nodes) within a given Search Radius (Search Radius input defines the buffer radius used for computing the specified measures) or the default infinite radius. The same computations can be run for any other preferred attribute of a node such us building volumes or inhabitants number, etc. to capture Reach to activities or land use destinations. One can use the number of jobs, the number of residents, or the number of business establishments in the surrounding buildings.
Whereas the Reach measure simply counts the number of destinations around each building within a given Search Radius (optionally weighted by building attributes), the Gravity measure additionally factors in the spatial impedance required to travel to each of the destinations.
This index is based on the idea that a node is central if it lies between many other nodes, in the sense that it is traversed by many of the shortest path connecting couples of nodes. The Betweenness measure is typically used to estimate the potential of passersby at different buildings (nodes) on the network.
Closeness measures to what extent a certain node (building) is near all the other nodes in a system along the shortest path. In other words, itis defined as the inverse of cumulative distance required to reach from that building to all other buildings in the system that fall within the Search Radius along the shortest paths. High closeness will have many connections within a short distance while low closeness will have few connections at short distance.
Straightness centrality or “being central as being straight to the others”. This centrality index originates from the idea the efficiency in the communication between two nodes in a system increases when there is less deviation of their shortest path from the virtual straight line (straight Euclidean path) connecting them.