In this article we use Multiple Centrality Assessment (MCA) based on the idea of Sergio Porta [Por06]. We apply MCA for the urban streets of Tirana. MCA is based on primal, rather than dual, street graphs, it works within a metric, rather than topological, framework and it investigates a plurality of peer centrality indices. We show that, in the MCA primal approach, some centrality indices nicely capture the 'skeleton' of the urban structure that impacts so much on economic and cultural development of the city, which is also the purpose of our study. Moreover, the distributions of centrality in self-organized cities are different from those in planned cities. In our analysis we are considering all roads for vehicles, pedestrian paths and even the closed roads, which appear as leaves in the primary graph (vertex degree k=1). The area considered in this study is 4.392052∙106 m2 (1.7 miles2). We have built two databases for the sake of the study. One database contains information about edges like the vertex pairs of each edge and their weight (length in meters). The total number of edges is K = 1964. The other database contains information about geographical location of vertices (Longitude, Latitude for each vertex). The total number of edges is N=1565. After all the analysis, we'll see if Tirana is a self-organized city or a planned one.
The study of complex networks is a new area of scientific research (since 2000) inspired of real-world networks such as computer networks, technological networks, brain networks and social networks. But, spatial networks were actually the subject a long time ago of many studies in quantitative geography. Objects of studies in geography are locations, activities and flows of individuals and goods. Already in the 1970s the scientists working in quantitative geography focused on networks evolving in time and space. The network analysis of territorial cases has a long tradition, especially in transportation and economic geography. All these approaches, like ‘closeness’, ‘connectivity’, or ‘cost’ focus on the idea that some places (or streets) are more important than others because they are more central. [Por06]
Two coordinates (Longitude, Latitude)1, used in the database of vertices, uniquely determine a point in the area and specifically a geographic position on Earth. The picture below explains it better.
To calculate the Euclidean distance between two points in Earth, we apply the Haversine formula. This formula is a special case of spherical trigonometry and it is used for the first time in 1801 by spanish mathematician and astronomer José de Mendoza y Ríos2. To implement this formula, we need longitude, latitude and the radius of Earth R = 6371∙106 m.
For data processing we have used the software program R: package igraph for construction of graphs, package Geosphere to perform calculations and package RColorBrewer for coloring graphs according to different algorithms.
By using the primal approach, where networks of streets and intersections can be represented by spatial graphs in which zero-dimensional geographic entities (such as intersections) are turned into zero-dimensional graph entities (nodes) placed into a two-dimensional Euclidean space, and one-dimensional geographic entities (such as streets) are turned into onedimensional graph entities (edges or links). The primal approach is also the world standard in geospatial dataset construction. [Por06]
Moreover for us it is more important the metric distance rather than topological one. So, according to primal approach, the streets will be represented by the edges and intersections will be represented by the nodes. Of course, that the graph will be a weighted graph. So, each edge will be attached the weight that in this case will be the length (in meters). Each vertex of graph will be identified by a number that will be unique for each vertex. Also the graph will not be oriented, since we’re not considering the directions of motion. Four primal approaches (Figure 2, Figure 3) [Por06] namely Ahmedabad, Venice, Richmond, CA, and Walnut Creek, CA, are here given closer focus in order to frame the comparison between those cities and Tirana (Figure 4, Figure 5).
On one hand Ahmedabad and Venice are typical selforganized patterns, in that they `spontaneously' emerged from a historical process outside of any central coordination (the graph network visualization is disorganized). On the other hand Richmond and Walnut Creek are planned patterns, developed following one coordinating layout in a relatively short 2 https://en.wikipedia.org/wiki/Haversine_formula period of time (the graph network visualization is quadratic). [Por06] http://www.arcgis.com/home/webmap/viewer.html?webmap= 1cefeb8113724e489ef63f29629683c4 As we see the primal graph, we can judge that Tirana is a city, which can be regarded as self-organized.
Network’s cost W is the sum of all edges lengths [Por06]: Network’s cost for Tirana WTir = 112 468 m. The calculations are made for 1 mile2.
The characteristic path length L (Watts and Strogatz, 1998) [Wat98] is defined as the average length of the shortest paths (with the average being calculated over all the couples of nodes in the network): L is a good measure of the connectivity properties of the network. For Tirana, we got the result LTir=1456.764m.
Average path length <l>4 is a concept in network topology that is defined as the average number of steps along the shortest paths for all possible pairs of network nodes. It is a measure of the efficiency of information or mass transport on a network. Our result was <l>Tir=19.92. It means that it takes 20 steps on average to go along the shortest paths for all possible pairs of intersections in Tirana.
High values of W and L low values of <l> are properties of self-organized cities.
Diameter D5 is the shortest distance between the two most distant nodes in the network. In other words, once 4 https://en.wikipedia.org/wiki/Average_path_length 5 https://en.wikipedia.org/wiki/Network_science the shortest path length from every node to all other nodes is calculated, the diameter is the longest of all the calculated path lengths. Diameter of Tirana (Figure 6) is D=3590m, while the Euclidian distance between those nodes is dE=2867m. Thus , the diameter is 1.25 times greater than the Euclidian distance. In the tables below we have made a ranking of 19 cities [Cru06], including Tirana, according to the number of nodes N , the number edges K, the cost W and average path length <l>. It is obvious, that Tirana is ranked near the self-organized cities like Ahmedabad, Venice, Cairo and Seoul. Venice Ahmedabad Richmond Savannah Bologna London Vienna Paris New Delhi New York Los Angeles
Washington San Francisco Brasilia Walnut Creek 192 169 179 169 303 271 230 197 36342 38187 30910 25131
Closeness centrality, CC, [Por06] measures to which extent a node i is near to all the other nodes along the shortest paths, and is defined as [Por06]: where dij is the shortest path length between i and j, defined, in a valued graph, as the smallest sum of the edges length l throughout all the possible paths in the graph between i and j. [Por06]
Implemented on primal graphs, the spatial flow of CC is dominated by the so-called `border effect', in the sense that higher CC scores consistently group around the geo-metric center of the image. To some extent less evident in less dense cases such as Walnut Creek, the border effect is overwhelming in denser urban fabrics such as those of Ahmedabad and Venice (Figure 7 a,b) [Por06]. Also the primal graph of Tirana is dominated by this effect (Figure 8). The problem: as we have just shown, the CC integration index simply does not work on such primal graphs because CC is vulnerable to the border effect. But CC is not the only option. Centrality is a multifold concept and we have many indices at hand. Thus, to overcome this problem we can limit the analysis of CC to a local scale, at which it maintains a good potential. [Por06]
A review of Ahmedabad [Por06] and of my results for Tirana are offered in the figures below. 6 We notice that the graph coloring is done scalable from dark red to dark blue. With dark red are colored vertices with higher values of CC, while with dark blue are colored vertices with lower values CC.
Now, the concept of ‘being near others’ is more meaningful, because in this way are defined locations (intersections and roads) that are more important for certain areas of the city. This type of analysis can help entrepreneurs increase their economic activities in such areas, which can be considered optimal (for example supermarket chains).
Betweenness centrality, CB, [Por06] 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 paths connecting couples of nodes. The betweenness centrality of node i is [Por06]: where njk is the number of shortest paths between j and k, and njk(i) is the number of shortest paths between j and k that contain node i.
From the visual presentation of CB for Tirana, we notice that the most important streets, that influence most the rest of the street network, are "Fortuzi" street, Durrësi street, Dibra street, Kavaja street and "Myslym Shyri " street.
Straightness centrality, CS, [Por06] originates from the idea that the efficiency in the communication between two nodes i and j is equal to the inverse of the shortest path length dij. The straightness centrality of node i is defined as [Por06]: where dEucl ij is the Euclidean distance between nodes i and j along a straight line, and we have adopted a normalization recently proposed for geographic networks. This measure captures to which extent the connecting route between nodes i and j deviates from the virtual straight route.
From this visual representation of CS for Tirana, we notice that the straight streets, that directly link two intersections are mainly boulevards like "Dëshmorët e Kombit" boulevard, "Zhan D'Ark" boulevard and "Bajram Curri" boulevard. While some other streets are "Muhamet Gjollesha" street, "Asim Vokshi" street, etc. It was the first urban planning made in 1923, by the Austrians, which attempted to build straight streets for the capital city of Albania7.
Information centrality, CI , [Por06] relates the node centrality to the ability of the network to respond to the deactivation of the node. The information centrality of node i is defined as the relative drop in the network efficiency E[G] caused by the removal from G of the edges incident in i [Por06]: 7 Planet Urbanistike të Tiranës në vite, Arkivi Qendror Teknik i Ndërtimit (AQTN) where the efficiency of a graph G is defined as [Por06]: and where G′ is the graph with N nodes and K – ki edges obtained by removing from the original graph G the ki edges incident in node i. An advantange of using the efficiency to measure the performance of a graph is that E[G] is finite even for disconnected graphs. The spatial distribution of node centralities can be visualized as the one of Tirana reported in Figure 16. While figure 15 shows visual presentation of CS for Ahmedabad, Venice, Richmond and Walnut Creek [Por06]. From this visual representation of CI for Tirana, we notice that some of the most critical streets, that are very important for the connectivity of the street network in Tirana, playing in this way the role of ‘bridges’, are “Fortuzi” street, Durrësi street, Bogdanët street, “Myslym Shyri” street, Dibra street, “Qemal Stafa” street, Saraçët street, etc.
In this article we used MCA method and we concluded that Tirana is a self-organized city. We also defined some of the most important streets of Tirana that capture the ‘skeleton’ of the urban structure that impacts so much on economic and cultural development of the city. In the future, it will be of great interest to use MCA method even for other cities of Albania. For example we may use MCA for the city of Korça, that can be prejudged as a planned city. In this way we can make comparisons between different cities of Albania.
I am very thankful to DR. Eva NOKA (JANI) for leading me in this research and to the engineers of Department of Transportation and Mobility in the Municipality of Tirana, especially to the head of Traffic Control Sector Mr. Kreshnik Bajraktari for supporting me with updated information on network of urban streets of Tirana.