Published on March 14, 2008
Accessibility under competition: An implementation in a GIS environment : Accessibility under competition: An implementation in a GIS environment By: Binod Adhikari POEC 6389: GIS Masters Project Introduction: Introduction What is Accessibility? The degree to which a system is usable by as many people as possible Study of potential activity between facilities and those who will use them It is an indicator Social indicator – shows the availability of social opportunities (jobs, health care…) Introduction – Types of Accessibility Studies: Introduction – Types of Accessibility Studies Transportation and Planning Expansion of road network (Ahmed et al 2006) Traffic congestion study (Geurs, B Wee 2004) Operating speed (Geurs, B Wee 2004) Labor Market Spatial study between workers and jobs Study of spatial mismatch Real Estate Property value estimation Apartment rent estimation Location choice Scenario: Scenario Retail Site Selection What is the story? Where to locate a retail facility Competition with other retail facilities Competition for patrons (population) Seeking sites that provide access to population in the context of retail competition Introduction – Accessibility Under Competition: Introduction – Accessibility Under Competition People do not access opportunities in a vacuum There are competitors seeking to attract population Competition requires consideration of both demand and supply factors This research asserts that an approach that considers competition gives a more reasonable result for retail site selection Objective: Objective Implement two different methods of determining accessibility under competition in a GIS environment No such implementation exists Design a GIS extension Compare the results of the different methods Target audiences Geographic Information Scientists Real Estate developers Literature Review - Overview: Literature Review - Overview Accessibility has two components Location and Transportation Location defines the attractiveness or the opportunities that are present Transportation defines the impedance in order to access those opportunities Example of opportunities Job locations, shopping centers, day care centers, recreation centers etc. The population itself can be an opportunity if you are locating a facility, and other facilities are the competition Literature Review – Methods of computing accessibility: Literature Review – Methods of computing accessibility Regional Accessibility and Population Potential Demographic elements are considered as the opportunities E.g. population, household income, home owners etc. Note: in the following slides the symbols i, j, and k refer to locations…or zones where things are located, where there are n zones John Q. Stewart (1947) was the first to coin the term population potential population of area j distance between location i and area j Lit Review – Methods cont.: Lit Review – Methods cont. Mathematical formulation of Accessibility Distance based Simplest form of accessibility measurement Coined by Ingram Ingram refers to as the relative accessibility and the average (Ai ) as the integral accessibility Bigger the number less accessible is i distance between location i and j Lit Review - Methods cont: Lit Review - Methods cont Mathematical formulation of Accessibility Gravity based Based on the classical law of the Newtonian Gravity Model Accessibility measure depends upon the magnitude of the opportunities, and how far these opportunities are located from each other Ej = Opportunities in zone j = Impedance function Lit Review - Methods cont: Lit Review - Methods cont Impedance Function Different mathematical forms Simple negative function Drawback: Division by Zero when i = j Opportunities in zone i are excluded from the calculation Gaussian function Negative exponential function v is an empirical parameter is a distance decay parameter Lit Review - Methods cont: Lit Review - Methods cont Mathematical formulation of Accessibility Cumulative Opportunities Based Method Opportunities are cumulated based on travel time or travel distance The travel time based method is also known as the isochrone method or Drive-time method Ej = Opportunities in zone j Tj = Cut-off time (0 or 1). If greater than drive-time 0, otherwise 1 Lit Review - Methods cont: Lit Review - Methods cont Accessibility with competition Gravity Based Traditional accessibility measures consider only the supply side and not the demand side Shen’s formula explains the demand side Di = Demand Potential Pk = Population seeking opportunities in zone k = Impedance function Ai = Competitive accessibility Ej = Opportunities in zone j Lit Review - Methods cont: Lit Review - Methods cont Accessibility with competition Isochrone based Competitive accessibility based on the Isochrone method is a very rare topic to find in the accessibility literature Two-step floating catchment area (Luo, 2003) resembles the isochrone based competitive accessibility measurement First step: for every opportunity, measure the ratio of opportunity to opportunity seekers within an isochrone Second step: for every zone the ratios from the first step are accumulated to every isochrone Lit Review - Methods cont: Lit Review - Methods cont Lit Review - Methods cont: Lit Review - Methods cont Accessibility with competition Isochrone based Why is the isochrone method interesting? Opportunities are defined based on the actual road network Opportunities beyond the travel time threshold are excluded from the analysis Less complicated and pretty straight forward method of analysis Lit Review - Methods cont: Lit Review - Methods cont Retail Trade Analysis Accessibility analysis- Doesn’t take into account consumer behavior of choosing stores while shopping Doesn’t consider the attractiveness or the stores Huff model and Intervening Opportunity model try to overcome these deficiencies in Accessibility analysis Lit Review - Methods cont: Lit Review - Methods cont Huff Model Introduced by David Huff in 1963 A probabilistic model Gives the probability that a customer will shop in a certain shopping center Lit Review - Methods cont: Lit Review - Methods cont Intervening Opportunity Model Introduced by Samuel A. Stouffer in 1940 the number of persons going a given distance is directly proportional to the number of intervening opportunities nearer opportunities compete with more remote ones and thus the intervening opportunities act as a barrier Lit Review - Methods cont: Lit Review - Methods cont Intervening Opportunity Model Data: Data Boundary data (Block Groups): US Census Bureau Data: Data Road Network Street Map USA, ESRI Note: Streets not shown in the map Street Map USA comes with the speed limit data for the US roads. Otherwise, Census Feature Class Codes (CFCC) is used to estimate the speed limit for the roads. Data: Data Demographic Data US Census Bureau Data: Data Retail Data Research Department, Archon Group Major Retailers Wal-Mart, Target and H-E-B Methodology: Methodology Implement two methods Accessibility using the isochrone method Primary focus of the research First implementation of accessibility with competition for the isochrone method Accessibility using the gravity-based method Implementation of Shen’s method Helps to compare the results from the isochrone based method Methodology cont: Methodology cont Isochrone based method. Based on the floating catchment area method (Luo, Wang 2003) Implemented in two steps First Step: Voronoi diagram (catchment) is drawn around each competitor For each voronoi polygon j all the zones that lie inside are identified and the turnover population is calculated for all i within j New population at zone i is: Ei is calculated for all the zones i that are within the catchment j. Methodology cont: Methodology cont Isochrone based method. First Step cont: Methodology cont: Methodology cont Isochrone based method. Second Step: An iscochrone is drawn around each zone Zone centroids lying inside the isochrone are identified The population of each zone calculated in the first step is accumulated to the isochrone Alternative approach using Intervening Opportunity Model. The model could be implemented in the first step of the Isochrone method. Instead of Voronoi diagrams Intervening opportunity model could give more realistic figures in terms of population turnover for each zone. Methodology cont: Methodology cont Isochrone based method. Second Step cont: Accessibility at zone 8= 69.1+53.3+72.3+85.9+190.2+125.6+100.1 = 696.5 Methodology cont: Methodology cont Gravity based method. Implementation of the Shen’s formula Huff Model. The outcomes from the Accessibility analysis are not capable of pinpointing the new location Once the potential areas are identified, Huff Model could be run for these areas to pinpoint the suitable location Ej is the population in zone j. Pk is the retail stores present in zone k. f(Cij) and f(Ckj) are the impedance between the zones Implementation in GIS Env: Implementation in GIS Env Implementation in GIS Env: Implementation in GIS Env Geodatabase. Designed in ArcCatalog Four Feature classes Isochrone (Stores Isochrones) Network ( Stores Road Network data) ODMatrix (Table with Origin and Destination data) Zones ( Stores boundary data- Block Groups) Implementation in GIS Env: Implementation in GIS Env Isochrone Generation Network Analyst – Service Area feature Impedance – Minutes Isochrone generated for each block group centroid Analysis Setting Do not allow U-Turns Polygon type - Generalized Do not trim Polygons Implementation in GIS Env: Implementation in GIS Env OD Matrix Generation Network Analyst – OD Cost Matrix feature Impedance – Street Length (Miles) OD Matrix generated between all the block group centroids 765 block groups, 765 x 765 = 585225 total records Analysis Setting Do not allow U-Turns Implementation in GIS Env: Implementation in GIS Env Visual Basic Code Arc Objects Perform all the spatial related operations Isochrone generation Spatial join Copy objects Voronoi diagrams Implementation in GIS Env: Implementation in GIS Env Visual Basic Code Microsoft ActiveX Data Object (ADO) Perform all the attribute data related operations Computation of distance decay parameter, β Computation of Shen’s formula Why use ADO? Better performance Direct access to the geodatabase and manipulate the data Geodatabase can be accessed from other application, like Microsoft Excel Result: Result (4 min Isochrone) Result: Result (β = 0.2) Result: Result Comments on the maps Symbology based on “Natural Breaks” Central part of the city has higher accessibility values Better road network More people live in the central part Both methods give almost same accessibility values for the outer part of the city. Yellow color represents accessibility < 8300 Result: Result Statistical analysis of accessibility measures Result: Result Statistical analysis of accessibility measures Result: Result Statistical analysis of accessibility measures Result: Result Statistical analysis of accessibility measures Result: Result Statistical analysis of accessibility measures Pearson Correlation coefficients Result: Result Statistical analysis of accessibility measures Screenshots: Screenshots Conclusion: Conclusion Attempt has been made to implement two methods of competitive accessibility for retail site selection Both the methods gave similar results except for the inner part of the city Isochrone method tends to give higher accessibility values where there is better road network The correlation between the two methods is high but the median value and the standard deviation value did not match that well Block level data is recommended for Isochrone method Future Work: Future Work Include marketing and social variables Differentiate population with vehicle and without vehicle Differentiate population with socioeconomic and ethnicities Compare competitive accessibility for different time span Reference: Reference Ahmed M. 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