This set of tools in the UNA toolbar includes key functions that produce network analysis results — Accessibility Indices, Service Area, Redundant Paths, Betweenness, Closest Facility, Find Patronage, Distribute Weights and Clusters.
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<aside> 📽️ Video Tutorial
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The Accessibility Indices tool launches the accessibility calculations between network Origin and Destinations points that have been added. The UNA toolbox computes two different Accessibility Indices — Reach and Gravity — each of which offers a unique and complementary way of analyzing spatial relationships between Origin and Destination points in a network. In each case, results are returned to Origin points.
If the analysis is weighted, weights are applied to Destination objects. If you would like to analyze accessibility from buildings to subway stops, for instance, then buildings should be taken as Origins and subway stops as Destinations. You can optionally weigh subway stops by a numeric attribute, attached to these stops, indicating how many metro lines or trains per hour each stop serves.
Upon running the tool, a command line message will ask you to select the Origins for the analysis or to accept the pre-selection. The pre-selection automatically detects all Origin points you have previously added to the network. If you wish to use all of them for the analysis, then simply press Enter to accept the pre-selection. If you wish to only select a subset of points for analysis, then select those Origins you want to include. All accessibility results will be computed for the points you decide to select here. Selected Origins will be temporarily indicated with blue crosses.
Next, the tool prompts you to select Destinations in the same way. Selected Destinations are marked with red crosses. Once you have selected Origin and Destination points, you will be prompted for a set of options on the command line.
Accessibility Indices tool
Origin points are marked with blue crosses
Destination points are marked with red crosses
Search Radius <600> (Reach=*On* Gravity=*On* Alpha=*1* Plateau=*0*
Weight=*Count* DecayMode DecayOptions Nearest=*Off*):
Search Radius input defines the network radius used for computing accessibility measures you choose. For each Origin point, only Destination points whose shortest network distance from the Origin is less than the specified Search Radius are considered in the analysis. Search Radius units follow the drawing units — if your drawing is in meters, Search Radius is also in meters. The active Search Radius is shown at the beginning of the command line prompt; you can change the Search Radius by typing a new number on the command line.
Next, you can see a list of four Accessibility Indices that you can choose to include in the results:
Reach = Onoption turns the Reach analysis on or off.
Gravity = On **option turns the Gravity analysis on or off.
Turn them on or off by clicking each one on the command line. Each of these Accessibility Indices is explained in detail below.
Alpha = 1 **coefficient is an exponent that is applied to the destination weights to control how an increase in destination attractiveness affects accessibility outcomes. It only affects the Gravity index. If you are not performing Gravity accessibility analysis and set Gravity = Off, you can disregard this input. The alpha coefficient is explained as part of the Gravity index.
Plateau = 0 establishes a set distance from origins until which Gravity accessibility is considered “1” (100%) and beyond which a decay function will begin. If Plateau is set to “100 meters”, for instance, then all destinations that are within a 100-meter network radius from the origin are considered fully accessible, with no differentiation whether they are 1m away or 99m away. Starting at 100m from the origin, the chosen decay function (e.g. linear, exponential, logistic) will be applied when computing Gravity accessibility. See DecayMode and DecayOptions for setting the decay parameter below. If you are not performing Gravity accessibility analysis and set Gravity = Off, you can disregard this input.
Weight = Count **option allows you to weigh the accessibility results with Destination attributes. When using Reach analysis to measure how many jobs one can walk to from each Origin in a ten-minute walkshed, for instance, you might weight the Destination buildings with a “Jobs” weight, indicating the number of jobs at each Destination building. Reach results would then illustrate the number of jobs that can be reached from every Origin building in a ten-minute walkshed. When the weights are kept as Weight = Count, then each Destination point is simply counted as “1” and no additional weights are applied. You can choose weights by clicking on the Weight = Countoption, which will list all the numeric attribute weights that are available in the drawing. Click on the weight field you want to use and make sure that the weights you assign are actually associated with the Destination points in your network. See sections Add Numeric Attribute, Import Points, and Join Attributes on how to assign weights to points.
Decay Mode includes three modes of calculating the decay function. “Exponential” applies a Gravity weight based on an exponential curve defined by the value of Beta set in the DecayOptions. “Logistic” applies a Gravity weight based on a logistic curve defined by the value of Midpoint set in the DecayOptions. “Linear” applies a Gravity weight based on a linear function defined by the Intercept set in the DecayOptions. See the figure below for a visual representation of each of these functions. If you are not performing Gravity accessibility analysis and set Gravity = Off, you can disregard this input.
Visual representation of different decay mode options
DecayOptions allows for setting decay parameters based on the three different decay modes explained above. If you are not performing Gravity accessibility analysis and set Gravity = Off, you can disregard this input. Beta coefficients, midpoints, and intercepts are explained as part of the Gravity index.
Nearest = Onoption finds only the nearest Destination for each Origin. Nearest = Offdoes not limit the analysis to the nearest Destination.
The next section discusses both Accessibility Indices in detail.