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What is location-allocation?Location is often considered the most important factor leading to the success of a private- or public-sector organization. Private-sector organizations can profit from a good location, whether a small coffee shop with a local clientele or a multinational network of factories with distribution centers and a worldwide chain of retail outlets. Location can help keep fixed and overhead costs low and accessibility high. Public-sector facilities, such as schools, hospitals, libraries, fire stations, and emergency response services (ERS) centers, can provide high-quality service to the community at a low cost when a good location is chosen. Given facilities that provide goods and services and a set of demand points that consume them, the goal of location-allocation is to locate the facilities in a way that supplies the demand points most efficiently. As the name suggests, location-allocation is a twofold problem that simultaneously locates facilities and allocates demand points to the facilities. Initially, it may appear that all location-allocation analyses solve the same problem, but the best location is not the same for all types of facilities. For instance, the best location for an ERS center is different than the best location for a manufacturing plant. The next two examples demonstrate how the goals of location-allocation problems vary according to the type of facility being located. Example 1: Locating an ERS centerWhen someone calls for an ambulance, we trust it will come to their aid almost instantly; the emergency response time depends considerably on the distance between the ambulance and the patient. Typically, the goal for determining the best sites for ERS centers is to make it possible for ambulances to reach the most people within a defined time frame. The specific question may be: Where should three ERS facilities be placed so that the greatest number of people in the community can be reached within four minutes? Example 2: Locating a manufacturing plantMany retail outlets receive their goods from manufacturing plants. Whether producing automobiles, appliances, or packaged food, a manufacturing plant can spend a large percentage of its budget on transportation. Location-allocation can answer the following question: Where should the manufacturing plant be located to minimize overall transportation costs? Location-allocation problem typesThe ArcGIS Location-Allocation analysis layer offers seven different problem types to answer specific kinds of questions, including questions like those posed in the two examples above. The seven problem types are the following:
Details and examples of the individual problem types are described in the Location-allocation analysis layer properties section of this document. The workflow to perform a location-allocation analysis is similar to performing any other analysis in the ArcGIS Network Analyst extension. Learn more about the network analysis workflow Location-allocation analysis layerThe location-allocation analysis layer stores the inputs, parameters, and results for a given location-allocation problem. Creating a location-allocation analysis layerYou can create a location-allocation analysis layer from the Network Analyst toolbar by clicking Network Analyst > New Location-Allocation. When you create a location-allocation analysis layer, it shows up in the Network Analyst window along with its six network analysis classes—Facilities, Demand Points, Lines, Point Barriers, Line Barriers, and Polygon Barriers. The location-allocation analysis layer also appears in the table of contents as a composite layer containing six corresponding feature layers: Facilities, Demand Points, Lines, Point Barriers, Line Barriers, and Polygon Barriers. Each of the six feature layers has default symbology that can be modified in its Layer Properties dialog box. Location-allocation analysis classesThe location-allocation analysis layer is made up of six network analysis classes, which are feature layers stored within the analysis layer. They contain the network analysis objects used when performing a location-allocation analysis. The network analysis classes have attributes that specify the inputs and outputs for a given location-allocation problem. The attributes can be examined and edited through a network analysis class attribute table, where all the objects and their attributes for that class are listed, or through an object's Properties window, where only one object and its attributes are listed at a time. Learn more about viewing and editing properties of network analysis objects Within a network analysis class, there may be a combination of input fields, output fields, and input/output fields. You enter data into input fields, which are then used by the location-allocation solver to set up the problem. In contrast, output fields store results from the solve process and give you information about the solution. Last, input/output fields are a combination of the two: you can set values for these fields before solving and let the solver assign output values. Facilities classA facility in a location-allocation analysis is a point feature that represents a candidate or required site, but in some cases, it represents a competing facility. The location-allocation solver chooses the best candidate facilities to allocate demand to in the most efficient way according to the problem type and criteria you specify. A candidate facility should be a location that is suitable for the event or structure you are locating. For instance, if you are locating distribution centers, first, you might need to find parcels that are for sale, within your budget, properly zoned, and large enough to contain the distribution center you plan to build. You might also choose to include parcels that already have structures on them that are large enough to house your distribution center. There is no limit to the number of factors you might consider in determining suitability for your facilities. Choosing the right factors and finding sites that meet those factors is all part of a suitability analysis, which should be performed prior to setting up a location-allocation problem. Suitability analyses can be as rudimentary or elaborate as you want to make them. If finding good candidate facilities is neglected, however, the location-allocation solver may choose a facility that won't ultimately be viable. Returning to the distribution center example, if candidate facilities are scattered randomly over the study area without a proper suitability analysis, the location-allocation solver might choose a candidate facility in an inappropriate location, such as a residential neighborhood. The point to remember is you provide the location-allocation solver with candidate facilities that would fit your needs, and the location-allocation solver will choose among the candidate facilities to find the one facility, or set of facilities, that would minimize cost and maximize demand allocation while respecting the constraints of a specific location-allocation problem type. Another type of facility is the required facility, which must be included in the solution. If a city is growing to the point that new neighborhoods are not properly protected by the fire department, a goal could be to locate a new fire station without closing any existing stations. The potential sites for the new fire stations would be candidate facilities, and the current fire stations would be required facilities. Competitor facilities are specific to the maximize market share and target market share problem types and typically represent facilities of rival businesses that compete for the same clientele as your business. Each facility can have a weight that represents the importance or attractiveness of it. A facility weight value other than one can only be used with the maximize market share and target market share problem types—it is ignored by the other problem types. For example, it could be determined that a department store with a sales floor that is twice as large as a typical sales floor would be twice as attractive to customers. The larger store would have a weight of 2.0 and the normally sized stores would have a weight of 1.0. Determining what factors influence facility weight and quantifying them requires careful consideration. Facility propertiesInput fields of facilities
Input/Output fields of facilities
Output fields of facilities
Demand Points classThis feature layer stores demand points that are part of a given location-allocation analysis layer. A demand point is typically a location that represents the people or things requiring the goods and services your facilities provide. A demand point could be a ZIP Code centroid weighted by the number of people residing within it or by the expected consumption generated by those people. Demand points could also represent business customers. If you supply businesses with a high turnover of inventory, they would be weighted more heavily than those with a low turnover rate. Demand points can override the distance cutoff for the location-allocation problem type. This is useful if some demand points have different needs or behavior. For instance, when prepositioning ambulances, it may be acceptable to reach everyone within four minutes, except for areas with a high-density of elderly people, such as senior centers, which require a faster response time of two minutes. Demand point propertiesInput fields of demand points
Input/Output fields of demand points
Output fields of demand points
Lines classThe Lines class is an output-only network analysis class and therefore contains line features that are generated by the solver during the solve operation. It contains line features that connect demand points to the facilities they are allocated to. If a demand point is allocated to more than one facility, it has one line for each facility it is allocated to. If a demand point is not allocated to any facility, it won't have any corresponding lines. The output shape type can be either Straight Line or None; either way, a line feature always represents the shortest network path between the facility and the demand point; thus, the cost-related attributes reflect network costs, not straight-line distances. The reason the actual shape of the network paths are not output is that they are rarely needed in location-allocation, and generating the shape of the paths would require a substantial increase in the solve time and potentially exhaust your system's resources, especially for large problems. Line propertiesOutput fields of lines
Point, line, and polygon barriersBarriers serve to temporarily restrict, add impedance to, and scale impedance on parts of the network. When a new network analysis layer is created, the barrier classes are empty. They are populated only when you add objects into them—but adding barriers is not required. Barriers are available in all network analysis layers; therefore, they are described in a separate topic. Learn more about barriers Location-allocation analysis layer propertiesAnalysis parameters are set on the Layer Properties dialog box for the analysis layer. The dialog box can be accessed in different ways: Learn more about opening the network analysis Layer Properties dialog box The Analysis Settings tabImpedanceThis property specifies the network cost attribute used to define the traversal cost along the elements of the network. Note that specifying a start time doesn't require a traffic-enabled cost attribute; yet, if the network dataset includes traffic data, a time-dependent location-allocation analysis is solved. This allows you to see how results change for different starting times. Learn more about cost attributes Use Start TimeUse Start Time, in conjunction with the Time of Day and Day of Week or Specific Date properties, lets you specify what the start time from the facilities or demand points is. Time of Day The value you enter here represents the time of day for which you want the analysis to be solved. Specifically, the travel time is measured from either facilities or demand points for the time of day specified. The Travel From property determines whether the travel time is measured from facilities or origins. The time you specify in Time of Day must be associated with a date. You can choose between entering a floating day (Day of Week) or a calendar day (Specific Date). Specific Date For a calendar day, you provide the day, month, and year that the Time of Day value is associated with. Day of Week For a floating date, you can choose Today or any day of the week (Sunday through Saturday) relative to the current date. Floating days enable you to configure an analysis layer that can be reused, without having to remember to change the date. You can solve up to six days ahead relative to the current day when choosing Day of Week. Using a start time with traffic data and time zones If you use a time-based impedance attribute or accumulate attribute, the start time and date refers to the time zone of the edge or junction that the starting facility or demand point is located on. Note that all origins must be in the same time zone when solving an analysis across multiple time zones using a time-based impedance attribute. Two requirements to be aware of when solving a location-allocation analysis that spans across multiple time zones are listed below.
Travel FromWhen Network Analyst solves a location-allocation problem, it can calculate network costs from demand points to facilities or from facilities to demand points. Restrictions, such as one-way streets, and impedances, such as travel time, can be based on direction of travel, which can affect travel times. For instance, a facility may be a 15-minute drive from the demand point to the facility, but only a 10-minute trip when traveling from the facility to the demand point. The Travel From property can affect which facility the demand point is allocated to. Fire departments commonly use the Facility to Demand setting, since they are concerned with the time it takes to travel from the fire station to the location of the emergency. A retail store is more concerned with the time it takes the shoppers to reach the store; therefore, stores commonly use the Demand to Facility option. Travel From also determines the meaning of any start time that is provided. See Use Start Time above for more information. U-turns at JunctionsNetwork Analyst can allow U-turns everywhere, nowhere, only at dead ends (or culs-de-sac), or only at intersections and dead ends. Allowing U-turns implies the vehicle can turn around at a junction and double back on the same street. Learn more about U-turn policies Output Shape TypeThe results of the analysis can be represented without lines (None) or with straight lines:
In both cases, the impedance-related costs in the solution are the same and based on shortest network paths (straight-line distance is not used). Use HierarchyIf the network dataset has a hierarchy attribute, you can use the hierarchy during the analysis. Using a hierarchy results in the solver preferring higher-order edges to lower-order edges. Hierarchical solves are faster, and they can be used to simulate the driver preference of traveling on freeways instead of local roads—even if that means a longer trip. Not using a hierarchy, however, yields an exact route for the network dataset. Learn more about routing with hierarchy Ignore Invalid LocationsThis property allows you to ignore invalid network locations and solve the analysis layer from valid network locations only. If this option is not checked and you have unlocated network locations, the solve may fail. In either case, the invalid locations are ignored in the analysis. RestrictionsYou can choose which restriction attributes should be respected while solving the analysis. In most cases, restrictions cause roads to be prohibited, but they can also cause them to be avoided or preferred. A restriction attribute, such as Oneway, should be used when finding solutions for vehicles that must obey one-way streets (for instance, nonemergency vehicles). Other common restriction attributes include height or weight limits that prohibit some vehicles from traversing certain roads or bridges; hazardous materials restrictions that hazmat drivers need to completely bypass or at least try to avoid; and designated truck routes that truck drivers should try to follow. You can choose which restriction attributes should be respected while solving the analysis. (You can further specify whether the elements that use the restriction should be prohibited, avoided, or preferred in the Attribute Parameters tab.)
The Advanced Settings tabThe Advanced Settings tab of the Layer Properties dialog box is where you choose the problem type and set its properties. The problem type you should choose depends on the kind of facility you are locating, because different kinds of facilities have different priorities and constraints. For instance, a fire department may have a mandate that it locate facilities in such a way that it can reach everyone in the community within four minutes. A restaurant may try to maximize patronage by locating in such a way that as many people as possible are within a 10-minute drive and, furthermore, as many of those people are as close as possible to the restaurant. Both examples can be solved with a location-allocation analysis, but they require different problem types. (The minimize facilities problem type would best fit the fire department's objectives, while the maximize attendance problem type would work well for the restaurant.) The settings you make on the Advanced Settings tab change constraints and influence the solver's priorities when locating facilities. Problem TypeThis Problem Type property allows you to choose the location-allocation problem type. The problem types are listed and described below. Location-allocation problem types
Facilities To ChooseUse the Facilities To Choose property to specify the number of facilities the solver should locate. The facilities with a FacilityType value of Required are always part of the solution when there are more facilities to find than required facilities; any excess facilities to choose are picked from candidate facilities. Any facilities that have a FacilityType value of Chosen before solving are treated as candidate facilities at solve time. The Facilities To Choose property is disabled for the Minimize Facilities problem type, since the solver determines the minimum number of facilities to maximize coverage. The Facilities To Choose property is disabled in the Target Market Share problem, because the solver searches for the minimum number of facilities required to capture the specified market share. Impedance CutoffImpedance Cutoff specifies the maximum impedance at which a demand point can be allocated to a facility. The maximum impedance is measured by the least-cost path along the network. If a demand point is outside the cutoff, it is left unallocated. This property might be used to model the maximum distance that people are willing to travel to visit your stores or the maximum time that is permitted for a fire department to reach anyone in the community. Impedance TransformationThis sets the equation for transforming the network cost between facilities and demand points. This property, coupled with the Impedance Parameter, specifies how severely the network impedance between facilities and demand points influences the solver's choice of facilities. Applying a transformation can equalize the overall distances that demand points must travel to reach their nearest facility. Libraries and health clinics are concerned with equity of service, so they often locate facilities using a minimize impedance problem type with a power impedance transformation and an impedance parameter of 2.0. This way, a minority of faraway patrons or patients is not burdened with comparatively excessive travel distances. Some stores gather data on where their customers live; as they collect data, the effect distance has on customer behavior is revealed. One benefit of the data is that stores can establish and calibrate impedance transformations, which can lead to better site selections in the future. Accurately fitting an impedance transformation and parameter to describe your priorities or model the behavior of your demand points requires careful study, including research on topics like the Huff model and distance decay. The first step, however, is understanding how costs are transformed. In the following list of transformation options, d refers to demand points and f, facilities. So impedancedf is the shortest-path network impedance between demand point d and facility f, and costdf is the transformed network impedance between the facility and demand point. Lambda (λ) denotes the impedance parameter.
The next set of graphics and tables use Minimize Impedance to demonstrate the potential effects of using different impedance transformations and parameters. A linear transformation always uses a parameter value of one, so the cost is unchanged, and facility B minimizes that cost. Comparison of costs using a linear transformation
A power transformation with a parameter of two amplifies longer distances enough that facility A minimizes cost instead. Comparison of costs using a power transformation with a parameter of 2.0
An exponential transformation with an impedance parameter of 0.02 will favor nearby demand points, so facility B is the solution facility in this case. (The graphic is omitted, since it would look the same as the linear transformation graphic.) Comparison of costs using an exponential transformation with a parameter of 0.02
Impedance ParameterThis property allows you to set a parameter, λ, for use with the Impedance Transformation property. However, when Impedance Transformation is set to Linear, the impedance parameter value is ignored, and a value of one is used instead. See the impedance transformation property (above) for more information. Target Market ShareThis property is specific to the Target Market Share problem type. It is the percentage of the total demand weight that you want your solution facilities to capture. The solver chooses the minimum number of facilities required to capture the target market share specified by this numeric value. Default CapacityThis property is specific to the Maximize Capacitated Coverage problem type. It is the capacity assigned to all facilities in the analysis. You can override the default capacity for a facility by specifying a value in the facility's Capacity property. The Accumulation tabUnder the Accumulation tab, you can choose cost attributes from the network dataset to be accumulated on the line objects, which represent least-cost routes along the network. These accumulation attributes are purely for reference; the solver only uses the cost attribute specified by the analysis layer's Impedance parameter to calculate the results. For each cost attribute that is accumulated, a Total_[Impedance] property is added to the lines that are output by the solver, where [Impedance] is replaced with the name of the accumulated impedance attribute. Assume you set the impedance attribute to Minutes because you want to base the analysis on routes that minimize travel time. Even though minimizing travel time is most important to you, you would also like to know the length of the quickest routes. Suppose you have another cost attribute, Miles, that you check on the Accumulation tab. After solving, the output line features have fields named Total_Minutes and Total_Miles, which provide the time to travel along the quickest route and the distance of that route, respectively. Conversely, you can base the analysis on shortest routes and accumulate travel time to determine how long it takes to complete each trip between a facility and a demand point. If you have a traffic-enabled network dataset, you can even find this information for a specific time of day and account for variable traffic speeds. To do this, choose a distance-based cost attribute for the impedance of the analysis layer, use a start time, and accumulate time using a time-dependent cost attribute. The Network Locations tabThe parameters on the Network Locations tab are used to find network locations and set values for their properties. Learn more about network locations Solving and interpreting the results of a location-allocation analysisAfter creating a location-allocation analysis layer, populating the required network analysis objects, and setting appropriate analysis properties, the solution for the location-allocation problem can be determined by clicking the Solve button on the Network Analyst toolbar. After solving, if the Output Shape Type property is set to Straight Lines, the location-allocation solver draws lines between the solution facilities and their allocated demand points and sets the FacilityType property of a candidate facility to Chosen if it is part of the solution. The Network Analyst window also updates the name of the Lines class to show the number of line objects it contains. During the solve operation, the location-allocation solver creates an internally managed origin-destination (OD) cost matrix between the facilities and demand points using the active network cost attribute as impedance. The solver references the OD cost matrix when analyzing potential solutions to the problem. Learn more about OD cost matrix analysis The location-allocation problem is one of combinatorial optimization, which means the number of potential solutions can grow quickly: This table demonstrates the complexity of location-allocation problems and why heuristics are needed
Due to the combinatorial nature of the location-allocation problem, exhaustive search techniques are impractical for finding good solutions within reasonable search times (especially for large problem sets). Therefore, heuristics are employed to carry out faster searches. More information about the heuristic approach used by Network Analyst can be found in the following topic: Learn more about algorithms used by Network Analyst After successfully choosing facilities and allocating demand points, the location-allocation solver outputs results in the appropriate output fields of the network analysis objects. Interpreting the results of a location-allocation analysisAfter successfully solving a location-allocation problem, you can inspect the results by looking at the properties of the facilities, demand points, and lines. You may also want to make selections on the analysis classes to better understand the results. The following list shows a couple of common postanalysis selections:
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