Barriers

Barriers are feature classes in network analysis layers that restrict or alter impedances of the underlying edges and junctions of the associated network dataset. Barriers are split into three geometry types (point, line, and polygon) and are designed to model temporary changes to the network. The various types of barriers are introduced below:

Barriers are part of the network analysis layer, not the network dataset. Therefore, barriers only have an effect on the network analysis layer that contains them. If barriers are needed in other analyses, they should be loaded into the appropriate network analysis layer. Alternatively, the network dataset could be edited instead of using barriers.

Any kind of change barriers make to the traversability or impedance of the network can also be accomplished through edits to the network dataset. Yet barriers help you add and remove network changes quickly, which is ideal for modeling temporary impedance changes: the tree blocking traffic will eventually be removed, and the flood will ultimately recede. Once the event the barrier is modeling is over, the barrier can simply be deleted.

Since barriers can function as a substitute to editing, you can make network traversability and impedance changes even when you don't have the editing privileges. For instance, if you have a noneditable SDC network dataset, such as StreetMap North America, and think the impedance values do not accurately reflect your travel times in a given area, you can add a polygon barrier and scale the impedances to more appropriate values. However, expect slower solver performance with barriers.

The time it takes to create or load a line or polygon barrier is proportional to the number of network elements it covers. Using barriers to cover dense or large regions of your network may not be feasible.

Point, line, and polygon barriers

Barrier classes are present in all network analysis layers.

Barriers can be added into barrier classes via the Create Network Location ToolCreate Network Location Tool on the Network Analyst toolbar, the Load Locations dialog box, or the Add Locations geoprocessing tool. When barriers are created, loaded, or added, they affect only the items checked on the Network Locations tab of the network analysis layer's Layer Properties dialog box. This is true even when a barrier intersects layers that are unchecked on the Network Locations tab.

Learn more about creating network analysis objects with the Create Network Locations Tool

Barriers have attributes that can be examined and edited through an attribute table where all the barriers and their attributes in that barrier class are listed, or through a barrier's Properties table where only one barrier and its attributes are listed at a time.

Learn about opening the Properties window of network analysis objects

Point Barriers feature layer

When a point barrier is loaded or created, it snaps to the nearest network edge or junction within a search tolerance. The position of the barrier on the network is specified by the network location fields (SourceID, SourceOID, SideOfEdge, and PosAlong). When a restriction point barrier is on an edge, you can opt to restrict travel across the point barrier only, or restrict the edge entirely. A scaled cost point barrier adds a cost whenever it is crossed.

Point barrier properties

Input fields of point barriers

Input field

Description

ObjectID

The system-managed ID field.

Name

The name of the network analysis object.

BarrierType

Specifies whether the point barrier restricts travel completely or adds impedance when it is crossed. There are two options:

  • Restriction (0)—Prohibits traversing through the barrier. This is the default value.
  • Added Cost (2)—Traversing through the barrier increases the network cost by the amount specified in the Attr_[Impedance] property value.

FullEdge

This property is specific to restriction point barriers on edge elements. The default value is False.

  • False—Permits travel on the edge up to the barrier, but not through it
  • True—Restricts travel anywhere on the associated edge

Attr_[Impedance]

(for instance, Attr_Minutes, where Minutes is the impedance for the network)

This property is specific to added-cost barriers and is limited to values that are greater than or equal to zero. It indicates how much network impedance is added when the barrier is traversed.

Network location fields

  • SourceID
  • SourceOID
  • PosAlong
  • SideOfEdge

Together, these four properties describe the point on the network where the object is located.

Learn more about network location fields

CurbApproach

The CurbApproach property specifies the direction of traffic that is affected by the barrier. If a barrier is on an edge and has its curb approach property set to

  • Either side of vehicle(0)—The barrier affects travel over the edge in both directions.
  • Right side of vehicle (1)—Only affects vehicles traveling such that the barrier is on their right side. Vehicles that traverse the same edge but approach the barrier on their left side are not affected by the barrier.
  • Left side of vehicle (2)—Vehicles are only affected if the barrier is on their left side during the approach. Vehicles that traverse the same edge but approach the barrier on their right side are not affected by the barrier.

Since junctions are points and don't have a side, barriers on junctions affect all vehicles regardless of the CurbApproach.

Input/Output fields of point barriers

Input/output field

Description

Status

This field is constrained by a domain of values, which are listed below (their coded values are shown in parentheses).

  • OK (0)—The network location is valid.
  • Not located (1)—The location on the network can't be determined.
  • Network element not located (2)—The network element that the network location is supposed to be on can't be found. This can occur when a network edge is deleted and the network location is not recalculated.

After a solve operation, the status can be modified using one of the following status values:

  • OK (0)—The network location was successfully evaluated.
  • Invalid field values (4)—The field values of the network location fall outside the analysis layer's coded or range domains. For example, a negative number may exist where positive numbers are required.

Line Barriers feature layer

Whereas point barriers snap to the nearest edge or junction, line barriers don't snap to the network; therefore, line barriers need to overlap edges and junctions to have any effect on them.

If the purpose of the line barrier is to cut across network elements and block travel where the barrier intersects edges and junctions, the Create Network Location Tool Create Network Location Tool is usually sufficient to accomplish this goal. Alternatively, if the purpose is to restrict a span along an edge, a line feature should be created in an edit session and loaded into the line barriers class. Editing tools, such as the Trace tool, give you more control than the Create Network Location tool and ensures that the line barrier is coincident with the network source features.

Line barrier properties

Input fields of line barriers

Input field

Description

ObjectID

The system-managed ID field.

Name

The name of the network analysis object.

BarrierType

Specifies whether the barrier restricts travel completely or scales the cost of traveling through it. There are two options:

  • Restriction (0)—Prohibits traversing through any part of the barrier. This is the default value.
  • Scaled Cost (1)—Scales the impedance of underlying edges by multiplying them by the value of the Attr_[Impedance] property. If edges are partially covered by the barrier, the impedance is apportioned and multiplied.

Attr_[Impedance]

(for instance, Attr_Minutes, where Minutes is the impedance for the network)

This property is specific to scaled-cost barriers. It is the factor by which the impedance of edges underlying the barrier are multiplied.

Values must be zero or greater; however, note that if you set this value to zero, solves will likely return nonsensical results since the underlying edge costs become free to traverse. Therefore, it is recommended to use values greater than zero.

Polygon Barriers feature class

Polygon barriers are similar to line barriers in that they don't snap to the network. Therefore, line barriers need to overlap edges and junctions to have any effect on them.

Polygon barrier properties

The properties for polygon barriers are identical to line barriers.

Input fields of polygon barriers

Input field

Description

ObjectID

The system-managed ID field.

Name

The name of the network analysis object.

BarrierType

Specifies whether the barrier restricts travel completely or scales the cost of traveling through it. There are two options:

  • Restriction (0)—Prohibits traversing through any part of the barrier. This is the default value.
  • Scaled Cost (1)—Scales the impedance of underlying edges by multiplying them by the value of the Attr_[Impedance] property. If edges are partially covered by the barrier, the impedance is apportioned and multiplied.

Attr_[Impedance]

(for instance, Attr_Minutes, where Minutes is the impedance for the network)

This property is specific to scaled-cost barriers. It is the factor by which the impedance of edges underlying the barrier are multiplied.

Values must be zero or greater; however, note that if you set this value to zero, solves will likely return nonsensical results since the underlying edge costs become free to traverse. Therefore, it is recommended to use values greater than zero.

Precedence of overlapping barriers

When two or more barriers overlap, ArcGIS Network Analyst follows a set of rules to guarantee consistent behavior and results:

Methods of creating barriers

This section highlights some of the options you have when creating barriers and cites some of their advantages and disadvantages to help you understand the different approaches and choose the one that best fits your needs.

Point barriers are associated with only one network element, even if the element is coincident with another element. Alternatively, line and polygon barriers affect by default all network elements they cover. Unintended consequences can occur if any of the network elements under line or polygon barriers are overlooked. For example, if the road on an overpass is being resurfaced and a restriction line barrier is added along the overpass, both the span of the overpass would be restricted as well as the road under the overpass. This is due to the line barrier touching both roads in two-dimensional space. Careful consideration should be given to how you create barriers.

Here are some other methods to consider in the overpass scenario above that accomplish the goal of blocking the overpass without blocking the road below:

Other methods of creating line barriers can be valuable in different scenarios as well. Here are a couple of examples:


2/2/2012