TopologyBuilder

Computes topology on input point, line, and/or area features, and outputs significant nodes, edges, and faces with attributes describing topological relationships.

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Typical Uses

How does it work?

The TopologyBuilder computes topology on input point, line, and/or area features.

Topologically significant nodes and lines are computed using all input features and output with additional attributes which describe the topological relationships. The TopologyBuilder will intersect the inputs before building topology, provided the Generate From advanced parameter is set to End Nodes and Intersections. It takes any data and constructs the resulting topology after computing any intersections present in the input data.

It outputs the significant Nodes (points) and Edges (lines) with attributes describing their topological relationships. Faces (areas) are output with information about the Edges which form them.

This transformer is typically used to determine topological relationships to aid in decision making in later transformers.

* Red line indicates direction of flow.

 

ClosedExample: Computing topology on street centerlines

In this example, we start with a set of whole street centerlines, which are not split into individual features at intersections.

The features are routed into a TopologyBuilder.

The default parameter settings will produce correct and useful results. It is worth noting that the default setting of Generate From is left as End Nodes and Intersections, and so will create intersections where lines cross.

At intersections, the lines are split and nodes are created. Nodes receive attributes about connecting edge (line) features, including identifier and angle.

The Edges (line segments) also receive attributes about their relationship with adjacent features.

ClosedExample: Computing topology on areas

In this example, we compute topology on a set of neighborhood polygons.

The polygons are routed into a TopologyBuilder.

Edges and topologically significant nodes are added, and list attributes created containing topological relationship information, as shown for this selected Face (polygon).

Usage Notes

Choosing a Spatial Transformer

Many transformers can assess spatial relationships and perform spatial joins - analyzing topology, merging attributes, and sometimes modifying geometry. Generally, choosing the one that is most specific to the task you need to accomplish will provide the optimal performance results. If there is more than one way to do it (which is frequently the case), time spent on performance testing alternate methods may be worthwhile.

To correctly analyze spatial relationships, all features should be in the same coordinate system. The Reprojector may be useful for reprojecting features within the workspace.

ClosedSpatial Transformers Comparison Matrix

Transformer

Can Merge Attributes

Alters Geometry

Counts Related Features

Creates List

Supported Types*

Recommended For
SpatialFilter Yes No No No
  • Point
  • Text
  • Curve
  • Area
  • Testing for the existence of spatial relationships between two sets of features, and routing them according to whether they pass or fail the test(s).
SpatialRelator Yes No Yes Yes
  • Point
  • Text
  • Curve
  • Area
  • Identifying the nature of spatial relationships between two sets of features.
AreaOnAreaOverlayer Yes Yes Yes Yes
  • Area
  • Finding polygon overlaps and extracting them into new geometry.
LineOnAreaOverlayer Yes Yes Yes Yes
  • Curve and Area
  • Finding intersections between lines and polygons, and splitting the lines where they intersect with the polygons.
LineOnLineOverlayer Yes Yes Yes Yes
  • Curve
  • Finding intersections between line features, splitting them, and generating new line geometry as well as points representing the intersections.
PointOnAreaOverlayer Yes No Yes Yes
  • Point and Area
  • Text and Area
  • Identifying points that fall within polygons, and merging attributes between them
PointOnLineOverlayer Yes Yes Yes Yes
  • Point and Curve
  • Text and Curve
  • Identifying where points fall on lines, and splitting the lines into new geometry.
PointOnPointOverlayer Yes No Yes Yes
  • Point
  • Text
  • Identifying points in the same location (within a tolerance), and merging attributes between them.
Intersector Yes Yes Yes Yes
  • Point
  • Text
  • Curve
  • Area
  • Finding intersections between all input features, regardless of geometry (optionally including self-intersections), splitting features, and creating new geometry.
Clipper Yes Yes No No
  • Point
  • Text
  • Curve
  • Area
  • Solids
  • Raster
  • Point Cloud
  • Comparing features against a set of Clipper features, and splitting the features at or along the Clipper boundaries. Outputs both new and untouched geometry, identified as either Inside or Outside the Clipper.
NeighborFinder Yes In some cases No Yes
  • Point
  • Text
  • Curve
  • Area
  • Identifying the nearest other feature(s) to each feature being considered, either in another set of features or within the same feature set.
TopologyBuilder Yes Yes No Yes
  • Point
  • Text
  • Curve
  • Area
  • Analyzing spatial relationships between features to compute topology, splitting features and creating new geometry representing topologically significant nodes, edges, and faces, with associated attributes.

* NOTE: Curve includes Lines, Arcs, and Paths. Area includes Polygons, Donuts, and Ellipses.

ClosedThe Effect of Spatial Transformer Choice on Performance

Spatial analysis can be processing-intensive, particularly when a large number of features are involved. If you would like to tune the performance of your workspace, this is a good place to start.

When there are multiple ways to configure a workspace to reach the same goal, it is often best to choose the transformer most specifically suited to your task. As an example, when comparing address points to building polygons, there are a few ways to approach it.

The first example, using a SpatialFilter to test whether or not points fall inside polygons, produces the correct result. But the SpatialFilter is a fairly complex transformer, able to test for multiple conditions and accept a wide range of geometry types. It isn’t optimized for the specific spatial relationship we are analyzing here.

With a SpatialFilter:

The second example uses a PointOnAreaOverlayer, followed by a Tester. The features output are the same as in the first method, but the transformer is optimized for this specific task. The difference in processing time is substantial - from 54.3 seconds in the first configuration, down to 13.7 seconds in the second one.

With a PointOnAreaOverlayer and a Tester:

If performance is an issue in your workspace, look for alternative methods, guided by geometry.

Configuration

Input Ports

ClosedInput

The transformer accepts node, line, and area features. Aggregate and Multi features are invalid and will be rejected, unless Deaggregate Input is set to Yes.

Output Ports

Each topological primitive is output with attributes describing its topological relationships. Unlike most transformers, you cannot name these attributes.

ClosedNode

Topologically significant nodes (point geometries) are output through this port.

Attribute Description

_node_number

A unique identifier for each node.

_node_angle{}

List attribute describing the topological relationship of each edge connected to the node. The edges in this list are ordered counterclockwise.

_node_angle{}.fme_arc_id

The ID of the edge connected to this node. The magnitude of the ID corresponds to the _edge_id of the connected edge. If the ID is positive, the start of the edge is connected to the node. If the ID is negative, the end of the edge is connected to the node.

_node_angle{}.fme_arc_angle

The angle of the tangent line of the edge at the node.
ClosedEdge

Topologically significant edges (curve geometries) are output via this port.

Attribute Description

_edge_id

A unique identifier for each edge.

_right_face

The _face_id of the face to the right of this edge.

_left_face

The _face_id of the face to the left of this edge.

_right_edge

The _edge_id of the edge that is found when traveling along this edge and turning right at the _to_node. If the ID is positive, the right edge is going away from the _to_node. If the ID is negative, the right edge is coming towards the _to_node.
_left_edge The _edge_id of the edge that is found when traveling backwards along this edge and turning right at the _from_node. If the ID is positive, the left edge is going away from the _from_node. If the ID is negative, the left edge is coming towards the _from_node.
_from_node The _node_number of the node at the start of this edge.
_to_node The _node_number of the node at the end of this edge.
_faces Comma-separated list of the IDs of the faces this edge borders. The magnitude of the ID corresponds to the _face_id of the bordered face. If the ID is positive, the face boundary contains this edge. If the ID is negative, the face boundary contains the reverse of this edge.
ClosedFace

Topologically significant faces (area geometries) are output through this port.

Attribute Description

_face_id

A unique identifier for each face.

_faces

Comma-separated list of _face_ids for each face this face shares an edge with.

_perimeter

The length of the outline of the face in 2D.

_area

The area of the face in 2D.
_edges

Comma-separated list of the IDs of the edges that compose this face. A 0 entry separates edges of different boundaries. The magnitude of the ID corresponds to the _edge_id of the contained edge. If the ID is positive, the face boundary contains this edge. If the ID is negative, the face boundary contains the reverse of this edge. The edge IDs are in the same order that the edges are used to make the boundary of the face.

Note: The Provide Bounding Arcs on Output Polygons parameter changes the behaviour of the _edges attribute. For more information, see the section below that describes this parameter.
ClosedUniverse

The output area, by subtraction, represents everything not covered by the faces.

Attribute Description

_face_id

The unique face identifier of the universe is always 0.

_perimeter

The length of the outline of the universe in 2D.

_area

The area of the universe in 2D.
_edges

Comma-separated list of the IDs of the edges that compose the universe. A 0 entry separates edges of different boundaries. The magnitude of the ID corresponds to the _edge_id of the contained edge. If the ID is positive, the universe contains this edge. If the ID is negative, the universe contains the reverse of this edge. The edge IDs are in the same order that the edges are used to make the boundary of the universe.

Note: The Provide Bounding Arcs on Output Polygons parameter changes the behaviour of the _edges attribute. For more information, see the section below that describes this parameter.
Closed<Rejected>

Features without point, text, curve, or area geometries are output through this port along with an additional attribute, fme_rejection_code, to indicate the reason for rejection.

Rejected Feature Handling: can be set to either terminate the translation or continue running when it encounters a rejected feature. This setting is available both as a default FME option and as a workspace parameter.

Parameters

ClosedTransformer
Group By The default behavior is to use the entire set of features as the group. This option allows you to select attributes that define which groups to form.
Group By Mode

Process At End (Blocking): This is the default behavior. Processing will only occur in this transformer once all input is present.

Process When Group Changes (Advanced): This transformer will process input groups in order. Changes of the value of the Group By parameter on the input stream will trigger processing on the currently accumulating group. This may improve overall speed (particularly with multiple, equally-sized groups), but could cause undesired behavior if input groups are not truly ordered.

ClosedConsiderations for Using Group By

There are two typical reasons for using Process When Group Changes (Advanced) . The first is incoming data that is intended to be processed in groups (and is already so ordered). In this case, the structure dictates Group By usage - not performance considerations.

The second possible reason is potential performance gains.

Performance gains are most likely when the data is already sorted (or read using a SQL ORDER BY statement) since less work is required of FME. If the data needs ordering, it can be sorted in the workspace (though the added processing overhead may negate any gains).

Sorting becomes more difficult according to the number of data streams. Multiple streams of data could be almost impossible to sort into the correct order, since all features matching a Group By value need to arrive before any features (of any feature type or dataset) belonging to the next group. In this case, using Group By with Process At End (Blocking) may be the equivalent and simpler approach.

Note: Multiple feature types and features from multiple datasets will not generally naturally occur in the correct order.

As with many scenarios, testing different approaches in your workspace with your data is the only definitive way to identify performance gains.
ClosedParameters
Maximum Coords Per Edge The number indicates the maximum length to output any edge. If any line contains more than this number of coordinates, it will be broken into pieces which are output separately, each with their own edge IDs, and correctly noded. A value of 0 indicates an unlimited number of coordinates per edge.
Unify Attributes From Overlapping Input

If Yes, the transformer enters a mode where no collinear edges or overlapping nodes are output at all, whether they came from source linear features or from the borders of source area features or input points, or calculated as intersection points.

In this mode, all output edges or nodes which were overlapping with at least one direct input will contain a list attribute (_overlapping_input_data) with information about each input with which it was overlapping. This keyword sets the fieldname of the list attribute to contain all the attributes (except geometry attributes that start with fme_) from all of the input lines or points that were overlapping with the final output edge or node.

A side effect of this option is that only edges that form part of a face boundary will be considered in the calculation of _left_edge and _right_edge. (All edges originating only from line input will have their own ID supplied as their left edge ID, and the negation of this as their right edge ID.)
Provide Bounding Arcs on Output Faces

If All:

  • All of the edges from the output face will be listed in the comma-separated _edges attribute. A 0 entry separates edges of different boundaries. An example _edges attribute is "1,2,-3,0,5,0,7".
  • If the universe consists of multiple disjoint areas, it will be returned as a single feature with a MultiArea geometry.

If First Per Boundary:

  • The first edge of each boundary will be in the _edges list attribute. An example list is _edges{0} = 1, _edges{1} = 5, _edges{2} = 7.
  • The value will always be positive, regardless of the direction of the edge.
  • If the universe consists of multiple disjoint areas, a different feature will be returned for each area.

If First Per Outer and Disjoint Inner Boundary:

  • The first edge of each outer and disjoint inner boundary will be in the _edges list attribute. If an inner boundary is topologically connected to the outer boundary, it is not considered disjoint. If two inner boundaries are topologically connected to each other, only one is considered disjoint. An example list is _edges{0} = 1, _edges{1} = 5.
  • The value will always be positive, regardless of the direction of the edge.
  • If the universe consists of multiple disjoint areas, a different feature will be returned for each area.
Aggregate Handling

Deaggregate: All input aggregates will be deaggregated, and each split part will be processed independently. With this setting, the transformer might output more features than were given as inputs.

Reject: All input aggregates will be rejected.

ClosedList Accumulation
Generate List From Input Nodes Gives the option to create a list on the Face and Edge output ports that relate to information from the input nodes.
Generate List From Input Edges Gives the option to create a list on the Node and Face output ports that relate to information from the input curves.
Generate List From Input Faces Gives the option to create a list on the Node and Edge output ports that relate to information from the input faces.
ClosedAdvanced Parameters
Preserve Internal Edges If Yes, coordinate "cycles" within a face are allowable and will be preserved. A "cycle" is an edge that occurs twice in the same face's boundary (once in each direction); the edge's ID will appear twice in that face's edge list, positive in one instance and negative in the other.
Generate From

End Nodes and Collinear Segments: Topology nodes will then be placed at end nodes, and at the ends of collinear segments whose end nodes are present in both input geometries.

End Nodes and Intersections (default): New points will be added to the geometries where intersections occur, if necessary, prior to constructing the topology from all shared nodes and end nodes.

End Nodes Only: The Topology will be generated only from existing end nodes.
Tolerance

This parameter is only used when Generate From is set to End Nodes and Intersections.

The minimum distance between geometries in 2D before they are considered equal, in ground units. If the tolerance is None, the geometries must be exactly identical to be considered equal. If the tolerance is Automatic, a tolerance will be automatically computed based on the location of the input geometries. Additionally, a custom tolerance may be used.

Editing Transformer Parameters

Using a set of menu options, transformer parameters can be assigned by referencing other elements in the workspace. More advanced functions, such as an advanced editor and an arithmetic editor, are also available in some transformers. To access a menu of these options, click beside the applicable parameter. For more information, see Transformer Parameter Menu Options.

Defining Values

There are several ways to define a value for use in a Transformer. The simplest is to simply type in a value or string, which can include functions of various types such as attribute references, math and string functions, and workspace parameters. There are a number of tools and shortcuts that can assist in constructing values, generally available from the drop-down context menu adjacent to the value field.

ClosedHow to Set Parameter Values

Using the Text Editor

The Text Editor provides a convenient way to construct text strings (including regular expressions) from various data sources, such as attributes, parameters, and constants, where the result is used directly inside a parameter.

Text Editor

Using the Arithmetic Editor

The Arithmetic Editor provides a convenient way to construct math expressions from various data sources, such as attributes, parameters, and feature functions, where the result is used directly inside a parameter.

Arithmetic Editor

Conditional Values

Set values depending on one or more test conditions that either pass or fail.

Parameter Condition Definition Dialog

Content

Expressions and strings can include a number of functions, characters, parameters, and more - whether entered directly in a parameter or constructed using one of the editors.

ClosedContent Types

String Functions

These functions manipulate and format strings.

Special Characters

A set of control characters is available in the Text Editor.

Math Functions

Math functions are available in both editors.

Math Operators

These operators are available in the Arithmetic Editor.

FME Feature Functions

These return primarily feature-specific values.

FME Parameters

FME and workspace-specific parameters may be used.
Working with User Parameters Create your own editable parameters.

Reference

ClosedRelated Transformers

AreaOnAreaOverlayer

Bufferer

Clipper

Intersector

LineOnAreaOverlayer

LineOnLineOverlayer

NeighborFinder

PointOnAreaOverlayer

PointOnLineOverlayer

PointOnPointOverlayer

SpatialFilter

SpatialRelator

Processing Behavior

Group-Based

Feature Holding

Yes
Dependencies  
FME Licensing Level FME Base Edition and above
Aliases Topologizer
History  
Categories

Spatial Analysis

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Examples may contain information licensed under the Open Government Licence – Vancouver