AreaOnAreaOverlayer

Performs an area-on-area overlay (intersection of polygons) so that all input areas are intersected against each other and resultant area features are created and output. The resultant areas can accumulate attribute from any overlapping polygons.

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Typical Uses

Deconstructing overlapping polygons to produce the intersections and differences
Comparing multiple datasets for area overlaps
Performing area calculations based on overlapping area data

How does it work?

The AreaOnAreaOverlayer takes in area features. All polygons are considered against each other, and where they overlap, new polygons are created that represent both the overlapping area and the original areas with the overlap removed.

The new polygons can retain attributes from all original relevant features (performing a spatial join), and a count of the number of overlaps encountered during the overlay. This count starts at 1 for all features, as they are considered to overlap themselves.

Aggregates can either be deaggregated before processing or rejected.

Examples

Example: Overlaying polygons from multiple datasets

In this example, coastal zone polygons are overlaid with a parcel dataset, perhaps to identify where parcels may be subject to restrictions due to proximity to the coast. The original data looks like this:

In this portion of a workspace, the input features - three polygons representing different setbacks from the coast and the parcel fabric - are routed into the Area input port.

In the parameters dialog, we make the following selections:

Aggregate Handling: Deaggregate, to ensure all areas will be processed
Attribute Accumulation: Accumulation Mode is set to Merge Incoming Attributes.

As the overlay is processed, new area features are created, with the merged attributes of their source features. Some parcels are divided if they partially intersect a zone polygon, and areas representing the space between the parcels that falls within a zone polygon are also created (in this example, generally representing roads and laneways).

As we are interested in the parcels themselves, we have added a TestFilter to route the parcel polygons by zone and discard the non-parcel features.

By creating test clauses that check for the existence of the ParcelA_ID attribute, non-parcel areas are output via the <Unfiltered> port, and discarded.

The results, viewed in the FME Data Inspector, with color coding by Zone.

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.

Spatial Transformers Comparison

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, splitting either where they intersect.
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 (including self-intersections), splitting features, and creating new geometry.
Clipper	Yes	Yes	No	No	Point Text Curve Area Surface 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 that Curve includes Lines, Arcs, and Paths. Area includes Polygons, Donuts, and Ellipses.

Configuration

Input Ports

Output Ports

Parameters

Group Processing

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.

Complete Groups

Select the point in processing at which groups are processed:

When All Features Received	This is the default behavior. Processing will only occur in this transformer once all input is present.
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.

General

Tolerance

The minimum distance between geometries in 2D before they are considered equal, in ground units. 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.

See Geometry Concepts > Tolerance.

Connect Z Mode

If applicable, select a method for handling z values.

Connect Z Mode

When viewed in 2D (ignoring Z), a path (which may define the border of a polygon) may appear to be closed as shown in the left figure below. This same path, when viewed in 3D, may appear to be open as shown in the right figure below.

To specify how (and if) paths should be closed in 3D, select one of the listed modes.

Mode	Description	Example
Extend	The Curve is extended so that all vertices are left at their original location.
Average	Subsequent vertices that are not connected, but share an x and a y value are combined into one vertex, whose Z value is the average of the original two.
First Wins	Subsequent vertices that are not connected, but share an x and a y value are combined into one vertex, whose Z value is taken from the first encountered vertex.
Last Wins	Subsequent vertices that are not connected, but share an x and a y value are combined into one vertex, whose Z value is taken from the last encountered vertex.
Ignore	Z values are ignored. No change is made to the way the nodes are connected.

Aggregate Handling

Choose how aggregate geometries are to be handled.

Deaggregate: Decompose aggregates into their individual components. With this setting, the transformer might output more features than were given as inputs.

Reject: Do not process aggregates and output them via the <Rejected> port.

Attribute Accumulation

Accumulation Mode

Specifies how attributes should be accumulated.

Drop Incoming Attributes: All incoming attributes are removed from the features

Merge Incoming Attributes: Merges all attributes from overlapping areas

Use Attributes From One Feature: Takes all attributes from one representative feature

Generate List

When enabled, adds a list attribute to the output features, retaining attribute values for multiple overlay matches. This allows later inspection of overlapping area attributes.

List Name

Enter a name for the list attribute.

Note List attributes are not accessible from the output schema in FME Workbench unless they are first processed using a transformer that operates on them, such as ListExploder or ListConcatenator. Alternatively, AttributeExposer can be used.

Add To List

All Attributes: All attributes will be added to the output Area features.

Selected Attributes: Enables the Selected Attributes parameter, where specific attributes may be chosen to be added.

Selected Attributes

Enabled when Add To List is set to Selected Attributes. Specify the attributes you wish to be added.

Editing Transformer Parameters

Transformer parameters can be set by directly entering values, using expressions, or referencing other elements in the workspace such as attribute values or user parameters. Various editors and context menus are available to assist. To see what is available, click beside the applicable parameter.

How to Set Parameter Values

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.

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.

When setting values - whether entered directly in a parameter or constructed using one of the editors - strings and expressions containing String, Math, Date/Time or FME Feature Functions will have those functions evaluated. Therefore, the names of these functions (in the form @<function_name>) should not be used as literal string values.

Content 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.
Date/Time Functions	Date and time functions are available in the Text Editor.
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.
Creating and Modifying User Parameters	Create your own editable parameters.

Dialog Options - Tables

Table Tools

Transformers with table-style parameters have additional tools for populating and manipulating values.

Row Reordering

Enabled once you have clicked on a row item. Choices include:

Add a row
Remove a row
Move current row up one
Move current row down one
Move current row to top
Move current row to bottom

Cut, Copy, and Paste

Enabled once you have clicked on a row item. Choices include:

Cut a row - delete and copy to clipboard
Copy a row to the clipboard
Paste a row from the clipboard

Cut, copy, and paste may be used within a transformer, or between transformers.

Filter

Start typing a string, and the matrix will only display rows matching those characters. Searches all columns. This only affects the display of attributes within the transformer - it does not alter which attributes are output.

Import

Import populates the table with a set of new attributes read from a dataset. Specific application varies between transformers.

Reset/Refresh

Generally resets the table to its initial state, and may provide additional options to remove invalid entries. Behavior varies between transformers.

Note: Not all tools are available in all transformers.

For more information, see Transformer Parameter Menu Options.

Reference

Processing Behavior	Group-Based
Feature Holding	Yes
Dependencies
Aliases
History

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Examples may contain information licensed under the Open Government Licence – Vancouver, Open Government Licence - British Columbia, and/or Open Government Licence – Canada.