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 have all the attributes of the original and any overlapping polygons.
- 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.
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 segment 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 Data Inspector, with color coding by Zone.
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.
Can Merge Attributes
Counts Related Features
|NeighborFinder||Yes||In some cases||No||Yes||
* NOTE: Curve includes Lines, Arcs, and Paths. Area includes Polygons, Donuts, and Ellipses.
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.
This transformer accepts area features as input. Area features are expected to be non-self-intersecting.
Tip: If input areas are self-intersecting, first use the GeometryValidator’s Self-Intersection in 2D rule to remove self-intersections.
Area features resulting from the intersection of input areas are output through this port.
Geometries and attributes that are not part of the overlayed output. If the remnant feature has a geometry, this geometry was not used in any output area. If the remnant feature does not have a geometry, this feature contains attributes that were not used in any output area.
Features with invalid geometries will be rejected and output via this port.
Rejected features will have an fme_rejection_code attribute with one of the following values:
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.
|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.|
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.
Considerations 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.
|Tolerance||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.|
|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.
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.
|Overlap Count Attribute||The Overlap Count Attribute holds the number of features that the resultant feature overlapped, which will be at least one.|
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
When enabled, adds a list attribute to the output features, retaining attribute values for multiple overlay matches.
If specified, a list of the attributes of each area that overlapped an output area is made. This allows later inspection of overlapping area attributes.
Note: List attributes are not accessible from the output schema in 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
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.
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.
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.
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.
Set values depending on one or more test conditions that either pass or fail.
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.
|These functions manipulate and format strings.|
|A set of control characters is available in the Text Editor.|
|Math functions are available in both editors.|
|These operators are available in the Arithmetic Editor.|
|These return primarily feature-specific values.|
|FME and workspace-specific parameters may be used.|
|Working with User Parameters||Create your own editable parameters.|
|FME Licensing Level||FME Professional Edition and above|
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Examples may contain information licensed under the Open Government Licence – Vancouver