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Overview
In this exercise, you will learn how to perform a union between overlapping polygon features using the AreaOnAreaOverlayer. The AreaOnAreaOverlayer is typically used for three main operations: (1) deconstruct overlapping polygons to produce the intersections and differences, (2) compare multiple datasets for area overlaps, and (3) area calculations based on overlapping area data.
Downloads
polygon-in-polygon-overlay.fmwt
Note: Since you are working on a public facing database, tables can occasionally be overwritten and/or modified. If your translation produces unexpected results, see the Resetting the PostGIS Training Database article. Alternatively, you can download the provided file(s) and replace the PostGIS reader with a reader of the appropriate format.
Exercise
In this scenario, you will be performing area calculations based on overlapping historic zones and neighborhood polygons. This will allow you to identify which historic areas belong to each neighborhood and the total area of each historic zone.
Instructions
1. Start FME Workbench
- If it isn’t open already, launch FME Workbench.
2. Create a New Workspace
- In the Create Workspace part of the Start page, select the option to Create a New Workspace.
3. Add a PostGIS Reader
- Start typing “PostGIS” without anything selected on the canvas, then select the PostGIS format from the list of Readers by double-clicking or by using the arrow keys and the Enter key to add it to the canvas.
- Next, select the PostGIS Training Database from the Reader Connection list. If you have not already set up the database connection select Add Database Connection and enter the following parameters:
- Host: postgis.train.safe.com
- Port: 5432
- Database: fmedata
- Username: fmedata
- Password: fmedata
- Next, open the Reader Parameters and select the Neighborhoods and HistoricZones tables from the Table List.
4. Add an AreaOnAreaOverlayer Transformer
- Select the Neighborhoods and HistoricZones Reader Feature Types and type “AreaOnAreaOverlayer” then select the AreaOnAreaOverlayer from the list of Transformers in the Quick Add Search. This will automatically make the connection from the Neighborhood and HistoricZones Readers to the AreaOnAreaOverlayer. The Neighborhoods and HistoricZones Reader Feature Types should be connected to the AreaOnAreaOverlayer:Area port.
5. Set the Attribute Accumulation Mode
The AreaOnAreaOverlayer transformer will perform an area-on-area overlay for intersecting polygons - 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. For more technical details about the AreaOnAreaOverlayer. For more technical details about the AreaOnAreaOverlayer, see the Documentation.
- Once the AreaOnAreaOverlayer has been added, double-click the AreaOnAreaOverlayer or click on the gear icon to open the transformer parameters.
- Set the Attribute Accumulation Mode to Merge Incoming Attributes. This will allow you to join the attributes from both the HistoricZones and Neighborhood polygons where features overlap.
6. Calculate the Area of Each Polygon
- Add an AreaCalculator after the AreaOnAreaOverlayer - this will automatically calculate the area of each polygon that is created by the AreaOnAreaOverlayer.
- Connect the AreaCalculator to the AreaOnAreaOverlayer:Area port.
Note: area is calculated in square map units. For a more in-depth tutorial on area calculations, see the Area Measurements with the AreaCalculator article.
7. Add an Inspector
- Next, add an Inspector and connect it to the AreaOnAreaOverlayer:Output port.
8. Add a GeoJSON (Geographic JavaScript Object Notation) Writer
- Similar to adding a reader, type “GeoJSON” then select the GeoJSON format from the list of Writers.
- Click on the Dataset ellipsis to specify where to save your data and name the file: HistoricAreas (for example, C:\Users\Documents\FME\HistoricAreas\HistoricAreas.json)
- Set the Layer Definition to Automatic to ensure all attributes that are created or renamed will be written to the GeoJSON file.
- Connect the GeoJSON writer to the AreaOnAreaOverlayer:Output port.
9. Run the Workspace
- Run the workspace by clicking the run button on the toolbar, or by using Run > Run Translation on the menu bar. Since you attached an Inspector to the final transformer, the output dataset will automatically be opened in the FME Data Inspector.
Results
Input
Output
Transformers
- AreaCalculator - Calculates the area of a polygonal object and stores the value in an attribute.
- 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.
- Inspector - Sends features to the FME Data Inspector for display.
Data Attribution
Data used in this tutorial originates from open data made available by the City of Vancouver, British Columbia. It contains information licensed under the Open Government License - Vancouver.
