Overview
Extracting data using area masks in FME requires the use of both a raster and a vector dataset and the Clipper transformer. In this tutorial, you will be selecting raster data inside of polygon features; however, this tutorial can easily be modified to select vector features within a search envelope.
Downloads
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 are interested in clipping the extent of an existing DEM to the boundary of your city based on a polygon feature. This exercise will use a municipal land boundary as a search envelope; however, you can also use multiple polygon features.
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 VancouverLandBoundary table from the Table List.
4. Add a Canadian Digital Elevation Data (CDED) Reader
- Start typing “CDED” without anything selected on the canvas to bring up the list of FME Transformers in the Quick Add Search. Then select the Canadian Digital Elevation Data 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.
- Click on the Dataset ellipsis and navigate to the DEM-FULL.dem found in the ElevationModel folder of the provided sample data. Once selected, click Open, then click OK to add the reader to the canvas.
5. Reproject the Data
In this scenario, the provided datasets do not share the same coordinate system (one is currently in LL83 and the other is in UTM83-10). As a result, you cannot continue without reprojecting the DEM to a linear coordinate system. For more information on coordinate systems in FME, see the Using Coordinate Systems within FME article.
- Add a Reprojector to the canvas by typing “Reprojector” to bring up the list of FME Transformers in the Quick Add Search. Select the Reprojector from the list of Transformers by double-clicking or by using the arrow keys and the Enter key to add it to the canvas.
- Once the transformer has been added, connect the CDED Reader to the Reprojector.
6. Set the Destination Coordinate System
- Once the Reprojector connection has been made, double-click the Reprojector or click on the gear icon to open the transformer parameters dialog.
Notice the Source Coordinate System is automatically read from the feature. The only parameter that needs to be changed in this scenario is the Destination Coordinate System.
- Set the Destination Coordinate System to UTM83-10 by typing “UTM83-10” in the text box or by selecting it from the dropdown list and choosing “More Coord Systems”.
For more tips on Coordinate System Transformation, see the Desktop Basic Course Manual.
7. Add a Clipper
In FME, the Clipper performs a geometric clipping operation similar to a cookie cutter. With that in mind, you will clip the areas outside of the VancouverLandBoundary polygon because you are only interested in the DEM for the Vancouver area. For a more detailed description of how the Clipper works, see the Example Using the Clipper Transformer article.
- Connect the VancouverLandBoundary reader to the Clipper port.
- Next, Connect the Reprojector:Reprojected port to the Clipee port.
8. Add an Inspector
The Clipper:Inside port will output Clippee features that are completely within the Clipper, and Clippee features that intersect the Clipper which were broken into pieces. Conversely, the Clipper:Outside port will output Clippee features that are completely outside of the Clipper. You can learn more technical details, see the Clipper Documentation.
- Since you are only interested in the area within the clip boundary, add an Inspector and connect it to the Clipper:Inside port. This will also automatically open the output in the FME Data Inspector after running a successful translation
9. Add a PostGIS Raster Writer
- Similar to adding a reader, type “PostGIS Raster” then select the PostGIS Raster format from the list of Writers.
- Set the Writer Connection to the PostGIS Training Database (same as the connection listed in step 3) and set the Table Definition to Automatic, then click OK to add the writer to the canvas and the parameters.
An Automatic Attribute Definition is when Workbench automatically defines the list of attributes, depending on which Reader feature types are connected. Further, the list of attributes on the Writer feature type will update automatically whenever attributes are changed (i.e. renamed, removed, etc.) in the workspace. For more information on User Attributes and Attribute Definitions, see the Documentation.
- After the Writer Feature Type is added the Writer parameters dialog will open. By default, the table will be placed in the “public” schema.
- In the General section, set the Table Name to VanDEMClip.
- In the Table section, ensure the Feature Operation is set to Insert and set the Table Handling to Drop and Create, then click OK.
- Once the PostGIS writer appears on the canvas, connect it to the Clipper:Inside port to complete the workspace.
10. Run the workspace
- Run the workspace by clicking the run button on the toolbar, or by using Run > Run Translation on the menu bar.
After running your workspace, the output will open in the FME Data Inspector - you will be viewing a digital elevation model of the Vancouver area.
Results
Input
Output
Transformers
- Clipper - Performs a geometric clipping operation (sometimes called a cookie cutter).
- Inspector- Sends features to the FME Data Inspector for display.
- Reprojector - Reprojects feature coordinates from one coordinate system to another.
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.
