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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.
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.
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.
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.
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.
For more tips on Coordinate System Transformation, see the Desktop Basic Course Manual.
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.
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.
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 running your workspace, the output will open in the FME Data Inspector - you will be viewing a digital elevation model of the Vancouver area.
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.
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