Introduction
This article demonstrates how to create a buffer with a variable width extracted from an attribute. In this example, we have GPS Trace(point data) and vehicle speed to create geometry demonstrating traffic flow and some congestion points during the drive. Using an attribute value as the buffer size is useful in other areas such as the diameter of pipes, the width of roads, fire station/emergency services catchment areas, green belts zoning, etc.
This article helps you to understand the following:
- Use CSV reader to generate points(natively) from Latitude & Longitude.
- Create variable buffer polygons
- Merge multiple individual polygons in an area.
- Display congested area on the map
The Bufferer transformer works with point, line and polygon features. Based on workflow when using line features, it may require to use Chopper to create line into individual points. For very complex objects the Generalizer can be used before buffering. When buffering Line features(as is), a LineJoiner and Snapper works great in various scenarios.
Downloads
Step by step instructions
1. Add GPS data
- Create a new blank workspace.
- Add a CSV Reader. Select journey.csv as the input dataset.
- Open Reader Parameter dialog. Set Field Names Line to 1
- Under Attributes > Attribute Definition > Switch to Manual. The CSV reader can support spatial data, for more details see the article( Converting from CSV to spatial features).
CSV Reader Parameter dialog
2. Visualize data in FME Data Inspector
Before we begin our workflow, let’s visualize the data.
- Add a Reprojector after the reader.
- Source Coordinate System LL84 (CSV reader is not coordinate system aware)
- Destination Coordinate System to ESPG: 27700 (British National Grid).
- Connect an Inspector to the Reprojector Reprojected output port. Run the workspace and view the results.
3.Create a buffer size from the attribute
The next step is to create an attribute that store a variable value for buffer size.
- Add an ExpressionEvaluator and create a new attribute _buffer with the expression:
(@Value(Speed)*4)
- Add a Bufferer transformer and set Buffer Amount to the attribute _buffer.
Bufferer transformer properties dialog
4. Create buffer area
Next step is to convert individual polygons (see below) created in the previous step to a single polygon.
NOTE: Bufferer transformer has a Group By option which can be used to dissolve buffers together. In this case, since the size of the buffer is not equal (variable size), the option will not work.
For this, we are going to use the Dissolver transformer to dissolve buffer into areas. Add Dissolver transformer and connect "Buffered" output port to it. Use all the default settings.
5. Visualize Data
Add an Inspector to Dissolver Area output port and view our final result in FME Data Inspector to verify that we can visualize some slow moving traffic spots during the drive. Complete workspace is located under the download section (buffer-from-attribute.fmwt)
NOTE: If required change Pen Color or Area Fill Color in FME Data Inspector to change Area and GPS point color.
Workspace
Result
The narrow areas on the map(highlighted) indicates congested areas.
Data Attribution
The data used in this example originates from Open Street Map and is available under the Open Database License.
