Square Buffers With FME

Using FME has a lot of advantages, however every now and then a function which comes as standard in Quantum or Arc is not available. Today I was trying to clip a raster image of an OS map tile to a buffer of the minimum bounding rectangle of my data, so that I have some context map surrounding my areas of interest.

I knocked together what I assumed would do the trick at the end of my workflow, using the Bounding Box Accumulator to get a bounding box, the Bufferer to this polygon buffer by 500 metres and a clipper to clip the map tile to the buffered polygon.

This resulted in a rounded map output, which although not incorrect, looked a bit odd to me. Tinkering with the options of the Bufferer did not yield any different buffer shapes, so I began to think creatively. Using square end caps, it is possible to create rectangular buffers around lines, so the objective became to explode the bounding box out into its individual lines.

I achieved this through the TopologyBuilder transformer which allows an area to be input and a line output, and by setting the Maximum Coords Per Line parameter to 2, four individual lines are created. These lines could then be buffered to 500 metres each using square end caps to create 4 overlapping rectangles which covered my desired buffer extent.

Blue lines are the individual buffers and the red polygon is the original bounding box.

The final stage was to aggregate these individual polygons into one larger shape, which can then be used as the clipper to cut out the section of OS map I required. The final workflow is more convoluted than the original plan, but it works without any problems to give me the square buffer I required.

Update: I noticed an error with this workbench where on larger areas the centre of the clipping polygon is not filled in, creating a doughnut. This is easily fixed by connecting the TopologyBuilder's area output into the aggregator, as the orginal bounding box will always cover the centre of the tile.