Because there is no obvious way to merge two parallel lines into a single line in Adobe Illustrator, I needed to come up with a way to do this that doesn’t involve redrawing lots of lines with the pen tool.
So, here’s the process for converting two parallel lines into a single line.
- If needed, duplicate the layer so you preserve your original data.
- Make the parallel lines black.
- Merge the parallel lines graphically be increasing the stroke weight. This makes them look like a single fat line.
- Rasterize the line using Object->Rasterize. I don’t think the resolution should affect things much, but I go with 150 ppi.
- Use Image trace -> Line art option.
- Expand and style the line.
The result is a single line.
What to watch out for
I tried to automate this by creating an action. The result ended up with the line being outlined instead of resulting in a single line. I don’t know why the Image trace selection wasn’t preserved by the Action. As a result, I just do this manually.
Because I import the GIS data into Illustrator using MAPublisher, it retains attributes (like route number) that I may want to use. I usually move the original data to the bottom of the layers palette (below the base layer of my map) and query this data even though I don’t use it for presentation.