Creating edge lines between specific polygons

Question

I have a polygon layer containing habitat types. It looks something like this: where dark green is forest, yellow are open lands and the other signatures different other habitat types. In order to get something like a forest edge-line, I'd like to create a line shapefile representing those linear structures where forest and open land touch each other. I could of course use the polygons to lines tool in QGIS and then select the forest edge lines by hand - however, my data are too large for this procedure.

Any suggestions on how to approach this problem in QGIS or R? Please note that - unfortunately - I am not familiar with python at all.

Answer 1

In the end, I followed this workflow:

first step:

• dissolve original polygons using fields
• polygons to lines on the resulting layer
• delete all non-forest lines

second step in order to delete the outlines where the original layer extend ends:

• dissolve original polygons without using fields
• tiny negative buffer on the resulting layer
• clip lines layer with buffer layer

The result looks like this: