I have a topographic layer for a country. I want to select topographic lines of a certain altitude and transform the lines to polygons. All work fine except one thing: since the lines of the topographic map are not properly closed, the polygon are awful looking. See the image below.
As you can see, since the "topographic lines" are not joining the Country borders, it creates polygons that are closed from the perspective of the lines and not the country border. The blue line that I drew is the resulting polygon that I'd be looking for. So is there a way to tell QGIS to first close the topographic line to the border (small red line) and then to apply the line to polygon algorithm? Obviously, the small polygon would have the same treatment, but the image would be too cluttered if I drew another polygon to show what I'm looking for.