I'm creating maps using the Generic Mapping Tools (GMT) from the University of Hawaii. I get weird vertical and horizontal lines using pscoast. This is the command to reproduce the problem:
gmt pscoast -JM18c -R43/62/35/49 -G50/255/50 -Df -P > caspiansea.ps convert -density 300 caspiansea.ps caspiansea.png The lines are already present in the ps file, so the last step (convert) does not introduce the problem. It happens across all areas, the caspian sea area is just an example.
This is the image that's created by those commands: 
Version: GMT 5.2.1 (the most recent version provided by Macports) on Mac OS X 10.11.6.