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: enter image description here

Version: GMT 5.2.1 (the most recent version provided by Macports) on Mac OS X 10.11.6.


I could fix the problem. I assumed that the lines were already in the .ps file. This was a mistake. Double-clicking the .ps file on MacOS opens Preview that on-the-fly converts PS to PDF and in that PDF the strange lines were visible. The same thing seems to happen when using convert from ImageMagick.

Using gmt psconvert though does not create those weird lines. So, there must be something strange happening in converting from PS to something else when not using the GMT tools.

Why did I use convert in the first place? TBH I don't know ...

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