I want to calculate a buffer around the worlds coastlines. To do so, I downloaded the GSHHS data version 2.3.7, imported it into qgis, used "polygons to lines" and then the MMQGIS Plugin to produce a 50 km buffer. It produces the buffer, but also a buffer around strange horizontal lines (see image). Also Europe and Asia shouldn't be filled.
I also see these lines when I use the buffer from the Geoprocessing tools, but they disappear when I'm zooming in. Do you have an idea what the problem might be?
I'm using MacOS 10.13.2 and QGis 2.18.15.
Buffer with strange lines:
GSHHS dataset: polygons:
GSHHS dataset: polygons to lines: