[Matplotlib-users] Basemap: Coastlines intersecting with map boundary

[Joachim S. <sa...@gf...>]

Hi there,

in basemap coastlines are apparently (always?) drawn as closed polygons not exceeding the map boundary, i.e. when the coastline intersects with the map boundary the polygon is continued along the map boundary until the next intersection point. The somewhat annoying side effect of this is a map boundary that appears thicker where it crosses landmasses. See for instance on http://matplotlib.org/basemap/users/examples the example "Plot hurricane tracks from a shapefile" where clearly the upper and left map boundaries are thicker where they cross the western U.S. or northern Canada. Another example where this effect is particularly pronounced is the example "Draw great circle between NY and London" on the same page. The effect gets worse if running these examples without antialiasing. Apparently only the upper and left boundaries are affected, whereas the lower and right boundaries are plotted properly.

It looks to me as if this might simply be a bug due to the coastline not aligning perfectly with the map boundary, perhaps because of some roundoff error. Is there a way to avoid this? Wouldn't it be better to draw the coastlines not as closed polygons but as collections of line segments?

Cheers,
Joachim