Numerical precision issue in rotate_vector

[rewquey]

Hi,

I ran into a numerical precision issue when using the rotate_vector function with 32bit floats as input data. In this case, the lat/lon increments used to determine the relative rotation between map projection and the lat/lon grid is chosen too small. The result was a cristalline structure in the plotted wind barbs which patches of barbs pointing in the exact same direction.

basemap/lib/mpl_toolkits/basemap/__init__.py

Line 3118 in decfa95

dc = 1E-5 * np.exp(theta*1j)

Converting the input to 64bit floats solves the issue for me, as did an increase of the increment to 1.0e-4.

Best regards,
Clemens