Thomas Kühn Thomas Kühn - 1 month ago 11
Python Question

IndexError with Basemap.contour() when using certain projections

I have run into problems when using

Basemap.contour
with certain projections. Based on the example given in the Basemap documentation, I created the following working code which produces the expected result. The example uses the 'tmerc' projection.

from mpl_toolkits.basemap import Basemap
import matplotlib.pyplot as plt
import numpy as np


m2 = Basemap(projection='tmerc',
lat_0=0, lon_0=3,
llcrnrlon=1.819757266426611,
llcrnrlat=41.583851612359275,
urcrnrlon=1.841589961763497,
urcrnrlat=41.598674173123)
##m2 = Basemap(projection='kav7',lon_0=0)

x = np.linspace(0, m2.urcrnrx, 100)
y = np.linspace(0, m2.urcrnry, 100)
xx, yy = np.meshgrid(x, y)
data = np.sin(xx/100)*np.cos(yy/100)

levels = np.linspace(-1,1,8)
m2.contour(xx, yy, data, levels)

plt.show()


However, if I switch to using the 'kav7' projection in the alternative
m2=Basemap
declaration (commented out in the example code), the code fails with the following error:

Traceback (most recent call last):
File "basemap_contour.py", line 20, in <module>
m2.contour(xx, yy, data, levels)
File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/mpl_toolkits/basemap/__init__.py", line 521, in with_transform
return plotfunc(self,x,y,data,*args,**kwargs)
File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/mpl_toolkits/basemap/__init__.py", line 3542, in contour
xx = x[x.shape[0]/2,:]
IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices


Note that this also happens when I define
lon
and
lat
values 'properly', the example was only chosen to be as short as possible. Does anybody know how to resolve this?

EDIT:

In case this is relevant, I'm using
python
version 3.5.3 on an
osx
Sierra machine. The
matplotlib
version is 2.0.0 and the
basemap
version is 1.0.7 .

Answer Source

This behavior is according to python3 integer division. Look for examples:

1) python3:

n=100
print (n/2, (n+1)/2)

Output: 50.0 50.5

2) For python 2.7 this code returns 50 50

Solutions:

1) manually update contour and contourf function of basemap with division for python3.

You have to write for integer n: n//2 which is apply division from python2.

2) or run your program with python2.

For more information read this section: Integer Division Doc