[SciPy-User] Problems evaluating interpolated function at new points

[Matteo Niccoli]

I am relatively new to Scipy, at least when it comes to interpolation. I
am trying to replicate some of the functionality of Matlab's interp2.

I have a distance function map (available at this Google drive location)
https://drive.google.com/open?id=0B6acq_amk5e3X0Q5UG1ya1VhSlE&authuser=0

Values are normalized in the range 0-1. Size is 200 rows by 300 columns.

I can load it up with this code snippet:
import numpy as np
dstnc1=np.load('dstnc.npy')


Coordinates are defined by the next snippet:
xmin = 0.
xmax = 9000.
ymin = 0.
ymax = 6000.
r1,c1 = dstnc1.shape
x = np.linspace(xmin,xmax,c1)
y = np.linspace(ymin, ymax,r1)

I have three map points defined by vectors xnew1, ynew1 with this snippet:
xnew1=[3700.540199,3845.940199,3983.240199]
ynew1=[1782.8611,1769.862,1694.862]

I check their location with respect to the distance map with this:
import matplotlib.pyplot as plt
fig = plt.figure(figsize=(20, 16))
ax = fig.add_subplot(1, 1, 1)
plt.imshow(dstnc1, cmap=my_cmap_r,vmin=0,vmax=0.3,
           extent=[0, 9000, 0, 6000], origin='upper')
plt.scatter(xnew1, ynew1, s=50, linewidths=0.15)
plt.show()

They plot in the correct location. Now I would like to extract the
distance value at those three points. I tried first interp2d.

from scipy.interpolate import interp2d
x1 = np.linspace(xmin,xmax,c1)
y1 = np.linspace(ymin,ymax,r1)
f = interp2d(x1, y1, dstnc1, kind='cubic')

but when I try to evaluate with:
test=f(xnew,ynew)

I get this error message:
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-299-d0f42e609b23> in <module>()
----> 1 test=f(xnew,ynew)

C:\...\AppData\Local\Continuum\Anaconda\lib\site-packages\scipy\interpolate\interpolate.pyc
in __call__(self, x, y, dx, dy)
    270                                 (self.y_min, self.y_max)))
    271
--> 272         z = fitpack.bisplev(x, y, self.tck, dx, dy)
    273         z = atleast_2d(z)
    274         z = transpose(z)

C:\...\AppData\Local\Continuum\Anaconda\lib\site-packages\scipy\interpolate\fitpack.pyc
in bisplev(x, y, tck, dx, dy)
   1027     z,ier = _fitpack._bispev(tx,ty,c,kx,ky,x,y,dx,dy)
   1028     if ier == 10:
-> 1029         raise ValueError("Invalid input data")
   1030     if ier:
   1031         raise TypeError("An error occurred")

ValueError: Invalid input data

If I try RectBivariateSpline:
from scipy.interpolate import RectBivariateSpline
x2 = np.linspace(xmin,xmax,r1)
y2 = np.linspace(ymin,ymax,c1)
f = RectBivariateSpline(x2, y2, dstnc1)

I get this error:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-302-d0f42e609b23> in <module>()
----> 1 test=f(xnew,ynew)

C:\...\AppData\Local\Continuum\Anaconda\lib\site-packages\scipy\interpolate\fitpack2.pyc
in __call__(self, x, y, mth, dx, dy, grid)
    643                 z,ier = dfitpack.bispev(tx,ty,c,kx,ky,x,y)
    644                 if not ier == 0:
--> 645                     raise ValueError("Error code returned by
bispev: %s" % ier)
    646         else:
    647             # standard Numpy broadcasting

ValueError: Error code returned by bispev: 10


Any suggestion as to whether I am using the wrong functions or the right
function with wrong syntax, and how I may fix it is appreciated. Thank you