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

[Oleksandr Huziy]

Hi All:

This is not the answer to Matteo's question but probably a report on
unclear documentation here:

http://docs.scipy.org/doc/scipy-0.14.0/reference/generated/scipy.interpolate.interp2d.html

I am confused by the returns part, it says

"""
Returns:

*values_x* : ndarray, shape xi.shape[:-1] + values.shape[ndim:]

Interpolated values at input coordinates.

"""

But in the examples, the returned value f is used as a callable...

"""

Now use the obtained interpolation function and plot the result:
>>>

>>> xnew = np.arange(-5.01, 5.01, 1e-2)>>> ynew = np.arange(-5.01, 5.01, 1e-2)>>> znew = f(xnew, ynew)>>> plt.plot(x, z[0, :], 'ro-', xnew, znew[0, :], 'b-')>>> plt.show()

"""

Probably returns section corresponds to this version of scipy, but the
examples are from the later version, which has probably confused Matteo.


Cheers



2015-05-14 17:15 GMT-04:00 Matteo Niccoli <matteo at mycarta.ca>:

> 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
>
>
>
> _______________________________________________
> SciPy-User mailing list
> SciPy-User at scipy.org
> http://mail.scipy.org/mailman/listinfo/scipy-user
>



-- 
Sasha
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.scipy.org/pipermail/scipy-user/attachments/20150514/f915e221/attachment.html>