# [SciPy-User] Interpolating discrete values with splprep and splev

Matthew Cragun mcragun@gmail....
Thu Aug 19 16:08:49 CDT 2010

```I'm trying to fit a curve to some data points using splprep and splev.  Then
I would like to take a few discrete values and get the inerpolated values
back using the spline.

I am having a problem getting the correct y values out based on the spline
fit.  In my code below I would like a plot of the saex values against their
interpolated values, but can't get them out properly.

saex2 gives: [  37.27079882   55.12775627  106.64397968  142.10925103
158.67651813 176.27838508]
saey  gives:  [  1.76685635e-01  -3.89842080e+00  -5.48621169e+01
-1.42948834e+02  -2.03504645e+02  -2.83162159e+02]

Any thoughts?  Here is my code:

from numpy import arange, cos, linspace, pi, sin, random, r_, array
from scipy.interpolate import splprep, splev

# Original points
x = array([0,3,6,9,12,15])
y = array([0.530,0.546,0.586,0.658,0.727,0.801])

#Interpolated X locations
saex = array([2.484,3.671,7.063,9.354,10.408,11.515])

# spline parameters
s=3.0 # smoothness parameter
k=3 # spline order
nest=-1 # estimate of number of knots needed (-1 = maximal)

# find the knot points
tckp,u = splprep([x,y],s=s,k=k,nest=nest)

# evaluate spline
splinex,spliney= splev(linspace(0,1,400),tckp)

#evaluate at discrete points
saex2,saey= splev(saex,tckp,der=0)
print saex2
#plot
import pylab
#plot the data
data,=pylab.plot(x,y,'o',label='data')
#plot the fitted curve
fit,=pylab.plot(splinex,spliney,'r-',label='fit')
#plot the discrete locations
#sae,=pylab.plot(saex2,saey,'x',label='sae points')
pylab.legend()
pylab.xlabel('x')
pylab.ylabel('y')

pylab.savefig('splprep_demo.png')
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