[PYTHON] Interpolate 2D data with scipy.interpolate.griddata

What you want to do

How to use scipy.interpolate.griddata

Suppose you have x, y coordinates, and value data at those coordinates.

example.py

print(sample_df)
# >>>
#      X   Y  value
# 0    0   0     11
# 1    0   2     17
# 2    0   4     13
# 3    0   6     12
# 4    0   8     26
# ...
# 32  10   4      8
# 33  10   6     35
# 34  10   8     30
# 35  10  10     17

Plot only x, y coordinates

example.py

import matplotlib.pyplot as plt
import pandas as pd

fig = plt.figure()
ax1 = fig.add_subplot(111)
ax1.scatter(sample_df['X'], sample_df['Y'])
plt.show()

Use scipy.interpolate.griddata to interpolate the blanks in coordinates and values. Check the results on the contour map.

Interpolated data

environment

Mac OS Python 3.8.1

matplotlib 3.3.2 numpy 1.19.2 pandas 1.1.3 scipy 1.6.0

pip install matplotlib numpy pandas scipy

Details

Sample data

Use the following as sample data.

sample_data.py

import itertools

import numpy as np
import pandas as pd

# sample data
x_coord_range = [i for i in range(0, 11, 2)]
y_coord_range = [i for i in range(0, 11, 2)]

xy_coord = list(itertools.product(x_coord_range, y_coord_range))
values = np.random.randint(1, 50, len(xy_coord))

sample_df = pd.DataFrame()
sample_df['X'] = [xy[0] for xy in xy_coord]
sample_df['Y'] = [xy[1] for xy in xy_coord]
sample_df['value'] = values

print(sample_df)
# >>>
#      X   Y  value
# 0    0   0     11
# 1    0   2     17
# 2    0   4     13
# 3    0   6     12
# 4    0   8     26
# ...
# 32  10   4      8
# 33  10   6     35
# 34  10   8     30
# 35  10  10     17

code

It is assumed that the contents of sample_df are unknown

import itertools  #Used only for sample data creation

import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
from scipy.interpolate import griddata

#Sample data creation location omitted

#Get data range
x_min, x_max = sample_df['X'].min(), sample_df['X'].max()
y_min, y_max = sample_df['Y'].min(), sample_df['Y'].max()

#Create an array with new coordinates in the acquired data range
new_x_coord = np.linspace(x_min, x_max, 100)
new_y_coord = np.linspace(y_min, y_max, 100)

# x,Create grid array of y
xx, yy = np.meshgrid(new_x_coord, new_y_coord)

#Known x,y coordinate,Get that value
knew_xy_coord = sample_df[['X', 'Y']].values
knew_values = sample_df['value'].values

#Interpolate data between coordinates, method='nearest', 'linear' or 'cubic'
result = griddata(points=knew_xy_coord, values=knew_values, xi=(xx, yy), method='cubic')

#graph display
fig = plt.figure()
ax = fig.add_subplot(111)
ax.set_aspect('equal', adjustable='box')
ax.contourf(xx, yy, result, cmap='jet')
plt.show()

Get data range, create new coordinates

#Get data range
x_min, x_max = sample_df['X'].min(), sample_df['X'].max()
# >>>This time x_min = 0, x_max = 10
y_min, y_max = sample_df['Y'].min(), sample_df['Y'].max()
# >>>This time y_min = 0, y_max = 10

#Create an array with new coordinates in the acquired data range
new_x_coord = np.linspace(x_min, x_max, 101)
# >>> [ 0.   0.1  0.2, ..., 10]
new_y_coord = np.linspace(y_min, y_max, 101)
# >>> [ 0.   0.1  0.2, ..., 10]
 

Get the maximum and minimum values ​​of x and y and create the coordinates for creating the actual x and y two-dimensional array. I want to put the interpolated value in the array like the table below, so new_x_coord, new_y_coord is for that Coordinates

np.grid

# x,Create grid array of y
xx, yy = np.meshgrid(new_x_coord, new_y_coord)

This is easy to understand. How to use the numpy.meshgrid function to generate a grid sequence from array elements --DeepAge

To briefly explain the array created by np.meshgrid,

example_meshgrid.py

import numpy as np

x = np.array([1, 2, 3])  # [x1, x2, x3]To
y = np.array([1, 2, 3])  # [y1, y2, y3]To
xx, yy = np.meshgrid(x, y)

xx=
array([[1, 2, 3],
       [1, 2, 3],
       [1, 2, 3]])
# xx=
# array([[x1, x2, x3],
#        [x1, x2, x3],
#        [x1, x2, x3]])


yy=
array([[1, 1, 1],
       [2, 2, 2],
       [3, 3, 3]])
# yy=
# array([[y1, y1, y1],
#        [y2, y2, y2],
#        [y3, y3, y3]])

#Coordinates can be created by overlaying xx and yy.
# array([[(1, 1),(2, 1),(3, 1)],
#        [(1, 2),(2, 2),(3, 2)],
#        [(1, 3),(2, 3),(3, 3)]])
# =
# array([[(x1, y1),(x2, y1),(x3, y1)],
#        [(x1, y2),(x2, y2),(x3, y2)],
#        [(x1, y3),(x2, y3),(x3, y3)]])

It is transmitted ...

griddata

#Known coordinates and their values
knew_xy_coord = sample_df[['X', 'Y']].values
# >>>[[ 0  0]
#     [ 0  2]
#     ...
#     [10  8]
#     [10 10]]

knew_values = sample_df['value'].values
# >>> [14 32  4 35..., 9]

#Interpolate data between coordinates, method='nearest', 'linear' or 'cubic'
result = griddata(points=knew_xy_coord, values=knew_values, xi=(xx, yy), method='cubic')
# >>> [[43.         40.16511073 37.34503184 ... 15.18228356 13.55766151 12.]
#      ...
#      [32.         30.91846813 29.83829943 ...  7.6922715   4.85443981 2. ]]

The interpolated two-dimensional array is stored in result.

The argument of scipy.interpolate.griddata is

--points: Known coordinates type = np.ndarray --values: Coordinate values ​​entered in points type = np.ndarray --xi: x, y grid array type = tuple (np.ndarray, np.ndarray) --method: Specify the type of interpolation type = str,'nearest', 'linear' or 'cubic' --fill_value: You can specify the value of the coordinates that could not be interpolated. type = float Default is nan, has no effect on 'nearest'

• rescale: ???

If the lengths of points and values ​​do not match, an error will occur.

Interpolation type

--nearest: Re-neighbor interpolation (substitute the data value closest to the point you want to interpolate) --linear: linear interpolation --cubic: (in the case of a two-dimensional array) cubic spline interpolation

graph display

python

fig = plt.figure()
ax = fig.add_subplot(111)
ax.set_aspect('equal', adjustable='box')
ax.contourf(xx, yy, result, cmap='jet')
plt.show()

Since it is a two-dimensional array with coordinates, ax.contourf is used instead of ax.imshow. Pass the x grid array, the y grid array, and the 2D array you want to display in that order. cmap specifies the type of color map.

ax.set_aspect ('equal', adjustable ='box') wants the x and y aspect ratios to be the same.

reference

scipy.interpolate.griddata — SciPy v1.6.0 Reference Guide

How to use the numpy.meshgrid function to generate a grid sequence from array elements --DeepAge