Clash Royale CLAN TAG#URR8PPP

use np.where to subset 3d array

I am currently working on a satellite image, and I got a 3D array(6464,4064,3) like this

[[[ 3.61944046e+01 -6.91377335e+01 -1.50000001e-09]
[ 3.61942863e+01 -6.91287460e+01 1.32471696e-08]
[ 3.61941681e+01 -6.91197662e+01 9.53853174e-09]
...,
[ 3.11809139e+01 -3.63661194e+01 6.60078259e-09]
[ 3.11785698e+01 -3.63582687e+01 6.60078259e-09]
[ 3.11762199e+01 -3.63504028e+01 6.40588294e-09]]

[[ 3.61873817e+01 -6.91379166e+01 -1.50000001e-09]
[ 3.61872635e+01 -6.91289215e+01 1.43964334e-08]
[ 3.61871490e+01 -6.91199493e+01 1.12178125e-08]
...,
[ 3.11743488e+01 -3.63688583e+01 6.63846089e-09]
[ 3.11720028e+01 -3.63610077e+01 7.23354443e-09]
[ 3.11696529e+01 -3.63531456e+01 7.43190709e-09]]

[[ 3.61803589e+01 -6.91380997e+01 -1.50000001e-09]
[ 3.61802444e+01 -6.91291122e+01 1.69292687e-08]
[ 3.61801338e+01 -6.91201324e+01 1.33426239e-08]
...,
[ 3.11677856e+01 -3.63715935e+01 7.35317940e-09]
[ 3.11654358e+01 -3.63637428e+01 6.95529767e-09]
[ 3.11630821e+01 -3.63558846e+01 7.15423853e-09]]

...,
[[ -5.02645159e+00 -7.61433792e+01 -1.50000001e-09]
[ -5.02774668e+00 -7.61361847e+01 3.38870656e-08]
[ -5.02903891e+00 -7.61290054e+01 3.38870656e-08]
...,
[ -9.27992916e+00 -4.86378708e+01 9.09282427e-09]
[ -9.28078461e+00 -4.86308556e+01 9.09282427e-09]
[ -9.28179646e+00 -4.86225281e+01 7.49361462e-09]]

[[ -5.03337288e+00 -7.61447067e+01 -1.50000001e-09]
[ -5.03466558e+00 -7.61375122e+01 3.04580183e-08]
[ -5.03595591e+00 -7.61303253e+01 3.48006957e-08]
...,
[ -9.28699970e+00 -4.86376190e+01 8.94025476e-09]
[ -9.28782177e+00 -4.86308937e+01 8.15083290e-09]
[ -9.28873920e+00 -4.86233711e+01 8.34818881e-09]]

[[ -5.04029608e+00 -7.61460190e+01 -1.50000001e-09]
[ -5.04158545e+00 -7.61388321e+01 3.18825499e-08]
[ -5.04287243e+00 -7.61316452e+01 3.26812319e-08]
...,
[ -9.29387188e+00 -4.86390038e+01 8.31999980e-09]
[ -9.29480457e+00 -4.86313744e+01 8.51963478e-09]
[ -9.29572582e+00 -4.86238594e+01 8.71926975e-09]]]


which is [latitude, longtitude, radiance] * 6464rows * 4064colomns

I want to subset my interested area according to latitude and longtitude,
so I use

new= np.where((hh[:,:,0]<=13)& (hh[:,:,0]>=7) & (hh[:,:,1]>=-76) & (hh[:,:,1]<=-64))


(hh is my 3d array)

and it comes out the shape of new array is (1156142, 3),
which means it becomes a 2d array and lost its colomns and rows.

I don't know how to why and don't know how to plot the radiance figure with unknown rows and colomns.

1 Answer
1

when np.where feed only one argument, return the tuple condition.nonzero(), the indices where condition is True..

np.where

If you only want data points according to latitude and longtitude, index slicing is OK, more detail

Like:

import numpy as np

hh = np.random.randn(6464, 4064, 3)

# boolean
new_idx = (hh[:, :, 0] <= 0.13)& (hh[:, :, 0] >= 0.07) & (hh[:, :, 1] >= -0.76) &
(hh[:, :, 1] <= -0.64)
print(new_idx.shape) # (6464, 4064)

# Boolean array indexing
# data points (num, latitude, longtitude, radiance)
new_arr = hh[new_idx]
print(new_arr.shape) # (23175, 3)

# rows, cols
new_where = np.where(new_idx)
print(type(new_where)) # <class 'tuple'>
print([x.shape for x in new_where]) # [(23175,), (23175,)]


np.where(new_idx, hh, 0)

0

plt.contourf

plt.contour

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