Saturday, December 3, 2022

Is Lanzhou City the mass center of the map of China?

Almost!

It turns out that blue (Lanzhou city) is close to but not exact the mass center (red). I mean, if we only consider Lanzhou as a point.

An algorithmic approach

import json
import matplotlib.pyplot as plt
from matplotlib.patches import Polygon
import numpy as np
import cv2

# be aware to fix coordinates from 内蒙古 (add an extra bracket, download it from https://geo.datav.aliyun.com/areas_v3/bound/100000_full.json)
with open('100000_full.json', 'r') as f:
    content = json.load(f)

print(content.keys())


fig, ax = plt.subplots()

for feat in content['features']:
    if feat['properties']['name'] == '甘肃省':
        center = feat['properties']['center']
    if feat['properties']['name'] not in ['']:
        coordinates = feat['geometry']['coordinates']
        for c in coordinates:
            coord = np.array(c)
            if len(coord[0].shape) == 2:
                p = Polygon(coord[0], facecolor='k')
                ax.add_patch(p)
            else:
                print(coord[0])
                print(feat['properties']['name'])

ax.set_xlim([60, 140])
ax.set_ylim([0, 60])
cx, cy = center
print("center", center)
print("lim:", ax.get_xlim(), ax.get_ylim())
xmin, xmax = ax.get_xlim()
ymin, ymax = ax.get_ylim()
cx = (cx -60) * (640 / (xmax - xmin))
cy = 480 - cy * (480 / (ymax-ymin))
print("center scaled:", cx, cy)
plt.axis('off')
plt.tight_layout()
plt.savefig("china_map.png")
plt.show()


img = cv2.imread('china_map.png', 0)
print(img.shape, img.mean(), img.max(), img.min())
th, im_th = cv2.threshold(img, 100, 255, cv2.THRESH_BINARY_INV)

print(th)
print(im_th.shape)
cv2.imwrite('china_map_inverse.png', im_th)

moments = cv2.moments(im_th, True)
print(moments)

centeroid = (moments['m10'] / moments['m00'], moments['m01'] / moments['m00'])
print("centeroid:", centeroid)
x,y = centeroid
fig,ax = plt.subplots()

img = cv2.imread('china_map.png')
ax.imshow(img)

plt.scatter(x, y, c='r')
plt.scatter(cx, cy, c='b')
plt.tight_layout()
plt.axis('off')
plt.savefig('china_map_with_centeroid.png')
plt.show()

References

  • https://blog.csdn.net/qq826309057/article/details/70039397
  • https://www.zhoulujun.cn/html/GIS/WebGIS/8348.html
  • http://datav.aliyun.com/portal/school/atlas/area_selector#&lat=33.50475906922609&lng=104.2822265625&zoom=4

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