GIS：在 Python 中合并多面体

Question

我有一个不列颠群岛县的geojson file，我正在尝试用一个合并的县替换伦敦的各个县，然后将结果保存为 geojson。 （可以识别伦敦县，因为它们的TYPE_2 属性设置为London Borough。）

我认为我可以通过以下方式执行此任务：

from shapely.geometry import Polygon, MultiPolygon, asShape
from shapely.ops import unary_union
import json, geojson

j = json.load(open('british-isles.geojson'))

# find the london counties
indices = [idx for idx, i in enumerate(j['features']) if \
    i['properties']['TYPE_2'] == 'London Borough']

# transform each london county into a shapely polygon
polygons = [asShape(j['features'][i]['geometry']) for i in indices]

# get the metadata for the first county
properties = j['features'][indices[0]]['properties']
properties['NAME_2'] = 'London'

# get the union of the polygons
joined = unary_union(polygons)

# delete the merged counties
d = j
for i in indices:
  del d['features'][i]

# add the new polygon to the features
feature = geojson.Feature(geometry=joined, properties=properties)
d['features'].append(feature)

# save the geojson
with open('geojson-british-isles-merged-london.geojson', 'w') as out:
  json.dump(d, out)

但是，这并不能正确地合并伦敦县 - 它会导致伦敦县过去所在的多边形系列碎片化。

其他人知道我如何在 Python 中完成这项任务吗？任何建议都会非常有帮助！

Answer 1

上面有两个问题。第一个纯粹是一个疏忽：从d['features'] 删除时，我需要以相反的顺序删除数组成员（删除索引 0 然后 1 与删除 1 然后 0 不同）。

更根本的是，上面的 geojson 已经是有损的了。坐标值的小数位数有限，以减少 JSON 文件大小的字节数。但这使得合并几何图形只是近似的，并导致合并多边形之间的小间隙：

所以我得到的工作流程是获取高分辨率topojson file，将其转换为geojson，使用下面的代码合并几何，然后限制小数精度（如有必要）。

from shapely.geometry import Polygon, MultiPolygon, asShape
from shapely.ops import unary_union, cascaded_union
from geojson import Feature
import json

j = json.load(open('GBR_adm2.json'))

# find the london counties
indices = [idx for idx, i in enumerate(j['features']) if \
    'London Borough' in i['properties']['TYPE_2']]

# transform each london county into a shapely polygon
polygons = [asShape(j['features'][i]['geometry']) for i in indices]

# get the metadata for the first county
properties = j['features'][indices[0]]['properties']
properties['NAME_2'] = 'London'

# get the union of the polygons
joined = unary_union(polygons)

# delete the merged counties
d = j
for i in reversed(sorted(indices)):
  del d['features'][i]

# add the new polygon to the features
feature = Feature(geometry=joined, properties=properties)
d['features'].append(feature)

# save the geojson
with open('british-isles-merged-london.geojson', 'w') as out:
  json.dump(d, out)

结果：