Folium地图无法显示多边形问题排查求助

问题 #

通过API获取坐标后用Shapely创建多边形，点是否在多边形内的判断功能正常（能正确输出The point is inside a area），但使用Folium展示时，地图仅显示点，无法显示多边形。怀疑与多坐标系使用有关，需要排查原因并修复。

问题排查与解决 #

核心原因 #

问题确实出在坐标系不匹配：

• 从API获取的多边形坐标是SWEREF99（EPSG:3006）格式的平面坐标（单位为米），而Folium地图默认使用WGS84（EPSG:4326）的经纬度坐标（单位为十进制度）。
• 代码中仅转换了用于判断的点，未对多边形坐标做反向转换，导致多边形坐标值与地图的经纬度范围完全脱节，因此无法在地图上渲染出来。

修复步骤 #

1. 定义从SWEREF99到WGS84的坐标转换器
2. 遍历每个多边形，将其坐标从SWEREF99转换为WGS84格式
3. 转换时保持always_xy=True，确保坐标顺序为（经度，纬度）

修改后的完整代码 #

import requests
import folium
from shapely.geometry import Point, Polygon
from shapely.ops import transform
import pyproj

# Define the coordinate systems
wgs84 = pyproj.CRS('EPSG:4326')
sweref99 = pyproj.CRS('EPSG:3006')

# Fetch the areas from the API
url = 'examplecoordinates.com'
response = requests.get(url)
data = response.json()

# 定义SWEREF99转WGS84的转换器
project_sweref_to_wgs84 = pyproj.Transformer.from_crs(sweref99, wgs84, always_xy=True).transform

# Extract the polygons from the response and store them in a list
polygons = []
polygons_wgs84 = []  # 存储转换后的WGS84格式多边形
for feature in data['features']:
    coords = feature['geometry']['coordinates'][0]
    polygon = Polygon(coords)
    polygons.append(polygon)
    # 将多边形转换为WGS84格式
    polygon_wgs84 = transform(project_sweref_to_wgs84, polygon)
    polygons_wgs84.append(polygon_wgs84)

# Define the point to check in WGS84
point_wgs84 = Point(15.0560, 65.7346)

# Convert the point to SWEREF99
project_wgs84_to_sweref = pyproj.Transformer.from_crs(wgs84, sweref99, always_xy=True).transform
point_sweref99 = transform(project_wgs84_to_sweref, point_wgs84)

# Check if the point is inside any of the polygons
inside_risk_area = False
for polygon in polygons:
    if polygon.contains(point_sweref99):
        inside_risk_area = True
        print('The point is inside a area')
        break
else:
    print('The point is not inside any area')

# Create a map centered at the point of interest
m = folium.Map(location=[point_wgs84.y, point_wgs84.x], zoom_start=10)

# Add a marker for the point of interest
folium.Marker([point_wgs84.y, point_wgs84.x]).add_to(m)

# Add the areas as polygons to the map（使用转换后的WGS84多边形）
for polygon in polygons_wgs84:
    coords = [[p[1], p[0]] for p in polygon.exterior.coords]
    color = 'red' if inside_risk_area else 'green'
    folium.Polygon(locations=coords, color=color, fill_color=color, fill_opacity=0.3).add_to(m)

# Display the map
m.save('map.html')

内容的提问来源于stack exchange，提问作者Hugo Lundqvist