如何使用Cartopy在西南印度洋地图上标注国家名称？

问题说明 #

尝试用Cartopy绘制西南印度洋区域地图，添加国家名称标注（如莫桑比克）时报错，目标是为地图范围内所有国家添加标注。以下是尝试标注的报错代码和无标注的正常代码：

报错的标注尝试代码 #

import matplotlib.pyplot as plt
import cartopy.crs as ccrs
import cartopy.feature as cfeature
from cartopy.mpl.gridliner import LONGITUDE_FORMATTER, LATITUDE_FORMATTER

def plot_southwest_indian_ocean():
    # 定义西南印度洋的经纬度范围
    lat_range = [-40, 0]
    lon_range = [-340, -260]

    # 创建带投影的画布和轴
    fig, ax = plt.subplots(figsize=(8, 6), subplot_kw={'projection': ccrs.PlateCarree()})

    # 设置地图边界
    ax.set_extent(lon_range + lat_range, crs=ccrs.PlateCarree())

    # 添加陆地和海洋要素
    ax.add_feature(cfeature.LAND, edgecolor='black')
    ax.add_feature(cfeature.OCEAN)

    # 添加国界线
    ax.add_feature(cfeature.BORDERS, linestyle='-', alpha=0.5)

    # 尝试添加国家标签
    countries = cfeature.NaturalEarthFeature(
        category='cultural',
        name='admin_0_countries',
        scale='50m',
        facecolor='none',
        edgecolor='gray'
    )
    ax.add_feature(countries, linewidth=0.5)

    country_name = 'Mozambique'
    country_boundaries = [country for country in countries.geometries() if country.properties['ADMIN'] == country_name]

    for boundary in country_boundaries:
        centroid = boundary.centroid
        ax.annotate(
            country_name,
            xy=(centroid.x, centroid.y),
            xytext=(5, 5),
            textcoords='offset points',
            ha='left',
            va='bottom',
            fontsize=8,
            color='black',
            bbox=dict(boxstyle='round,pad=0.2', fc='white', ec='gray', lw=0.5)
        )

    # 添加网格线
    gl = ax.gridlines(crs=ccrs.PlateCarree(), draw_labels=True, linewidth=1, color='gray', alpha=0.5, linestyle='--')
    gl.ylabels_right = False
    gl.xformatter = LONGITUDE_FORMATTER
    gl.yformatter = LATITUDE_FORMATTER
    gl.xlabel_style = {'size': 12, 'color': 'gray'}
    gl.ylabel_style = {'size': 12, 'color': 'gray'}

    # 添加标题
    ax.set_title("西南印度洋区域")

    # 显示地图
    plt.show()

无标注的正常代码 #

def plot_southwest_indian_ocean():
    # 定义西南印度洋的经纬度范围
    lat_range = [-40, 0]
    lon_range = [-340, -260]

    # 创建带投影的画布和轴
    fig, ax = plt.subplots(figsize=(8, 6), subplot_kw={'projection': ccrs.PlateCarree()})

    # 设置地图边界
    ax.set_extent(lon_range + lat_range, crs=ccrs.PlateCarree())

    # 添加陆地和海洋要素
    ax.add_feature(cfeature.LAND, edgecolor='black')
    ax.add_feature(cfeature.OCEAN)

    # 添加国界线
    ax.add_feature(cfeature.BORDERS, linestyle='-', alpha=0.5)

    # 添加网格线
    gl = ax.gridlines(crs=ccrs.PlateCarree(), draw_labels=True, linewidth=1, color='gray', alpha=0.5, linestyle='--')
    gl.ylabels_right = False
    gl.xformatter = LONGITUDE_FORMATTER
    gl.yformatter = LATITUDE_FORMATTER
    gl.xlabel_style = {'size': 12, 'color': 'gray'}
    gl.ylabel_style = {'size': 12, 'color': 'gray'}

    # 添加标题
    ax.set_title("西南印度洋区域")

    # 显示地图
    plt.show()

错误原因 #

原代码中cfeature.NaturalEarthFeature的geometries()方法仅返回shapely几何对象，不包含国家属性（如ADMIN名称），直接访问country.properties会引发属性错误。此外，未判断国家是否在当前地图范围内，可能导致标注出现在视图外。

修正后的代码 #

使用cartopy.io.shapereader读取NaturalEarth的国家shapefile，获取带属性的要素，筛选出地图范围内的国家后添加标注：

import matplotlib.pyplot as plt
import cartopy.crs as ccrs
import cartopy.feature as cfeature
from cartopy.mpl.gridliner import LONGITUDE_FORMATTER, LATITUDE_FORMATTER
from cartopy.io import shapereader

def plot_southwest_indian_ocean():
    # 定义西南印度洋的经纬度范围
    lat_range = [-40, 0]
    lon_range = [-340, -260]
    # 转换为0-360经度范围（适配shapefile数据）
    lon_range_360 = [lon + 360 if lon < 0 else lon for lon in lon_range]

    # 创建带投影的画布和轴
    fig, ax = plt.subplots(figsize=(8, 6), subplot_kw={'projection': ccrs.PlateCarree()})

    # 设置地图边界
    ax.set_extent(lon_range + lat_range, crs=ccrs.PlateCarree())

    # 添加陆地和海洋要素
    ax.add_feature(cfeature.LAND, edgecolor='black')
    ax.add_feature(cfeature.OCEAN)

    # 添加国界线
    ax.add_feature(cfeature.BORDERS, linestyle='-', alpha=0.5)

    # 读取NaturalEarth国家数据
    shp_filename = shapereader.natural_earth(
        category='cultural',
        name='admin_0_countries',
        scale='50m'
    )
    reader = shapereader.Reader(shp_filename)

    # 遍历所有国家，筛选出地图范围内的并添加标注
    for country in reader.records():
        # 获取国家名称和几何
        name = country.attributes['ADMIN']
        geom = country.geometry

        # 判断国家几何是否与地图范围相交
        bbox = geom.bounds
        # 处理跨0度经线的情况
        geom_lon_min = bbox[0] if bbox[0] >=0 else bbox[0]+360
        geom_lon_max = bbox[2] if bbox[2] >=0 else bbox[2]+360
        if (geom_lon_max < lon_range_360[0] or geom_lon_min > lon_range_360[1]):
            continue
        if (bbox[1] > lat_range[1] or bbox[3] < lat_range[0]):
            continue

        # 计算几何中心
        centroid = geom.centroid
        # 转换回负经度（如果需要）
        centroid_lon = centroid.x if centroid.x <=180 else centroid.x -360

        # 添加标注
        ax.annotate(
            name,
            xy=(centroid_lon, centroid.y),
            xytext=(5, 5),
            textcoords='offset points',
            ha='left',
            va='bottom',
            fontsize=8,
            color='black',
            bbox=dict(boxstyle='round,pad=0.2', fc='white', ec='gray', lw=0.5),
            transform=ccrs.PlateCarree()
        )

    # 添加网格线
    gl = ax.gridlines(crs=ccrs.PlateCarree(), draw_labels=True, linewidth=1, color='gray', alpha=0.5, linestyle='--')
    gl.ylabels_right = False
    gl.xformatter = LONGITUDE_FORMATTER
    gl.yformatter = LATITUDE_FORMATTER
    gl.xlabel_style = {'size': 12, 'color': 'gray'}
    gl.ylabel_style = {'size': 12, 'color': 'gray'}

    # 添加标题
    ax.set_title("西南印度洋区域")

    # 显示地图
    plt.show()

代码说明 #

1. 使用shapereader读取带属性的国家shapefile，能直接获取国家名称等信息
2. 处理了经度范围转换（原代码用-340到-260，对应20到100东经），确保筛选逻辑正确
3. 判断国家几何是否与地图范围相交，只标注视图内的国家
4. 添加transform=ccrs.PlateCarree()确保标注坐标正确匹配地图投影

内容的提问来源于stack exchange，提问作者MateaMar