如何移除Cartopy投影外框？投影与经纬度刻度重叠问题求助

运行代码后出现Cartopy投影默认的0-1范围外框与自定义经纬度刻度重叠的问题，需移除该多余外框。

原代码 #

import matplotlib.pyplot as plt
import cartopy.crs as ccrs
import cartopy.feature as cfeature
import numpy as np
import pandas as pd
from matplotlib.colors import Normalize
from matplotlib.cm import ScalarMappable

# 假设years、ca_fires、cmap、norm已定义
fig, ax = plt.subplots(figsize=(10, 8))

projection = ccrs.PlateCarree()

ax = plt.axes(projection=projection, frameon = False)

ax.add_feature(cfeature.BORDERS, edgecolor='black')
ax.add_feature(cfeature.STATES, edgecolor='black')

# Set map limits for California
ax.set_xlim(-125, -113)  # Longitude bounds for California
ax.set_ylim(32, 42)  # Latitude bounds for California

for i, year in enumerate(years):
    subset = ca_fires[ca_fires["FIRE_YEAR"] == year]
    color = cmap(norm(year))  # Get color from colormap
    # Convert GeoDataFrame to arrays for plotting
    x, y = subset.geometry.x, subset.geometry.y  # Get longitude and latitude
    ax.scatter(x, y, s=5, c=[color], alpha=0.7, transform=projection)


ax.set_title("Wildfires in California (Past 10 Years)", fontsize=14)
ax.grid(True, linestyle='--')

ax.set_xlabel('Longitude')
ax.set_ylabel('Latitude')

ax.set_xticks([])  # Remove the default ticks
ax.set_yticks([])  # Remove the default ticks

ax.set_xticks(np.arange(-125, -112, 1))  # Longitude ticks
ax.set_yticks(np.arange(32, 43, 1))  # Latitude ticks

plt.show()

问题根源 #

代码开头先通过plt.subplots()创建了一个默认Axes（范围0-1），随后又用plt.axes()重新创建带Cartopy投影的Axes，但原默认Axes未被清除，导致其外框叠加在地图上，出现0-1范围的多余框线。

解决方案 #

方案1：直接在创建子图时指定投影（推荐） #

避免重复创建Axes，在plt.subplots()中通过subplot_kw参数直接设置投影和frameon=False：

import matplotlib.pyplot as plt
import cartopy.crs as ccrs
import cartopy.feature as cfeature
import numpy as np
import pandas as pd
from matplotlib.colors import Normalize
from matplotlib.cm import ScalarMappable

# 假设years、ca_fires、cmap、norm已定义
fig, ax = plt.subplots(figsize=(10, 8), 
                       subplot_kw={'projection': ccrs.PlateCarree(), 'frameon': False})

ax.add_feature(cfeature.BORDERS, edgecolor='black')
ax.add_feature(cfeature.STATES, edgecolor='black')

# Set map limits for California
ax.set_xlim(-125, -113)  # Longitude bounds for California
ax.set_ylim(32, 42)  # Latitude bounds for California

for i, year in enumerate(years):
    subset = ca_fires[ca_fires["FIRE_YEAR"] == year]
    color = cmap(norm(year))  # Get color from colormap
    # Convert GeoDataFrame to arrays for plotting
    x, y = subset.geometry.x, subset.geometry.y  # Get longitude and latitude
    ax.scatter(x, y, s=5, c=[color], alpha=0.7, transform=ccrs.PlateCarree())


ax.set_title("Wildfires in California (Past 10 Years)", fontsize=14)
ax.grid(True, linestyle='--')

ax.set_xlabel('Longitude')
ax.set_ylabel('Latitude')

# 设置刻度时需指定transform，确保刻度对应经纬度
ax.set_xticks(np.arange(-125, -112, 1), transform=ccrs.PlateCarree())
ax.set_yticks(np.arange(32, 43, 1), transform=ccrs.PlateCarree())

plt.show()

方案2：清除原默认Axes #

若需保留分步创建逻辑，可在创建新Axes前清除原Axes：

fig, ax = plt.subplots(figsize=(10, 8))
ax.remove()  # 清除默认Axes

projection = ccrs.PlateCarree()
ax = plt.axes(projection=projection, frameon=False)

# 后续代码同原代码（注意添加刻度的transform参数）

额外优化点 #

设置经纬度刻度时必须添加transform=ccrs.PlateCarree()，否则Cartopy会将刻度值视为投影坐标而非地理经纬度，导致刻度位置错误。

内容的提问来源于stack exchange，提问作者Katherine Jeanette Benison