matplotlib绘图中卡托地轴如何正确设置投影和变换

Question

在使用Python geopandas、cartopy和matplotlib以正确绘制shapefile数据时，我面临着严重的困难。

问题来自于在正确设置转换和投影对象的shapefile数据的困难。

我在这里描述的示例是相对于SIRGAS 2000投影中的SHP而言的，其WKT格式为：

GEOGCS[“SIRGAS 2000”，基准[“D_SIRGAS_2000”，球体[“GRS_1980”，6378137298.257222101]]，PRIMEM[“格林威治”，0]，单位[“度”，0.017453292519943295]]

从上面的WKT可以看出，我的坐标是以度为单位的。地球仪不是WGS84，而是GRS_1980，与WGS84相似（但不相等）。

该crs尚未在cartopy中实施。因此，尝试使用cartopy.crs.EPSG（4674）这样的函数是行不通的，特别是因为这不是投影crs，而是地理crs。

在打印问题之后，我从逻辑上假设我应该使用四个cartopy.crs选项中的一个来打印我的数据：

transform_option_1 = cartopy.crs.PlateCarree() # works well

transform_option_2 = cartopy.crs.PlateCarree(globe=ccrs.Globe(ellipse='GRS80')) # works well

transform_option_3 = cartopy.crs.Geodetic(globe=ccrs.Globe(ellipse='GRS80')) # doesn't work

# using wkt:

WKT = """GEOGCS["SIRGAS 2000",DATUM["D_SIRGAS_2000",SPHEROID["GRS_1980",6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]"""

transform_option_4 = ccrs.PlateCarree(WKT) # results in an error message (see message in annex)

但是，当我尝试使用四个变换选项打印数据时，我的变换选项3和变换选项4出现错误。

前两个选项（transform_option_1，transform_option_2），一切运作良好。此外，当我比较图中的前两个转换选项时，它们会产生相同的图形，尽管它们会导致略有不同的图（由于全球对象）。这是我的第一个问题：

问题1：尽管Globe对象不同，为什么ccrs.PlateCarree（）对象对我的数据产生相同的绘图？

问题2：由于cartopy接受WKT字符串格式用于转换对象实例，为什么会出现错误？请参阅本信息下方所附的错误信息（错误信息编号1）。

问题3：在cartopy中，大地测量投影和platecarree投影之间有什么区别？我不知道什么时候该用这个或那个。由于这两个投影的描述都暗示了以度为单位的坐标，因此我对它们的差异感到奇怪。此外，当我尝试使用cartopy绘制SHP的两个选项时，只有PlateCarree起作用。另一个结果是黑色的地轴。

下面是一段代码片段，可以更好地澄清我的问题3：

# importing libraries:
import geopandas as gpd
import matplotlib.pyplot as plt
import cartopy.crs as ccrs


# importing data:

SHP_path = 'Data_SHP.shp'

SHP = gpd.read_file(SHP_path)


# setting projection to PlateCarre:

projection = ccrs.PlateCarree() # projection.proj4_init


# setting SHP data transform

Transform2 = ccrs.Geodetic(globe=ccrs.Globe(ellipse='GRS80'))


# creating figure:

fig, ax = plt.subplots(1, subplot_kw={'projection':projection})


SHP.plot(ax=ax, transform=Transform2, 
         color='k', alpha=0.5)

# defining a small function to add grilines to the plot:

def custom_plot(geoaxes, projection):



    gl = geoaxes.gridlines(crs=projection)

    tick_axis_positions={'xlabels_top':False,
                     'ylabels_left':True,
                     'ylabels_right':False,
                     'xlabels_bottom':True}

    gl.xlabels_top = tick_axis_positions['xlabels_top']
    gl.ylabels_left = tick_axis_positions['ylabels_left']
    gl.ylabels_right= tick_axis_positions['ylabels_right']
    gl.xlabels_bottom = tick_axis_positions['xlabels_bottom']


    return geoaxes


# adding gridlines to the geoaxes:

ax = custom_plot(ax, projection)

fig.show()

# end of code

问题4：为什么我不能使用crs（WKT格式）实例化cartopy投影对象？当我尝试这样做时，会出现一条错误消息（见附件中的错误消息2）。

Projection = ccrs.Projection(proj4_params=WKT, globe=ccrs.Globe(ellipse='GRS80'))

参考文献：

1） 关于如何从WKT或proj4坐标系投影格式创建cartopy.crs变换对象的讨论

2） 将proj4映射到cartopy CRS

3）在Cartopy中绘制直线，Robinson投影

4）用cartopy绘制SHP的例子

5） cartopy变换与投影理论

附件：

错误消息1）

TypeError: unsupported operand type(s) for -: 'str' and 'float'


Traceback (most recent call last):
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\backends\backend_qt5.py", line 519, in _draw_idle
    self.draw()
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\backends\backend_agg.py", line 402, in draw
    self.figure.draw(self.renderer)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\artist.py", line 50, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\figure.py", line 1649, in draw
    renderer, self, artists, self.suppressComposite)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\image.py", line 138, in _draw_list_compositing_images
    a.draw(renderer)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\artist.py", line 50, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\cartopy\mpl\geoaxes.py", line 385, in draw
    inframe=inframe)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\artist.py", line 50, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\axes\_base.py", line 2628, in draw
    mimage._draw_list_compositing_images(renderer, self, artists)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\image.py", line 138, in _draw_list_compositing_images
    a.draw(renderer)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\artist.py", line 50, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\collections.py", line 262, in draw
    transform, transOffset, offsets, paths = self._prepare_points()
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\collections.py", line 240, in _prepare_points
    for path in paths]
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\collections.py", line 240, in <listcomp>
    for path in paths]
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\transforms.py", line 2451, in transform_path_non_affine
    return self._a.transform_path_non_affine(path)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\cartopy\mpl\geoaxes.py", line 173, in transform_path_non_affine
    src_path.vertices, self.source_projection)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\cartopy\crs.py", line 746, in quick_vertices_transform
    bboxes, proj_offset = self._bbox_and_offset(src_crs)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\cartopy\crs.py", line 709, in _bbox_and_offset
    lon_0_offset = other_lon_0 - self_lon_0
TypeError: unsupported operand type(s) for -: 'str' and 'float'
Traceback (most recent call last):
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\backends\backend_qt5.py", line 519, in _draw_idle
    self.draw()
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\backends\backend_agg.py", line 402, in draw
    self.figure.draw(self.renderer)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\artist.py", line 50, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\figure.py", line 1649, in draw
    renderer, self, artists, self.suppressComposite)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\image.py", line 138, in _draw_list_compositing_images
    a.draw(renderer)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\artist.py", line 50, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\cartopy\mpl\geoaxes.py", line 385, in draw
    inframe=inframe)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\artist.py", line 50, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\axes\_base.py", line 2628, in draw
    mimage._draw_list_compositing_images(renderer, self, artists)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\image.py", line 138, in _draw_list_compositing_images
    a.draw(renderer)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\artist.py", line 50, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\collections.py", line 262, in draw
    transform, transOffset, offsets, paths = self._prepare_points()
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\collections.py", line 240, in _prepare_points
    for path in paths]
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\collections.py", line 240, in <listcomp>
    for path in paths]
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\matplotlib\transforms.py", line 2451, in transform_path_non_affine
    return self._a.transform_path_non_affine(path)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\cartopy\mpl\geoaxes.py", line 173, in transform_path_non_affine
    src_path.vertices, self.source_projection)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\cartopy\crs.py", line 746, in quick_vertices_transform
    bboxes, proj_offset = self._bbox_and_offset(src_crs)
  File "C:\Users\lealp\AppData\Local\conda\conda\envs\Python_3.7\lib\site-packages\cartopy\crs.py", line 709, in _bbox_and_offset
    lon_0_offset = other_lon_0 - self_lon_0
TypeError: unsupported operand type(s) for -: 'str' and 'float'

错误消息（2）

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-165-9916b2ddcc67> in <module>
----> 1 ccrs.Projection(proj4_params=Transform_from_wkt, globe=ccrs.Globe(ellipse='GRS80'))

TypeError: Can't instantiate abstract class Projection with abstract methods boundary, threshold, x_limits, y_limits

Answer 1

>

我不知道CartoPy在任何地方都接受WKT。我认为从Proj.4字符串或WKT创建投影是一个悬而未决的问题。

PlateCarree的工作原理是将lat/lon作为平面上的坐标，所以lat/lon点之间的线是平面上的直线。Geodetic将lat/lons视为球体上的坐标，所以连接lat/lons的线是大圆。这是正确的做事方式，但它使数学更加复杂，对一些绘图选项不起作用。看看这个例子，看看它是如何变化的。

见上文第（2）段

我很好奇将自定义椭球体传递到大地测量时遇到了什么问题。我对该代码和Cartopy 0.17没有任何问题：

import cartopy.crs as ccrs
globe = ccrs.Globe(ellipse='GRS80')
crs = ccrs.Geodetic(globe=globe)