问题
I am not sure on how to plot a dotted line from a shapefile in Python. It appears that readshapefile() does not have any linestyle for me to set. Below I have a working code where I take a shapefile and plot it, but it only plots a solid line. Any ideas to set me in the right direction? Thanks!
The shapefile can be found here: http://www.natice.noaa.gov/products/daily_products.html, where the Start Date is Feb 15th, end date is Feb 17th, and the Date Types is Ice Edge. It should be the first link.
#!/awips2/python/bin/python
from mpl_toolkits.basemap import Basemap
import matplotlib.pyplot as plt
map = Basemap(llcrnrlon=-84.37,llcrnrlat=42.11,urcrnrlon=-20.93,urcrnrlat=66.48,
resolution='i', projection='tmerc', lat_0 = 55., lon_0 = -50.)
map.drawmapboundary(fill_color='aqua')
map.fillcontinents(color='#ddaa66',lake_color='aqua')
map.drawcoastlines(zorder = 3)
map.readshapefile('nic_autoc2018046n_pl_a', 'IceEdge', zorder = 2, color = 'blue')
plt.show()
回答1:
From the Basemap documentation:
A tuple (num_shapes, type, min, max) containing shape file info is returned. num_shapes is the number of shapes, type is the type code (one of the SHPT* constants defined in the shapelib module, see http://shapelib.maptools.org/shp_api.html) and min and max are 4-element lists with the minimum and maximum values of the vertices. If drawbounds=True a matplotlib.patches.LineCollection object is appended to the tuple.
drawbounds is True by default, so all you have to do is collect the return value of readshapefile and alter the linestyle of the returned LineCollection object, which can be done with LineCollection.set_linestyle(). So in principle you can change the linestyle of your plotted shape file with something like this:
result = m.readshapefile('shapefiles/nic_autoc2018046n_pl_a', 'IceEdge', zorder = 10, color = 'blue')#, drawbounds = False)
col = result[-1]
col.set_linestyle('dotted')
plt.show()
However, your shapefile contains 5429 separate line segments of different length and somehow matplotlib does not seem to be able to deal with this large amount of non-continuous lines. At least on my machine the plotting did not finish within one hour, so I interrupted the process. I played a bit with your file and it seems like many of the lines are broken into segments unnecessarily (I'm guessing this is because the ice sheet outlines are somehow determined on tiles and then pieced together afterwards, but only the providers will really know). Maybe it would help to piece together adjacent pieces, but I'm not sure.
I was also wondering whether the result would even look that great with a dotted line, because there are so many sharp bends. Below I show a picture where I only plot the 100 longest line segments (leaving out drawcoastlines and with thicker lines) using this code:
import numpy as np
result = m.readshapefile('shapefiles/nic_autoc2018046n_pl_a', 'IceEdge', zorder = 10, color = 'blue')#, drawbounds = False)
col = result[-1]
segments = col.get_segments()
seglens = [len(seg) for seg in col.get_segments()]
segments = np.array(segments)
seglens = np.array(seglens)
idx = np.argsort(seglens)
seglens = seglens[idx]
segments = segments[idx]
col.remove()
new_col = LineCollection(segments[-100:],linewidths = 2, linestyles='dotted', colors='b')
ax.add_collection(new_col)
plt.show()
And the result looks like this:
来源：https://stackoverflow.com/questions/48949192/how-to-plot-dotted-lines-from-a-shapefile-in-python