Using Cartopy instead of Basemap to create map traces in Dash app - python-3.x

Question

I am currently creating a Dash app that uses drop down menus to determine what data to plot on a global map (usually contour plots).
I am aware that Basemap is being phased out, so want to change to Cartopy as the source of the maps (Mapbox is ace but not suitable) but am unsure how to get the map information as a trace in order to use it with Dash?
I have no problems changing over for a simple plot- its just within the Dash interface where the map data needs to go in as a trace.
The tutorial I followed is Basemap only and I can't find any information on how to convert the method into Cartopy whilst using Dash.
Any ideas much appreciated!
Code taken from: https://plot.ly/ipython-notebooks/basemap-maps/ Full example available there- I've just copied what I think is the key bit I need to alter
getting contours into a trace
trace1 = Contour(
z=air,
x=lon,
y=lat,
colorscale="RdBu",
zauto=False, zmin=-5, zmax=5 )
making the map
m = Basemap()
def make_scatter(x,y):
return Scatter(
x=x,
y=y,
mode='lines',
line=Line(color="black"),
name=' ' # no name on hover
)
def polygons_to_traces(poly_paths, N_poly):
'''
pos arg 1. (poly_paths): paths to polygons
pos arg 2. (N_poly): number of polygon to convert
'''
traces = [] # init. plotting list
for i_poly in range(N_poly):
poly_path = poly_paths[i_poly]
# get the Basemap coordinates of each segment
coords_cc = np.array(
[(vertex[0],vertex[1])
for (vertex,code) in poly_path.iter_segments(simplify=False)]
)
# convert coordinates to lon/lat by 'inverting' the Basemap projection
lon_cc, lat_cc = m(coords_cc[:,0],coords_cc[:,1], inverse=True)
# add plot.ly plotting options
traces.append(make_scatter(lon_cc,lat_cc))
return traces
def get_coastline_traces():
poly_paths = m.drawcoastlines().get_paths() # coastline polygon paths
N_poly = 91 # use only the 91st biggest coastlines (i.e. no rivers)
return polygons_to_traces(poly_paths, N_poly)
def get_country_traces():
poly_paths = m.drawcountries().get_paths() # country polygon paths
N_poly = len(poly_paths) # use all countries
return polygons_to_traces(poly_paths, N_poly)
traces_cc = get_coastline_traces()+get_country_traces()
data = Data([trace1]+traces_cc)

Answer 1

You can create a trace with the coastline as a graph object for example like this:
import plotly.graph_objects as go
import cartopy.feature as cf
# create the list of coordinates separated by nan to avoid connecting the lines
x_coords = []
y_coords = []
for coord_seq in cf.COASTLINE.geometries():
x_coords.extend([k[0] for k in coord_seq.coords] + [np.nan])
y_coords.extend([k[1] for k in coord_seq.coords] + [np.nan])
## in your app callback for dash
fig = go.Figure()
fig.add_trace(
go.Scatter(
x = x_coords,
y = y_coords,
mode = 'lines'))
The result looks like this
You can add more traces besides the COASTLINE, like BORDERS. More seem to be available here:
https://scitools.org.uk/cartopy/docs/latest/matplotlib/feature_interface.html#cartopy.feature.BORDERS