Graphics

Two options are offered to you when plotting geolocalised data on maps: you can use cartopy , or you can use salem’s Map object.

Plotting with cartopy

Plotting on maps using xarray and cartopy is extremely convenient.

With salem you can keep your usual plotting workflow, even with more exotic map projections:

In [1]: import matplotlib.pyplot as plt

In [2]: import cartopy

In [3]: from salem import open_wrf_dataset, get_demo_file

In [4]: ds = open_wrf_dataset(get_demo_file('wrfout_d01.nc'))

In [5]: ax = plt.axes(projection=cartopy.crs.Orthographic(70, 30))

In [6]: ax.set_global();

In [7]: ds.T2C.isel(time=1).plot.contourf(ax=ax, transform=ds.salem.cartopy());

In [8]: ax.coastlines();
_images/cartopy_orthographic.png

You can also use the salem accessor to initialise the plot’s map projection:

In [9]: proj = ds.salem.cartopy()

In [10]: ax = plt.axes(projection=proj)

In [11]: ax.coastlines();

In [12]: ax.add_feature(cartopy.feature.BORDERS, linestyle=':');

In [13]: ax.set_extent(ds.salem.grid.extent, crs=proj);
_images/cartopy_base.png

Plotting with salem

Salem comes with a homegrown plotting tool. It is less flexible than cartopy, but it was created to overcome some of cartopy’s limitations (e.g. the impossibility to add tick labels to lambert conformal maps), and to make nice looking regional maps:

In [14]: ds.T2C.isel(time=1).salem.quick_map()
Out[14]: <salem.graphics.Map at 0x7f15ec09d710>
_images/salem_quickmap.png

Salem maps are different from cartopy’s in that they don’t change the matplotlib axes’ projection. The map background is always going to be a call to imshow(), with an image size decided at instanciation:

In [15]: from salem import mercator_grid, Map, open_xr_dataset

In [16]: grid = mercator_grid(center_ll=(10.76, 46.79), extent=(9e5, 4e5))

In [17]: grid.nx, grid.ny  # size of the input grid
Out[17]: (1350, 600)

In [18]: smap = Map(grid, nx=500)
I am densified (external_values, 17 elements)

In [19]: smap.grid.nx, smap.grid.ny  # size of the "image", and thus of the axes
Out[19]: (500, 222)

In [20]: smap.visualize(addcbar=False)
_images/map_central_europe.png

The map has it’s own grid, wich is used internally in order to transform the data that has to be plotted on it:

In [21]: ds = open_xr_dataset(get_demo_file('histalp_avg_1961-1990.nc'))

In [22]: smap.set_data(ds.prcp)  # histalp is a lon/lat dataset

In [23]: smap.visualize()
_images/map_histalp.png

Refer to Recipes for more examples on how to use salem’s maps.