Geographic Transforms and Projections

Loading the GEOS data

For this analysis we’ll be loading some global climate data into yt. A frontend does not exist for this dataset yet, so we’ll load it in as a uniform grid with netcdf4.

import os
import re

import netCDF4 as nc4
import numpy as np

import yt

def get_data_path(arg):
    if os.path.exists(arg):
        return arg
    else:
        return os.path.join(yt.config.ytcfg.get("yt", "test_data_dir"), arg)

n = nc4.Dataset(get_data_path("geos/GEOS.fp.asm.inst3_3d_aer_Nv.20180822_0900.V01.nc4"))

Using the loaded data we’ll fill arrays with the data dimensions and limits. We’ll also rename vertical level to altitude to be clearer.

dims = []
sizes = []
bbox = []
ndims = len(n.dimensions)
for dim in n.dimensions.keys():
    size = n.variables[dim].size
    if size > 1:
        bbox.append([n.variables[dim][:].min(), n.variables[dim][:].max()])
        dims.append(n.variables[dim].long_name)
        sizes.append(size)
dims.reverse()  # Fortran ordering
sizes.reverse()
bbox.reverse()
dims = [f.replace("vertical level", "altitude") for f in dims]
bbox = np.array(bbox)

We’ll also load the data into a container dictionary and create a lookup for the short to the long names

w_regex = re.compile(r"([a-zA-Z]+)(.*)")


def regex_parser(s):
    try:
        return "**".join(filter(None, w_regex.search(s).groups()))
    except AttributeError:
        return s

data = {}
names = {}
for field, d in n.variables.items():
    if d.ndim != ndims:
        continue
    units = n.variables[field].units
    units = " * ".join(map(regex_parser, units.split()))
    data[field] = (np.squeeze(d), str(units))
    names[field] = n.variables[field].long_name.replace("_", " ")

Now the data can be loaded with yt’s load_uniform_grid function. We also need to say that the geometry is a geographic type. This will ensure that the axes created are matplotlib GeoAxes and that the transform functions are available to use for projections.

ds = yt.load_uniform_grid(
    data, sizes, 1.0, geometry="geographic", bbox=bbox, axis_order=dims
)

Default projection with geographic geometry

Now that the data is loaded, we can plot it with a yt SlicePlot along the altitude. This will create a figure with latitude and longitude as the plot axes and the colormap will correspond to the air density. Because no projection type has been set, the geographic geometry type assumes that the data is of the PlateCarree form. The resulting figure will be a Mollweide plot.

p = yt.SlicePlot(ds, "altitude", "AIRDENS")
p.show()

Note that this doesn’t have a lot of contextual information. We can add annotations for the coastlines just as we would with matplotlib. Before the annotations are set, we need to call p._setup_plots to make the axes available for annotation.

p = yt.SlicePlot(ds, "altitude", "AIRDENS")
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

Using geographic transforms to project data

If a projection other than the default Mollweide is desired, then we can pass an argument to the set_mpl_projection() function to set a different projection than the default. This will set the projection to a Robinson projection.

p = yt.SlicePlot(ds, "altitude", "AIRDENS")
p.set_mpl_projection("Robinson")
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

geo_projection accepts a string or a 2- to 3- length sequence describing the projection the second item in the sequence are the args and the third item is the kwargs. This can be used for further customization of the projection.

p = yt.SlicePlot(ds, "altitude", "AIRDENS")
p.set_mpl_projection(("Robinson", (37.5,)))
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

We don’t actually need to keep creating a SlicePlot to change the projection type. We can use the function set_mpl_projection() and pass in a string of the transform type that we desire after an existing SlicePlot instance has been created. This will set the figure to an Orthographic projection.

p.set_mpl_projection("Orthographic")
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

set_mpl_projection() can be used in a number of ways to customize the projection type. * If a string is passed, then the string must correspond to the transform name, which is exclusively cartopy transforms at this time. This looks like: set_mpl_projection('ProjectionType')

• If a tuple is passed, the first item of the tuple is a string of the transform name and the second two items are args and kwargs. These can be used to further customize the transform (by setting the latitude and longitude, for example. This looks like:

  • set_mpl_projection(('ProjectionType', (args)))

  • set_mpl_projection(('ProjectionType', (args), {kwargs}))

• A transform object can also be passed. This can be any transform type – a cartopy transform or a matplotlib transform. This allows users to either pass the same transform object around between plots or define their own transform and use that in yt’s plotting functions. With a standard cartopy transform, this would look like:

  • set_mpl_projection(cartopy.crs.PlateCarree())

To summarize: The function set_mpl_projection can take one of several input types: * set_mpl_projection('ProjectionType') * set_mpl_projection(('ProjectionType', (args))) * set_mpl_projection(('ProjectionType', (args), {kwargs})) * set_mpl_projection(cartopy.crs.MyTransform())

For example, we can make the same Orthographic projection and pass in the central latitude and longitude for the projection:

p.set_mpl_projection(("Orthographic", (90, 45)))
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

Or we can pass in the arguments to this function as kwargs by passing a three element tuple.

p.set_mpl_projection(
    ("Orthographic", (), {"central_latitude": -45, "central_longitude": 275})
)
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

A few examples of different projections

This next section will show a few of the different projections that one can use. This isn’t meant to be complete, but it’ll give you a visual idea of how these transforms can be used to illustrate geographic data for different purposes.

p.set_mpl_projection(("RotatedPole", (177.5, 37.5)))
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

p.set_mpl_projection(
    ("RotatedPole", (), {"pole_latitude": 37.5, "pole_longitude": 177.5})
)
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

p.set_mpl_projection("NorthPolarStereo")
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

p.set_mpl_projection("AlbersEqualArea")
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

p.set_mpl_projection("InterruptedGoodeHomolosine")
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

p.set_mpl_projection("Robinson")
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

p.set_mpl_projection("Gnomonic")
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

Modifying the data transform

While the data projection modifies how the data is displayed in our plot, the data transform describes the coordinate system that the data is actually described by. By default, the data is assumed to have a PlateCarree data transform. If you would like to change this, you can access the dictionary in the coordinate handler and set it to something else. The dictionary is structured such that each axis has its own default transform, so be sure to set the axis you intend to change. This next example changes the transform to a Miller type. Because our data is not in Miller coordinates, it will be skewed.

ds.coordinates.data_transform["altitude"] = "Miller"
p = yt.SlicePlot(ds, "altitude", "AIRDENS")
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()

Because the transform type shouldn’t change as we make subsequent figures, once it is changed it will be the same for all other figures made with the same dataset object. Note that this particular dataset is not actually in a Miller system, which is why the data now doesn’t span the entire globe. Setting the new projection to Robinson results in Miller-skewed data in our next figure.

p.set_mpl_projection("Robinson")
p._setup_plots()
p.plots["AIRDENS"].axes.set_global()
p.plots["AIRDENS"].axes.coastlines()
p.show()