plot_rasterio.py

"""
.. _recipes.rasterio:

=================================
Parsing rasterio's geocoordinates
=================================


Converting a projection's cartesian coordinates into 2D longitudes and
latitudes.

These new coordinates might be handy for plotting and indexing, but it should
be kept in mind that a grid which is regular in projection coordinates will
likely be irregular in lon/lat. It is often recommended to work in the data's
original map projection (see :ref:`recipes.rasterio_rgb`).
"""

import cartopy.crs as ccrs
import matplotlib.pyplot as plt
import numpy as np
from rasterio.warp import transform

import xarray as xr

# Read the data
url = "https://github.com/mapbox/rasterio/raw/master/tests/data/RGB.byte.tif"
da = xr.open_rasterio(url)

# Compute the lon/lat coordinates with rasterio.warp.transform
ny, nx = len(da["y"]), len(da["x"])
x, y = np.meshgrid(da["x"], da["y"])

# Rasterio works with 1D arrays
lon, lat = transform(da.crs, {"init": "EPSG:4326"}, x.flatten(), y.flatten())
lon = np.asarray(lon).reshape((ny, nx))
lat = np.asarray(lat).reshape((ny, nx))
da.coords["lon"] = (("y", "x"), lon)
da.coords["lat"] = (("y", "x"), lat)

# Compute a greyscale out of the rgb image
greyscale = da.mean(dim="band")

# Plot on a map
ax = plt.subplot(projection=ccrs.PlateCarree())
greyscale.plot(
    ax=ax,
    x="lon",
    y="lat",
    transform=ccrs.PlateCarree(),
    cmap="Greys_r",
    add_colorbar=False,
)
ax.coastlines("10m", color="r")
plt.show()