Add gallery example for grdlandmask (GenericMappingTools#1469)

Co-authored-by: Dongdong Tian <[email protected]>

examples/gallery/images/grdlandmask.py

@@ -0,0 +1,34 @@
+"""
+Create 'wet-dry' mask grid
+--------------------------
+The :meth:`pygmt.grdlandmask` method allows setting
+all nodes on land or water to a specified value using
+the ``maskvalues`` parameter.
+"""
+
+import pygmt
+
+fig = pygmt.Figure()
+
+# Define region of interest
+region = [-65, -40, -40, -20]
+
+# Assign a value of 0 for all water masses and a value of 1 for all land masses.
+# Use shoreline data with (l)ow resolution and set the grid spacing to
+# 5 arc minute in x and y direction.
+grid = pygmt.grdlandmask(region=region, spacing="5m", maskvalues=[0, 1], resolution="l")
+
+# Plot clipped grid
+fig.basemap(region=region, projection="M12c", frame=True)
+
+# Define a colormap to be used for two categories, define the range of the
+# new discrete CPT using series=(lowest_value, highest_value, interval),
+# use color_model="+cwater,land" to write the discrete color palette
+# "batlow" in categorical format and add water/land as annotations for the
+# colorbar.
+pygmt.makecpt(cmap="batlow", series=(0, 1, 1), color_model="+cwater,land")
+
+fig.grdimage(grid=grid, cmap=True)
+fig.colorbar(position="JMR+o0.5c/0c+w8c")
+
+fig.show()