Plotting data points

GMT shines when it comes to plotting data on a map. We can use some sample data that is packaged with GMT to try this out. PyGMT provides access to these datasets through the pygmt.datasets package. If you don’t have the data files already, they are automatically downloaded and saved to a cache directory the first time you use them (usually ~/.gmt/cache).

import pygmt

For example, let’s load the sample dataset of tsunami generating earthquakes around Japan using pygmt.datasets.load_sample_data. The data are loaded as a pandas.DataFrame.

data = pygmt.datasets.load_sample_data(name="japan_quakes")

# Set the region for the plot to be slightly larger than the data bounds.
region = [
    data.longitude.min() - 1,
    data.longitude.max() + 1,
    data.latitude.min() - 1,
    data.latitude.max() + 1,
]

print(region)
print(data.head())

[np.float64(131.29), np.float64(150.89), np.float64(34.02), np.float64(50.77)]
   year  month  day  latitude  longitude  depth_km  magnitude
0  1987      1    4     49.77     149.29       489        4.1
1  1987      1    9     39.90     141.68        67        6.8
2  1987      1    9     39.82     141.64        84        4.0
3  1987      1   14     42.56     142.85       102        6.5
4  1987      1   16     42.79     145.10        54        5.1

We’ll use the pygmt.Figure.plot method to plot circles on the earthquake epicenters.

fig = pygmt.Figure()
fig.basemap(region=region, projection="M15c", frame=True)
fig.coast(land="black", water="skyblue")
fig.plot(x=data.longitude, y=data.latitude, style="c0.3c", fill="white", pen="black")
fig.show()

We used the style c0.3c which means “circles with a diameter of 0.3 centimeters”. The pen parameter controls the outline of the symbols and the fill parameter controls the fill.

We can map the size of the circles to the earthquake magnitude by passing an array to the size parameter. Because the magnitude is on a logarithmic scale, it helps to show the differences by scaling the values using a power law.

fig = pygmt.Figure()
fig.basemap(region=region, projection="M15c", frame=True)
fig.coast(land="black", water="skyblue")
fig.plot(
    x=data.longitude,
    y=data.latitude,
    size=0.02 * (2**data.magnitude),
    style="cc",
    fill="white",
    pen="black",
)
fig.show()

Notice that we didn’t include the size in the style parameter this time, just the symbol c (circles) and the unit c (centimeters). So in this case, the size will be interpreted as being in centimeters.

We can also map the colors of the markers to the depths by passing an array to the fill parameter and providing a colormap name (cmap). We can even use the new matplotlib colormap “viridis”. Here, we first create a continuous colormap ranging from the minimum depth to the maximum depth of the earthquakes using pygmt.makecpt, then set cmap=True in pygmt.Figure.plot to use the colormap. At the end of the plot, we also plot a colorbar showing the colormap used in the plot.

fig = pygmt.Figure()
fig.basemap(region=region, projection="M15c", frame=True)
fig.coast(land="black", water="skyblue")
pygmt.makecpt(cmap="viridis", series=[data.depth_km.min(), data.depth_km.max()])
fig.plot(
    x=data.longitude,
    y=data.latitude,
    size=0.02 * 2**data.magnitude,
    fill=data.depth_km,
    cmap=True,
    style="cc",
    pen="black",
)
fig.colorbar(frame="xaf+lDepth (km)")
fig.show()