2. Extend a region on all sides

Sometimes, we have the values of a region (bounding box) that contains our data and need to pad it on all sides for some reason. This could be the boundaries of a plot or map that we’re making, or a larger region of influence that needs to be considered, or something else. Bordado offers bordado.pad_region to do this for you in n-dimensions.

import ensaio
import pygmt
import pandas as pd
import bordado as bd

Let’s illustrate this using a sample dataset of ground-based gravity observations across Southern Africa which we’ll download with ensaio.fetch_southern_africa_gravity and open with pandas.read_csv:

fname = ensaio.fetch_southern_africa_gravity(version=1)
data = pd.read_csv(fname)
data

	longitude	latitude	height_sea_level_m	gravity_mgal
0	18.34444	-34.12971	32.2	979656.12
1	18.36028	-34.08833	592.5	979508.21
2	18.37418	-34.19583	18.4	979666.46
3	18.40388	-34.23972	25.0	979671.03
4	18.41112	-34.16444	228.7	979616.11
...	...	...	...	...
14354	21.22500	-17.95833	1053.1	978182.09
14355	21.27500	-17.98333	1033.3	978183.09
14356	21.70833	-17.99166	1041.8	978182.69
14357	21.85000	-17.95833	1033.3	978193.18
14358	21.98333	-17.94166	1022.6	978211.38

14359 rows × 4 columns

We can then use get_region to find the exact bounding box of the data:

coordinates = (data.longitude, data.latitude)
region = bd.get_region(coordinates)
print(region)

(11.90833, 32.74667, -34.996, -17.33333)

If we plot the data using this bounding box, it may not look very good since some data points will be exactly at the boundary. Let’s make a quick map of the point locations with pygmt and the region that we found above:

fig = pygmt.Figure()
fig.coast(region=region, projection="M20c", land="#cccccc", frame=True)
fig.plot(x=data.longitude, y=data.latitude, style="c0.1c", fill="maroon")
fig.show()

The points are where we have data. But when we use the exact region, the map boundaries are touching some of the points, making it more difficult to see them all. The map would be better if we padded the region a bit on all sides. Let’s add some padding of 1.5° to each side using pad_region:

region_pad = bd.pad_region(region, pad=1.5)
print("Original:", region)
print("Padded:", region_pad)

Original: (11.90833, 32.74667, -34.996, -17.33333)
Padded: (10.40833, 34.24667, -36.496, -15.83333)

Now let’s make a new map with the padded region:

fig = pygmt.Figure()
fig.coast(region=region_pad, projection="M20c", land="#cccccc", frame=True)
fig.plot(x=data.longitude, y=data.latitude, style="c0.1c", fill="maroon")
fig.show()

That’s better! Now we can see the full outline of the survey.