Cross-section along a transect

pygmt.project and pygmt.grdtrack can be used to focus on a quantity and its variation along a desired survey line. In this example, the elevation is extracted from a grid provided via pygmt.datasets.load_earth_relief. The figure consists of two parts, a map of the elevation in the study area showing the survey line and a Cartesian plot showing the elevation along the survey line.

This example is orientated on an example in the GMT/China documentation: https://docs.gmt-china.org/latest/examples/ex026/

import pygmt

# Define region of study area
# lon_min, lon_max, lat_min, lat_max in degrees East and North
region_map = [122, 149, 30, 49]

# Create a new pygmt.Figure instance
fig = pygmt.Figure()

# ----------------------------------------------------------------------------
# Bottom: Map of elevation in study area

# Set up basic map
fig.basemap(
    region=region_map,
    projection="M12c",  # Mercator projection with a width of 12 centimeters
    frame="af",
)

# Download grid for Earth relief with a resolution of 10 arc-minutes and
# gridline registration [Default]
grid_map = pygmt.datasets.load_earth_relief(
    resolution="10m",
    region=region_map,
)

# Plot the downloaded grid with color-coding based on the elevation
fig.grdimage(grid=grid_map, cmap="oleron")

# Add a colorbar for the elevation
fig.colorbar(
    # Place the colorbar inside the plot (lower-case "j") in the Bottom
    # Right (BR) corner with an offset ("+o") of 0.7 centimeters and
    # 0.3 centimeters in x or y directions, respectively
    # Move the x label above the horizontal colorbar ("+ml")
    position="jBR+o0.7c/0.8c+h+w5c/0.3c+ml",
    # Add a box around the colobar with a fill ("+g") in "white" color and
    # a transparency ("@") of 30 % and with a 0.8-points thick black
    # outline ("+p")
    box="+gwhite@30+p0.8p,black",
    # Add x and y labels ("+l")
    frame=["x+lElevation", "y+lm"],
)

# Choose a survey line
fig.plot(
    x=[126, 146],  # Longitude in degrees East
    y=[42, 40],  # Latitude in degrees North
    # Draw a 2-points thick red dashed line for the survey line
    pen="2p,red,dashed",
)

# Add labels "A" and "B" for the start and end points of the survey line
fig.text(
    x=[126, 146],
    y=[42, 40],
    text=["A", "B"],
    offset="0c/0.2c",  # Move text 0.2 centimeters up (y direction)
    font="15p",  # Use a font size of 15 points
)

# ----------------------------------------------------------------------------
# Top: Elevation along survey line

# Shift plot origin 12.5 centimeters to the top
fig.shift_origin(yshift="12.5c")

fig.basemap(
    region=[0, 15, -8000, 6000],  # x_min, x_max, y_min, y_max
    # Cartesian projection with a width of 12 centimeters and
    # a height of 3 centimeters
    projection="X12c/3c",
    # Add annotations ("a") and ticks ("f") as well as labels ("+l")
    # at the west or left and south or bottom sides ("WSrt")
    frame=["WSrt", "xa2f1+lDistance+u°", "ya4000+lElevation / m"],
)

# Add labels "A" and "B" for the start and end points of the survey line
fig.text(
    x=[0, 15],
    y=[7000, 7000],
    text=["A", "B"],
    no_clip=True,  # Do not clip text that fall outside the plot bounds
    font="10p",  # Use a font size of 10 points
)

# Generate points along a great circle corresponding to the survey line
# and store them in a pandas.DataFrame
track_df = pygmt.project(
    center="126/42",  # Start point of survey line (longitude/latitude)
    endpoint="146/40",  # End point of survey line (longitude/latitude)
    generate="0.1",  # Output data in steps of 0.1 degrees
)

# Extract the elevation at the generated points from the downloaded grid
# and add it as new column "elevation" to the pandas.DataFrame
track_df = pygmt.grdtrack(
    grid=grid_map,
    points=track_df,
    newcolname="elevation",
)

# Plot water masses
fig.plot(
    x=[0, 15],
    y=[0, 0],
    fill="lightblue",  # Fill the polygon in "lightblue"
    # Draw a 0.25-points thick black solid outline
    pen="0.25p,black,solid",
    close="+y-8000",  # Force closed polygon
)

# Plot elevation along the survey line
fig.plot(
    x=track_df.p,
    y=track_df.elevation,
    fill="gray",  # Fill the polygon in "gray"
    # Draw a 1-point thick black solid outline
    pen="1p,black,solid",
    close="+y-8000",  # Force closed polygon
)

fig.show()