2. Create a contour map

This tutorial page covers the basics of creating a figure of the Earth relief, using a remote dataset hosted by GMT, using the method pygmt.datasets.load_earth_relief. It will use the pygmt.Figure.grdimage, pygmt.Figure.grdcontour, pygmt.Figure.colorbar, and pygmt.Figure.coast methods for plotting.

import pygmt

Loading the Earth relief dataset

The first step is to use pygmt.datasets.load_earth_relief. The resolution parameter sets the resolution of the remote grid file, which will affect the resolution of the plot made later in the tutorial. The registration parameter determines the grid registration.

This grid region covers the islands of Guam and Rota in the western Pacific Ocean.

grid = pygmt.datasets.load_earth_relief(
    resolution="30s", region=[144.5, 145.5, 13, 14.5], registration="gridline"
)

Plotting Earth relief

To plot Earth relief data, the method pygmt.Figure.grdimage can be used to plot a color-coded figure to display the topography and bathymetry in the grid file. The grid parameter accepts the input grid, which in this case is the remote file downloaded in the previous step. If the region parameter is not set, the region boundaries of the input grid are used.

The cmap parameter sets the color palette table (CPT) used for portraying the Earth relief. The pygmt.Figure.grdimage method uses the input grid to relate the Earth relief values to a specific color within the CPT. In this case, the CPT “oleron” is used; a full list of CPTs can be found at https://docs.generic-mapping-tools.org/6.5/reference/cpts.html.

fig = pygmt.Figure()
fig.grdimage(grid=grid, frame="a", projection="M10c", cmap="oleron")
fig.show()

Adding a colorbar

To show how the plotted colors relate to the Earth relief, a colorbar can be added using the pygmt.Figure.colorbar method.

To control the annotation and labels on the colorbar, a list is passed to the frame parameter. The value beginning with "a" sets the interval for the annotation on the colorbar, in this case every 1,000 meters. To set the label for an axis on the colorbar, the argument begins with either "x+l" (x-axis) or "y+l" (y-axis), followed by the intended label.

By default, the CPT for the colorbar is the same as the one set in pygmt.Figure.grdimage.

fig = pygmt.Figure()
fig.grdimage(grid=grid, frame="a", projection="M10c", cmap="oleron")
fig.colorbar(frame=["a1000", "x+lElevation", "y+lm"])
fig.show()

Adding contour lines

To add contour lines to the color-coded figure, the pygmt.Figure.grdcontour method is used. The frame and projection are already set using pygmt.Figure.grdimage and are not needed again. However, the same input for grid (in this case, the variable named “grid”) must be input again. The interval parameter sets the spacing between adjacent contour lines (in this case, 500 meters). The annotation parameter annotates the contour lines corresponding to the given interval (in this case, 1,000 meters) with the related values, here elevation or bathymetry. By default, these contour lines are drawn thicker. Optionally, the appearance (thickness, color, style) of the annotated and the not-annotated contour lines can be adjusted (separately) by specifying the desired pen.

fig = pygmt.Figure()
fig.grdimage(grid=grid, frame="a", projection="M10c", cmap="oleron")
fig.grdcontour(grid=grid, interval=500, annotation=1000)
fig.colorbar(frame=["a1000", "x+lElevation", "y+lm"])
fig.show()

Color in land

To make it clear where the islands are located, the pygmt.Figure.coast method can be used to color in the landmasses. The land is colored in as “lightgray”, and the shorelines parameter draws a border around the islands.

fig = pygmt.Figure()
fig.grdimage(grid=grid, frame="a", projection="M10c", cmap="oleron")
fig.grdcontour(grid=grid, interval=500, annotation=1000)
fig.coast(shorelines="2p", land="lightgray")
fig.colorbar(frame=["a1000", "x+lElevation", "y+lm"])
fig.show()

Additional exercises

This is the end of the second tutorial. Here are some additional exercises for the concepts that were discussed:

1. Change the resolution of the grid file to either "01m" (1 arc-minute, a lower resolution) or "15s" (15 arc-seconds, a higher resolution). Note that higher resolution grids will have larger file sizes. Available resolutions can be found here.

2. Create a contour map of the area around Mt. Rainier. A suggestion for the region would be [-122, -121, 46.5, 47.5]. Adjust the pygmt.Figure.grdcontour and pygmt.Figure.colorbar settings as needed to make the figure look good.

3. Create a contour map of São Miguel Island in the Azores; a suggested region is [-26, -25, 37.5, 38]. Instead of coloring in land, set water to “lightblue” to only display Earth relief information for the land.

4. Try other CPTs, such as “SCM/fes” or “geo”.