Note
Go to the end to download the full example code
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()
![02 contour map](../_images/sphx_glr_02_contour_map_001.png)
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()
![02 contour map](../_images/sphx_glr_02_contour_map_002.png)
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()
![02 contour map](../_images/sphx_glr_02_contour_map_003.png)
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()
![02 contour map](../_images/sphx_glr_02_contour_map_004.png)
Additional exercises
This is the end of the second tutorial. Here are some additional exercises for the concepts that were discussed:
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.Create a contour map of the area around Mt. Rainier. A suggestion for the
region
would be[-122, -121, 46.5, 47.5]
. Adjust thepygmt.Figure.grdcontour
andpygmt.Figure.colorbar
settings as needed to make the figure look good.Create a contour map of São Miguel Island in the Azores; a suggested
region
is[-26, -25, 37.5, 38]
. Instead of coloring inland
, setwater
to “lightblue” to only display Earth relief information for the land.Try other CPTs, such as “SCM/fes” or “geo”.
Total running time of the script: (0 minutes 1.001 seconds)