With the tmap package, thematic maps can be generated with great flexibility. The syntax for creating plots is similar to that of
ggplot2. The tmap package also contains many facility functions for reading and processing shape files (see overview). This vignette will focus on the core business of tmap, which is plotting maps.
Each map can be plotted as a static map and shown interactively. These two modes, called the
"plot" mode and the
"view" mode respectively, are described in
vignette("tmap-modes"). In the remainder of this vignette, the
"plot" mode is used.
We refer to shape objects as objects from the class
Raster, respectively from the
sp and the
raster package. The supported subclasses are:
|Without data||With data|
Obviously, shape objects with data (the right-hand side column) are recommended, since data is what we want to show.
Load shape object of Europe (contained in this package):
Shape objects in ESRI format can be read with
read_shape and written with
write_shape. Projection can be get and set with
set_projection respectively. Note: projections can also directly (and temporarily) be set in the plotting method (as argument of
tm_shape, see below).
The plotting syntax is based on that of
qtm function is
tmap’s equivalent to
qplot. The first, and only required argument is a shape object:
So, by default, the polygons (in case the shape object is a SpatialPolygonsDataFrame) are filled with light grey, and the polygon borders are drawn in dark grey.
A choropleth is created with the following code:
qtm(Europe, fill="well_being", text="iso_a3", text.size="AREA", format="Europe", style="gray", text.root=5, fill.title="Well-Being Index", fill.textNA="Non-European countries")
In this code,
text.size serve as aesthetics. Both
iso_a3 are variables of the data contained in the shape object
Europe. A color palette, in this case the qualitative palette from yellow to brown, is mapped to the values of
well_being. The variable
iso_a3 contains the text labels, in this case the country codes. The value
"AREA" is a derived variable that contains the polygon area sizes. So text is sized increasingly with coutry area size.
The two arguments
style are predefined layout settings (see layout).
The other arguments are passed on to the layer functions, which are described below. The argument
text.root determines how text size is increased; in this case, the fifth root of the area sizes are taken. The result is that the text label for Russia does not dominate the other text labels. The
fill.title argument is the title for the
fill-legend. The argument
fill.textNA is the legend text for missing values.
qtm offers the same flexibility as the main plotting method (to be described next). However, for more complex plots, the main plotting method is recommended for tis readability.
The main plotting method, the equivalent to
ggplot, consists of elements that start with
tm_. The first element to start with is
tm_shape, which specifies the shape object. Next, one, or a combination of the following drawing layers should be specified:
||Draws symbols||size, col, shape|
||Draws polylines||col, lwd|
||Draws a raster||col|
||Add text labels||text, size, col|
||Fills the polygons||see tm_polygons|
||Draws polygon borders||none|
||Draws bubbles||see tm_symbols|
||Draws squares||see tm_symbols|
||Draws dots||see tm_symbols|
||Draws markers||see tm_symbols and tm_text|
||Draws iso/contour lines||see tm_lines and tm_text|
Each aesthetic can take a constant value or a data variable name. For instance,
tm_fill(col="blue") colors all polygons blue, while
"var1" is the name of a data variable in the shape object, creates a choropleth. If a vector of constant values or variable names are provided, small multiples are created.
The following layers are map attributes:
||Add coordinate grid lines|
||Add credits text label|
||Add map compass|
||Add scale bar|
The last plot is reproduced as follows:
tm_shape(Europe) + tm_polygons("well_being", textNA="Non-European countries", title="Well-Being Index") + tm_text("iso_a3", size="AREA", root=5) + tm_format_Europe() + tm_style_grey()
We refer to
tm_shape and its subsequent drawing layers as a group. Multiple groups can be stacked. To illustrate this, let’s create a topographic map of Europe:
data(land, rivers, metro) tm_shape(land) + tm_raster("trees", breaks=seq(0, 100, by=20), legend.show = FALSE) + tm_shape(Europe, is.master = TRUE) + tm_borders() + tm_shape(rivers) + tm_lines(lwd="strokelwd", scale=5, legend.lwd.show = FALSE) + tm_shape(metro) + tm_bubbles("pop2010", "red", border.col = "black", border.lwd=1, size.lim = c(0, 11e6), sizes.legend = c(1e6, 2e6, 4e6, 6e6, 10e6), title.size="Metropolitan Population") + tm_text("name", size="pop2010", scale=1, root=4, size.lowerbound = .6, bg.color="white", bg.alpha = .75, auto.placement = 1, legend.size.show = FALSE) + tm_format_Europe() + tm_style_natural()
Things to learn from this code:
tm_shape, but in this case in the second
is.master=TRUE. Notice that the other shapes, i.e.
metroalso contains outside Europe: see for instance
tm_layoutcontrols all layout options such as fonts, legends, margins, and colors. The element
tm_format_Europeis a wrapper function with some other defaults that are tailored for Europe: for instance, the legend is placed top right. The element
tm_layout_naturalis another wrapper function of
tm_layoutused to specify map-independent layout layout settings, such as default colors. See also layout.
Small multiples are generated in two ways:
tm_shape(Europe) + tm_polygons(c("HPI", "gdp_cap_est"), style=c("pretty", "kmeans"), palette=list("RdYlGn", "Purples"), auto.palette.mapping=FALSE, title=c("Happy Planet Index", "GDP per capita")) + tm_format_Europe() + tm_style_grey()
In this case, two independent maps are created, with different scales. All arugments of the layer functions can be vectorized, one for each small multiple. Arugments that normally can take a vector, such as
palette should be placed in a
This method is normally used to show two totally different variables, such as in this example Happy Planet Index and GDP. However, it is also possible to show variables that are related, as if they are subsets from the same data:
tm_shape(metro) + tm_bubbles(size=c("pop1970", "pop2020"), title.size="Population") + tm_facets(free.scales=FALSE) + tm_layout(panel.labels=c("1970", "2020"))
Notice that this plot uses panels and that the common legend is plot outside of the maps.
tm_shape(Europe) + tm_polygons("well_being", title="Well-Being Index") + tm_facets("part") + tm_style_grey()
This plot also uses the panel layout with the common legend drawn outside the maps. These options can be changed with the arguments
tm_layout. By default, the panel/external legend layout is used when the group-by variable is specified, since in that case, the multiples share a common legend.
The scales of each aesthetic argument can be set to either fixed or free, and also, the coordinate ranges of the small multiples:
tm_shape(Europe[Europe$continent=="Europe",]) + tm_fill("part", legend.show = FALSE) + tm_facets("name", free.coords=TRUE, drop.units=TRUE)