With the tmap package, thematic maps can be generated with great flexibility. The syntax for creating plots is similar to that of
ggplot2. The add-on package tmaptools contains tool functions for reading and processing shape files.
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|
Also simple features from the new
sf package are supported. 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):
tmaptools package contains functions to read ESRI shape files and process them. Many of these processing steps can also be done directly in
tmap, e.g. setting to a different map projection.
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() ## Warning: The legend is too narrow to place all symbol sizes.
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 three 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)
drop.units is used to drop all non-selected spatial units. If
drop.shapes=FALSE then neighboring countries are also visible.
tm1 <- qtm(Europe, fill = "red") tm2 <- qtm(Europe, fill = "blue") tmap_arrange(tm1, tm2, asp = NA)
The layout of the thematic map can be changed with
tm_layout or one of its wrapper functions. In the next example we use two of these wrapper functions, one for the overall format of world maps, and one for the legend.
data(land) data(World) pal8 <- c("#33A02C", "#B2DF8A", "#FDBF6F", "#1F78B4", "#999999", "#E31A1C", "#E6E6E6", "#A6CEE3") tm_shape(land, ylim = c(-88,88), relative=FALSE) + tm_raster("cover_cls", palette = pal8, title="Global Land Cover", legend.hist=TRUE, legend.hist.z=0) + tm_shape(World) + tm_borders() + tm_format_World(inner.margins=0) + tm_legend(text.size=1, title.size=1.2, position = c("left","bottom"), bg.color = "white", bg.alpha=.2, frame="gray50", height=.6, hist.width=.2, hist.height=.2, hist.bg.color="gray60", hist.bg.alpha=.5)
The wrapper functions starting with
tm_format_ specify the format for a specifc shape. In the tmap package, a couple of them are included, for instance
tm_format_World that is taylored for world maps. It’s also possible to create your own wrapper function for shapes that you will use frequently.
Besides the shape-dependent
tm_format_ wrapper functions, tmap also contains wrapper functions for shape-independent styles.
qtm(Europe, style="natural", title="Natural style") # equivalent to: qtm(Europe) + tm_style_natural(title="Natural style")
qtm(Europe, style="cobalt", title="Cobalt style") # equivalent to: qtm(Europe) + tm_style_cobalt(title="Cobalt style")
style_catalogue() to create an extensive catalogue of the available styles. The default style is
tm_style_white. This default can be changed with the global option called
tmap.style, which can be get and set with
# make a categorical map qtm(Europe, fill="economy", title=paste("Style:", tmap_style())) ## current tmap style is "white"
# change to color-blind-friendly style current_style <- tmap_style("col_blind") ## tmap style set to "col_blind" # make a categorical map qtm(Europe, fill="economy", title=paste("Style:", tmap_style())) ## current tmap style is "col_blind"
# change back tmap_style(current_style) ## tmap style set to "white"
Also, the outer and inner margins as well as the aspect ratio are determined with
(tm <- qtm(World) + tm_layout(outer.margins=c(.05,0,.05,0), inner.margins=c(0,0,.02,0), asp=0))
The behaviour of
asp are correlated. To see the rectangles that these arguments determine, the design mode can be enabled:
tm + tm_layout(design.mode=TRUE) ## ----------------aspect ratios----------------------- ## | specified (asp argument of tm_layout) 0.000000 | ## | device (yellow) 2.500000 | ## | frame (blue) 2.500000 | ## | master shape, World, (red) 1.979637 | ## ----------------------------------------------------
The red rectangle is the bounding box of the shape object. Both
asp determine the measurements of the frame, indicated by the blue rectagle. Setting the left inner margin is useful to have extra space for the legend.
Setting the aspect ratio is handy when the plot is saved to an image with a specific resolution. For instance, to save a thematic World map as a png image of 1920 by 1080 pixels, the setting
outer.margins=0, asp=1920/1080 can be used. When
asp=0, as in the example above, the aspect ratio of the device (given the outer margins) is taken. See
save_tmap, which uses these tricks under the hood.
The following demo shows how a world map can be enhanced with map attributes such as grid lines and a map compass.
tm_shape(land, projection="eck4") + tm_raster("elevation", breaks=c(-Inf, 250, 500, 1000, 1500, 2000, 2500, 3000, 4000, Inf), palette = terrain.colors(9), title="Elevation", auto.palette.mapping=FALSE) + tm_shape(World) + tm_borders("grey20") + tm_grid(projection="longlat", labels.size = .5) + tm_text("name", size="AREA") + tm_compass(position = c(.65, .15), color.light = "grey90") + tm_credits("Eckert IV projection", position = c(.85, 0)) + tm_style_classic(inner.margins=c(.04,.03, .02, .01), legend.position = c("left", "bottom"), legend.frame = TRUE, bg.color="lightblue", legend.bg.color="lightblue", earth.boundary = TRUE, space.color="grey90")
A handy function for saving maps is
tm <- tm_shape(World) + tm_fill("well_being", id="name", title="Well-being") + tm_format_World()
save_tmap(tm, "World_map.png", width=1920, height=1080)
This function can also save interactive maps to stand-alone HTML files:
vignette("tmap-modes") for more on interactive maps.
tm_shape(Europe[Europe$name=="Austria", ]) + tm_polygons()
data(World) tm_shape(World) + tm_fill() + tm_shape(rivers) + tm_lines(col="dodgerblue3") + tm_add_legend(type="line", col="dodgerblue3", title="World map") + tm_format_World()
Each drawing element has a scalar argument called
scale. The overall scaling and font sizes can be set by the
scale argument in
tm_shape(World, bbox = "India") + tm_polygons("MAP_COLORS", palette="Pastel2") + tm_shape(metro) + tm_bubbles("pop2010", title.size = "Population") + tm_text("name", size = "pop2010", legend.size.show = FALSE, root=8, size.lowerbound = .7, auto.placement = TRUE)