Interactive Maps with tmap and Shiny

Session 2: The interactive view mode

Martijn Tennekes

Modes in tmap

Two built-in modes

tmap supports two output modes using the same map code:

"plot" — static maps (default), rendered via grid
"view" — interactive maps, powered by Leaflet

Additional modes via extensions:

"mapbox" and "maplibre" — via tmap.mapgl (covered in Session 7)

Switching modes

# Switch to interactive view mode
tmap_mode("view")

# Switch back to static plot mode
tmap_mode("plot")

# Toggle between the two built-in modes
ttm()

When more than two modes are loaded (e.g. after loading tmap.mapgl), use rtm() to rotate through all available modes.

The same map, two modes

library(tmap)

tm <- tm_shape(World) +
  tm_polygons(fill = "HPI", fill.legend = tm_legend("Happy Planet Index"))

# Static
tmap_mode("plot"); tm

# Interactive
tmap_mode("view"); tm

In "view" mode, a basemap is added by default and the map becomes interactive — zoom, pan, tooltips, popups, and more. Basemaps are covered in the next section.

Mode-specific options

Use tm_view() to control interactive-mode behaviour:

tmap_mode("view")

tm_shape(World) +
  tm_polygons("HPI") +
  tm_view(
    control.position = c("left", "bottom"),
    set_view = 2          # default zoom level
  )

See ?tm_view for all options (control box position, zoom limits, etc.).

Basemaps

What are basemaps?

A basemap is a pre-built map used as a geographic background
Drawn at the bottom, below all data layers
In "view" mode, basemaps are added by default
In "plot" mode, you need to add them explicitly via tm_basemap()

Use tm_basemap() to control which basemap is shown.

Basemap providers

Most basemaps are generated from OpenStreetMap data
Each provider styles OSM elements differently (colours, line widths, labels…)
The available providers depend on the mode: many work across modes, but there are differences — use tmap_providers() to see what is available for the active mode
Provider previews: Leaflet providers | MapLibre styles

# List providers available for the current mode
tmap_providers()

# Or use tab-completion:
.tmap_providers$

Using `tm_basemap()`

tmap_mode("view")

tm_shape(NLD_muni) +
  tm_polygons("employment_rate") +
  tm_basemap("OpenStreetMap")

# Disable basemap entirely
tm_shape(World) +
  tm_polygons("HPI") +
  tm_basemap(NULL)

Multiple basemaps — adding layers

Start with a choropleth and multiple switchable basemaps:

tmap_mode("view")

tm_shape(NLD_muni) +
  tm_polygons("employment_rate") +
tm_shape(NLD_prov) +
  tm_borders(lwd = 3) +
tm_basemap(c("OpenStreetMap",
             "CartoDB.Positron",
             "Esri.WorldImagery"))

The three basemaps appear as radio buttons — only one visible at a time.

Multiple basemaps — combining into a layer group

Assign both data layers to the same group so they toggle together:

tmap_mode("view")

tm_shape(NLD_muni) +
  tm_polygons("employment_rate", group = "Choropleth") +
tm_shape(NLD_prov) +
  tm_borders(lwd = 3, group = "Choropleth") +
tm_basemap(c("OpenStreetMap",
             "CartoDB.Positron",
             "Esri.WorldImagery"))

Now the municipality fill and province borders share a single “Choropleth” checkbox in the layer control.

Multiple basemaps — radio toggle between basemap and choropleth

Use tm_group() with control = "radio" to make the choropleth switch like a basemap — so only one of the basemap or the choropleth is visible at a time:

tmap_mode("view")

tm_shape(NLD_muni) +
  tm_polygons("employment_rate", group = "Choropleth") +
tm_shape(NLD_prov) +
  tm_borders(lwd = 3, group = "Choropleth") +
tm_basemap("OpenStreetMap") +
tm_group("Choropleth", control = "radio")

Overlay maps with `tm_tiles()`

An overlay map contains only vector elements (roads, labels, boundaries) with a transparent background
Use tm_tiles() — drawn in call order, unlike tm_basemap() which is always at the bottom
Multiple overlays become check box toggles (vs radio buttons for basemaps)

tmap_mode("view")

tm_shape(NLD_muni) +
  tm_polygons("employment_rate") +
  tm_basemap("Esri.WorldImagery") +
  tm_tiles("CartoDB.PositronOnlyLabels")

Tile servers

Basemaps and overlays are served as map tiles — small PNG images assembled by the browser:

URL template example: "http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png"
Variables: {s} = server, {z} = zoom, {x} and {y} = tile coordinates
You can pass a URL template directly to tm_basemap() or tm_tiles()

Plot mode + basemaps

Basemaps also work in "plot" mode — tmap downloads the tiles at the requested zoom level:

tmap_mode("plot")

tm_shape(NLD_muni) +
  tm_polygons("employment_rate") +
  tm_basemap("OpenStreetMap", zoom = 8)

Note: Thunderforest and Stadia maps require a free API key.

CRS in view mode

Leaflet and Web Mercator

In "view" mode, Leaflet uses Web Mercator (EPSG:3857) by default — because the vast majority of basemap tiles are served in this CRS, Leaflet adopts it as the map’s projection:

Your spatial data is reprojected on the fly to Web Mercator for display
Coordinates are shown as latitude/longitude (EPSG:4326)
The original CRS of your data is preserved in R — only the display uses 3857

tmap_mode("view")

# NLD_muni is in RD New (EPSG:28992), the official CRS for the Netherlands
tm_shape(NLD_muni) +
  tm_polygons("employment_rate")

Using a different CRS in view mode

You can set a different CRS via tm_crs(), but basemaps will usually disappear:

tmap_mode("view")

tm_shape(NLD_muni) +
  tm_polygons("employment_rate") +
tm_crs(28992) +       # RD New — virtually no tile providers support this
tm_basemap(NULL)      # disable basemap explicitly

Tile servers for CRS other than 3857 do exist, but require a custom tile URL template — not covered here
In practice: if you need a non-Web-Mercator CRS in view mode, disable basemaps

CRS in view mode — summary

By default, "view" mode uses Web Mercator (EPSG:3857) with basemaps. When you switch to any other CRS, basemaps are usually not available:

Mode	Default CRS	Basemaps
`"view"` (default)	Web Mercator (3857)	✅ available
`"view"` + custom `tm_crs()`	any	❌ usually not available
`"maplibre"` / `"mapbox"`	Web Mercator (3857)	✅ available
`"maplibre"` / `"mapbox"` + Globe	Globe	✅ some — even in 3D!

The "maplibre" and "mapbox" modes from tmap.mapgl work the same way, but add a Globe CRS and support 3D basemaps — more on that in Sessions 7 and 8. 🌐

World maps in view mode

Web Mercator distorts the poles heavily — fine for navigation and location, not recommended for thematic world maps.

Option 1 — use a world CRS and disable basemaps:

tmap_mode("view")
tm_shape(World) +
  tm_polygons("HPI") +
tm_crs("+proj=eqearth") +   # Equal Earth, Robinson, etc.
tm_basemap(NULL)             # basemaps not available for this CRS

Option 2 — use the "maplibre" mode from tmap.mapgl (Globe CRS by default, no distortion):

library(tmap.mapgl)
tmap_mode("maplibre")
tm_shape(World) +
  tm_polygons("HPI")

Recap

Use tmap_mode("view") or ttm() to switch to interactive mode
The same tmap code works in both "plot" and "view" mode
In "view" mode, a basemap is added by default; use tm_basemap() to change or disable it
tm_tiles() adds overlay maps; multiple basemaps → radio buttons, multiple tiles → checkboxes
Use tmap_providers() to see which providers are available for the active mode
Most basemaps are served in Web Mercator — so Leaflet uses this as its default CRS
For thematic world maps: use a world CRS + tm_basemap(NULL), or use the "maplibre" mode from tmap.mapgl
Provider previews: Leaflet providers | MapLibre styles