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Create Map Insets with mapinsetr

R-CMD-check

Map insets are small zoom-in or zoom-out maps that focus on particular territories within a general map. mapinsetr provides a set of functions that helps to create such insets.

Installation

require(devtools)
devtools::install_github("riatelab/mapinsetr")

Version

The previous version of the package was based on sp, rgeos and maptools.
This version (0.2.0) is based on the sf package only.

Example

The aim of this example is to create a base map of Algeria’s municipalities with an inset on the Algiers wilaya.
Here are the few steps of the base map creation process:

  • download data on Algerian administrative boundaries,
  • create a mask that will help to extract the municipalities,
  • create an inset of Alger municipalities within the previously created mask,
  • merge the original basemap and inset

Download data on Algerian administrative boundaries.

The raster package offers an access to the GADM database through the getData() function. The ISO3 code of Algeria is DZA and we will need 2 levels of administrative data (0 for the state and 2 for the municipalities).
Basemaps (SpatialPolygonsDataFrame) are downloaded unprojected in WGS84. We have to transform them into simple feature collections (sf objects) and project them in a valid projection (the EPSG code 30791 stands for “Nord Sahara 1959 / Voirol Unifie Nord”).

library(raster)
library(sf)
# download data from gadm.org
adm0 <-st_as_sf( getData(name = "GADM", country = "DZA", level = 0, path = tempdir() ))
adm0 <- st_transform(adm0, 30791)
adm2 <- st_as_sf(getData(name = "GADM", country = "DZA", level = 2, path = tempdir()))
adm2 <- st_transform(adm2, 30791)
# plot
plot(st_geometry(adm0), lwd = 2)
plot(st_geometry(adm2), add = T, col = "grey80", lwd = 0.2)

Create a mask that will help to extract the municipalities

Our goal is to extract Alger municipalities. To extract those munuicipalities we will use a mask.
This mask has to be an sf object. The create_mask function create a mask based on a bbox, a sf or sfc extent or an interactively defined rectangle.
In this example we use a mask defined by Alger willaya extent + a 2 km buffer.

library(mapinsetr)
alger_extent <- st_buffer(adm2[adm2$NAME_1=="Alger", ],dist = 2000)
box_alger <- create_mask(bb = alger_extent)
plot(st_geometry(adm0), lwd = 2)
plot(st_geometry(adm2), add=T, col = "grey80", lwd = 0.2)
plot(st_geometry(box_alger), border = "red", add=T, lwd = 2)

Alternatively, one can define the mask interactively on the plot :

library(mapinsetr)
plot(st_geometry(adm0), lwd = 2)
plot(st_geometry(adm2), add=T, col = "grey80", lwd = 0.2)
box_alger_interactive <- create_mask(interactive = TRUE, add=TRUE, 
                                     prj =  st_crs(adm0))

Create an inset of Alger municipalities within the previously created mask

The move_and_resize function extracts polygons or multipolygons (x) within a mask (mask), moves them to coordinates (xy, bottom left corner of the inset) and resizes them according to a factor (k).

zoom_alger <- move_and_resize(x = adm2, mask = box_alger, xy = c(-1000000,  -100000), k = 15)
box <- move_and_resize(x = box_alger, mask = box_alger, xy = c(-1000000,  -100000), k = 15)
plot(st_geometry(adm0), lwd = 2)
plot(st_geometry(adm2), add=T, col = "grey80", lwd = 0.2)
plot(st_geometry(box_alger), border = "red", add=T, lwd = 2)
plot(st_geometry(zoom_alger), add=T, col = "grey80", lwd = 0.2)
plot(st_geometry(box), border = "red", lwd = 1, add=T)

Merge the original basemap and inset

We currently have 2 sf objects, the next step is to merge them in a single one inset_rbinder merge the initial basemap and the inset.

adm2final <- inset_rbinder(list(adm2, zoom_alger))
plot(st_geometry(adm2final), col = "grey80", lwd = 0.2)
title("Final basemap with inset!")

Plot an example

The following example build a map of Municipalities types.

library(cartography)
adm2final[adm2final$CCA_2==2223, "TYPE_2"] <- "Commune"
plot(st_geometry(adm0), lwd = 2)
typoLayer(adm2final, var = "TYPE_2", 
          legend.title.txt = "Commune Type",
          border = "white", lwd = 0.1, 
          col = carto.pal("multi.pal", 3), 
          legend.pos = "topright", add = T)
plot(st_geometry(box_alger), border = "red", add=T, lwd = 2)
plot(st_geometry(box), border = "red", lwd = 2, add=T)
layoutLayer(title = "Commune Types in Algeria with a Zoom on Alger Region", 
            sources = "GADM 2.8, 2015", author = "@rCarto", theme = "green.pal")