6  Coordinate Reference Systems (CRS)

6.1 Coordinate Reference Systems

“Map projections try to portray the surface of the earth or a portion of the earth on a flat piece of paper or computer screen. A coordinate reference system (CRS) then defines, with the help of coordinates, how the two-dimensional, projected map in your GIS is related to real places on the earth. The decision as to which map projection and coordinate reference system to use, depends on the regional extent of the area you want to work in, on the analysis you want to do and often on the availability of data.” From qgis.org

6.2 Call Libraries

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library(sf) # simple (spatial) features

library(ggplot2) # beautiful graphs

6.3 Open wrld_simpl Shapefile

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world <- read_sf("./shapefiles/wrld_simpl/wrld_simpl.shp")
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head(world) # show the top (head) of the data
Simple feature collection with 6 features and 10 fields
Geometry type: MULTIPOLYGON
Dimension:     XY
Bounding box:  xmin: -61.88722 ymin: -18.01639 xmax: 50.37499 ymax: 42.61805
Geodetic CRS:  GCS_unknown
# A tibble: 6 × 11
  FIPS  ISO2  ISO3     UN NAME                AREA REGION SUBREGION    LON   LAT
  <chr> <chr> <chr> <int> <chr>              <int>  <int>     <int>  <dbl> <dbl>
1 AC    AG    ATG      28 Antigua and Barb…     44     19        29 -61.8   17.1
2 AG    DZ    DZA      12 Algeria           238174      2        15   2.63  28.2
3 AJ    AZ    AZE      31 Azerbaijan          8260    142       145  47.4   40.4
4 AL    AL    ALB       8 Albania             2740    150        39  20.1   41.1
5 AM    AM    ARM      51 Armenia             2820    142       145  44.6   40.5
6 AO    AO    AGO      24 Angola            124670      2        17  17.5  -12.3
# ℹ 1 more variable: geometry <MULTIPOLYGON [°]>

6.4 Find Out the CRS of wrld_simpl

As with many global data sets (and many other data sets), wrld_simpl uses World Geodetic System 1984.

Show the code 
st_crs(world)
Coordinate Reference System:
  User input: GCS_unknown 
  wkt:
GEOGCRS["GCS_unknown",
    DATUM["World Geodetic System 1984",
        ELLIPSOID["WGS 84",6378137,298.257223563,
            LENGTHUNIT["metre",1]],
        ID["EPSG",6326]],
    PRIMEM["Greenwich",0,
        ANGLEUNIT["Degree",0.0174532925199433]],
    CS[ellipsoidal,2],
        AXIS["longitude",east,
            ORDER[1],
            ANGLEUNIT["Degree",0.0174532925199433]],
        AXIS["latitude",north,
            ORDER[2],
            ANGLEUNIT["Degree",0.0174532925199433]]]

6.5 Plot The wrld_simpl Data

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ggplot(world) + 
  geom_sf() +
  theme_minimal()