2.8 Geographic and projected coordinate reference systems (CRS)

2.8.1 Geographic coordinate systems

Longitude is location in the East-West direction in angular distance from the Prime Meridian plane. Latitude is angular distance North or South of the equatorial plane.

Earth is not flat! and not exactly a sphere (neither an ellipsoids)

To define an ellipsoids we need the polar (semi-minor axis) and equatorial radius (semi-major axis).

The datum (geocentruc / local) contains information on what kind of ellispoid use and the relation with Cartesian coordinates on earth surface.

2.8.2 Projected coordinate reference systems

  • based on a geographic CRS

  • Cartesian coordinates, flat surfaces with x(easting)/y(northing) origin

  • they can be conic, cylindrical, planar

2.8.3 Units

CRS have specific units, sf use units to keep track of them.

luxembourg = world[world$name_long == "Luxembourg", ]
st_area(luxembourg) / 1000000
## 2416.87 [m^2]
units::set_units(st_area(luxembourg), km^2)
## 2416.87 [km^2]
#> 2409 [km^2]

For a raster you can use res()