Vector data I/O

The library {sf} can import data from various sources:
1. A multitude of file formats from disk (powered gdal / ogr2ogr)
2. Various geospatial databases such as PostGIS
3. From other sources (such as WKT, WKB other geospatial R-libraires)
We will only introduce the first method, since this is the most common use case

GDAL / ogr2ogr

GDAL:

is an open source translator library for raster and vector geospatial data formats.
stands for Geospatial Data Abstraction Library
is used in most geospatial software, be it FOSS or proprietary. The list includes: ArcGIS, QGIS, R (sf) and Python (geopandas)

GDAL Vector drivers (read)

Since GDAL supports a long list of different geospatial file formats, most (all?) are in turn supported by {sf}
Run the function st_drivers for a full list
{sf} tries to guess the correct driver based on the file extension (see below)

# sf uses the geojson driver, based on the file extension
# (data from moodle →  Spatial Databases II → Database Files)
stationen_schweiz <- read_sf("data/uebung1/stationen_schweiz.geojson")

# sf uses the shapefile driver, based on the file extension
# (data from moodle →  Spatial Analysis I → Exercise Data) 
hoheitsgebiet <- read_sf("data/Spatial_Analysis_I/Hoheitsgebiet/HOHEITSGEBIET_FR.shp")

Multiple layers in one file

Some file formats, e.g. Geopackages (*.gpkg) or Geodatabases (*.gdb) support multiple datasets in a single file.
If no specific layer is requested read_sf() will import the first available layer
If more than 1 layers are available, read_sf() will return a warning
The function st_layers() will list all available layers

tlm3d_path <- "data/Spatial_Analysis_II/swiss_TLM3D.gpkg"

# Note the warning:
tlm3d <- read_sf(tlm3d_path)

Warning in CPL_read_ogr(dsn, layer, query, as.character(options), quiet, :
automatically selected the first layer in a data source containing more than
one.

# This will list all layers, including some metadata
st_layers(tlm3d_path)

Driver: GPKG 
Available layers:
    layer_name        geometry_type features fields       crs_name
1       tlm_bb     3D Multi Polygon    49321     13 CH1903+ / LV95
2 tlm_strassen 3D Multi Line String   191644     27 CH1903+ / LV95

# To import a single layer
tlm_bb <- read_sf(tlm3d_path, "tlm_bb")

SQL queries during import

read_sf() can parse a (spatialite) SQL query provided in the query = argument
In case query = is used, the layers = argument should be skipped (since the layers is specified in the query)

tlm_seen <- read_sf(
  tlm3d_path, 
  query = "SELECT objektart, geom FROM tlm_bb WHERE objektart = 'Stehende Gewaesser'"
  )

GDAL Vector drivers (write)

The function st_write() is used to export an sf object to file
Most vector drivers support reading and writing (see st_drivers / column write)
Many file formats support appending to the dataset (see append =)

st_write(tlm_seen, "data-out/seen.geojson")

🪕 Tasks

Import the datasets from Spatial Analysis I and II using R
Explore these datasets by printing the sf objects to the console
Try different visualization methods (see Plotting simple features)