st_read: Read simple features or layers from file or database

In sf: Simple Features for R

Read simple features or layers from file or database

Description

Read simple features from file or database, or retrieve layer names and their geometry type(s)

Read PostGIS table directly through DBI and RPostgreSQL interface, converting Well-Know Binary geometries to sfc

Usage

st_read(dsn, layer, ...)

## S3 method for class 'character'
st_read(
  dsn,
  layer,
  ...,
  query = NA,
  options = NULL,
  quiet = FALSE,
  geometry_column = 1L,
  type = 0,
  promote_to_multi = TRUE,
  stringsAsFactors = sf_stringsAsFactors(),
  int64_as_string = FALSE,
  check_ring_dir = FALSE,
  fid_column_name = character(0),
  drivers = character(0),
  wkt_filter = character(0),
  optional = FALSE,
  use_stream = default_st_read_use_stream()
)

read_sf(..., quiet = TRUE, stringsAsFactors = FALSE, as_tibble = TRUE)

## S3 method for class 'DBIObject'
st_read(
  dsn = NULL,
  layer = NULL,
  query = NULL,
  EWKB = TRUE,
  quiet = TRUE,
  as_tibble = FALSE,
  geometry_column = NULL,
  ...
)

Arguments

dsn	data source name (interpretation varies by driver - for some drivers, dsn is a file name, but may also be a folder, or contain the name and access credentials of a database); in case of GeoJSON, dsn may be the character string holding the geojson data. It can also be an open database connection.
layer	layer name (varies by driver, may be a file name without extension); in case layer is missing, st_read will read the first layer of dsn, give a warning and (unless quiet = TRUE) print a message when there are multiple layers, or give an error if there are no layers in dsn. If dsn is a database connection, then layer can be a table name or a database identifier (see Id). It is also possible to omit layer and rather use the query argument.
...	parameter(s) passed on to st_as_sf
query	SQL query to select records; see details
options	character; driver dependent dataset open options, multiple options supported. For possible values, see the "Open options" section of the GDAL documentation of the corresponding driver, and https://github.com/r-spatial/sf/issues/1157 for an example.
quiet	logical; suppress info on name, driver, size and spatial reference, or signaling no or multiple layers
geometry_column	integer or character; in case of multiple geometry fields, which one to take?
type	integer; ISO number of desired simple feature type; see details. If left zero, and promote_to_multi is TRUE, in case of mixed feature geometry types, conversion to the highest numeric type value found will be attempted. A vector with different values for each geometry column can be given.
promote_to_multi	logical; in case of a mix of Point and MultiPoint, or of LineString and MultiLineString, or of Polygon and MultiPolygon, convert all to the Multi variety; defaults to TRUE
stringsAsFactors	logical; logical: should character vectors be converted to factors? Default for read_sf or R version >= 4.1.0 is FALSE, for st_read and R version < 4.1.0 equal to default.stringsAsFactors()
int64_as_string	logical; if TRUE, Int64 attributes are returned as string; if FALSE, they are returned as double and a warning is given when precision is lost (i.e., values are larger than 2^53).
check_ring_dir	logical; if TRUE, polygon ring directions are checked and if necessary corrected (when seen from above: exterior ring counter clockwise, holes clockwise)
fid_column_name	character; name of column to write feature IDs to; defaults to not doing this
drivers	character; limited set of driver short names to be tried (default: try all)
wkt_filter	character; WKT representation of a spatial filter (may be used as bounding box, selecting overlapping geometries); see examples
optional	logical; passed to as.data.frame; always TRUE when as_tibble is TRUE
use_stream	Use TRUE to use the experimental columnar interface introduced in GDAL 3.6.
as_tibble	logical; should the returned table be of class tibble or data.frame?
EWKB	logical; is the WKB of type EWKB? if missing, defaults to TRUE

Details

for geometry_column, see also https://gdal.org/en/latest/development/rfc/rfc41_multiple_geometry_fields.html

for values for type see https://en.wikipedia.org/wiki/Well-known_text#Well-known_binary, but note that not every target value may lead to successful conversion. The typical conversion from POLYGON (3) to MULTIPOLYGON (6) should work; the other way around (type=3), secondary rings from MULTIPOLYGONS may be dropped without warnings. promote_to_multi is handled on a per-geometry column basis; type may be specified for each geometry column.

Note that stray files in data source directories (such as *.dbf) may lead to spurious errors that accompanying *.shp are missing.

In case of problems reading shapefiles from USB drives on OSX, please see https://github.com/r-spatial/sf/issues/252. Reading shapefiles (or other data sources) directly from zip files can be done by prepending the path with /vsizip/. This is part of the GDAL Virtual File Systems interface that also supports .gz, curl, and other operations, including chaining; see https://gdal.org/en/latest/user/virtual_file_systems.html for a complete description and examples.

For query with a character dsn the query text is handed to 'ExecuteSQL' on the GDAL/OGR data set and will result in the creation of a new layer (and layer is ignored). See 'OGRSQL' https://gdal.org/en/latest/user/ogr_sql_dialect.html for details. Please note that the 'FID' special field is driver-dependent, and may be either 0-based (e.g. ESRI Shapefile), 1-based (e.g. MapInfo) or arbitrary (e.g. OSM). Other features of OGRSQL are also likely to be driver dependent. The available layer names may be obtained with st_layers. Care will be required to properly escape the use of some layer names.

read_sf and write_sf are aliases for st_read and st_write, respectively, with some modified default arguments. read_sf and write_sf are quiet by default: they do not print information about the data source. read_sf returns an sf-tibble rather than an sf-data.frame. write_sf delete layers by default: it overwrites existing files without asking or warning.

if table is not given but query is, the spatial reference system (crs) of the table queried is only available in case it has been stored into each geometry record (e.g., by PostGIS, when using EWKB)

The function will automatically find the geometry type columns for drivers that support it. For the other drivers, it will try to cast all the character columns, which can be slow for very wide tables.

Value

object of class sf when a layer was successfully read; in case argument layer is missing and data source dsn does not contain a single layer, an object of class sf_layers is returned with the layer names, each with their geometry type(s). Note that the number of layers may also be zero.

Note

The use of system.file in examples make sure that examples run regardless where R is installed: typical users will not use system.file but give the file name directly, either with full path or relative to the current working directory (see getwd). "Shapefiles" consist of several files with the same basename that reside in the same directory, only one of them having extension .shp.

See Also

st_layers, st_drivers

Examples

nc = st_read(system.file("shape/nc.shp", package="sf"))
summary(nc) # note that AREA was computed using Euclidian area on lon/lat degrees

## only three fields by select clause
## only two features by where clause
nc_sql = st_read(system.file("shape/nc.shp", package="sf"),
                     query = "SELECT NAME, SID74, FIPS FROM \"nc\" WHERE BIR74 > 20000")
## Not run: 
  library(sp)
  example(meuse, ask = FALSE, echo = FALSE)
  try(st_write(st_as_sf(meuse), "PG:dbname=postgis", "meuse",
       layer_options = "OVERWRITE=true"))
  try(st_meuse <- st_read("PG:dbname=postgis", "meuse"))
  if (exists("st_meuse"))
    summary(st_meuse)

## End(Not run)

## Not run: 
## note that we need special escaping of layer  within single quotes (nc.gpkg)
## and that geom needs to be included in the select, otherwise we don't detect it
layer <- st_layers(system.file("gpkg/nc.gpkg", package = "sf"))$name[1]
nc_gpkg_sql = st_read(system.file("gpkg/nc.gpkg", package = "sf"),
   query = sprintf("SELECT NAME, SID74, FIPS, geom  FROM \"%s\" WHERE BIR74 > 20000", layer))

## End(Not run)
# spatial filter, as wkt:
wkt = st_as_text(st_geometry(nc[1,]))
# filter by (bbox overlaps of) first feature geometry:
st_read(system.file("gpkg/nc.gpkg", package="sf"), wkt_filter = wkt)
# read geojson from string:
geojson_txt <- paste("{\"type\":\"MultiPoint\",\"coordinates\":",
   "[[3.2,4],[3,4.6],[3.8,4.4],[3.5,3.8],[3.4,3.6],[3.9,4.5]]}")
x = st_read(geojson_txt)
x
## Not run: 
library(RPostgreSQL)
try(conn <- dbConnect(PostgreSQL(), dbname = "postgis"))
if (exists("conn") && !inherits(conn, "try-error")) {
  x = st_read(conn, "meuse", query = "select * from meuse limit 3;")
  x = st_read(conn, table = "public.meuse")
  print(st_crs(x)) # SRID resolved by the database, not by GDAL!
  dbDisconnect(conn)
 }

## End(Not run)

sf documentation built on Oct. 11, 2024, 9:08 a.m.