sf (version 0.6-0)

st_read: 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 binary

Usage

st_read(dsn, layer, ...)
# S3 method for default
st_read(dsn, layer, ..., options = NULL, quiet = FALSE,
  geometry_column = 1L, type = 0, promote_to_multi = TRUE,
  stringsAsFactors = default.stringsAsFactors(), int64_as_string = FALSE,
  check_ring_dir = FALSE)
read_sf(..., quiet = TRUE, stringsAsFactors = FALSE)
st_read_db(conn = NULL, table = NULL, query = NULL, geom_column = NULL,
  EWKB, ...)

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

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.

...

parameter(s) passed on to st_as_sf

options

character; driver dependent dataset open options, multiple options supported.

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? The `factory-fresh' default is TRUE, but this can be changed by setting options(stringsAsFactors = FALSE).

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)

conn

open database connection

table

table name

query

SQL query to select records; see details

geom_column

character or integer: indicator of name or position of the geometry column; if not provided, the last column of type character is chosen

EWKB

logical; is the WKB is of type EWKB? if missing, defaults to TRUE if conn is of class codePostgreSQLConnection or PqConnection, and to FALSE otherwise

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.

Details

for geometry_column, see also https://trac.osgeo.org/gdal/wiki/rfc41_multiple_geometry_fields; for type values 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.

In case of problems reading shapefiles from USB drives on OSX, please see https://github.com/r-spatial/sf/issues/252.

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. write_sf delete layers by default: it overwrites existing files.

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)

in case geom_column is missing: if table is missing, this function will try to read the name of the geometry column from table geometry_columns, in other cases, or when this fails, the geom_column is assumed to be the last column of mode character. If table is missing, the SRID cannot be read and resolved into a proj4string by the database, and a warning will be given.

Examples

# NOT RUN {
nc = st_read(system.file("shape/nc.shp", package="sf"))
summary(nc) # note that AREA was computed using Euclidian area on lon/lat degrees

# }
# 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)
# }
# NOT RUN {
# 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 = read_sf(geojson_txt)
x
# }
# NOT RUN {
library(RPostgreSQL)
try(conn <- dbConnect(PostgreSQL(), dbname = "postgis"))
if (exists("conn") && !inherits(conn, "try-error")) {
  x = st_read_db(conn, "meuse", query = "select * from meuse limit 3;")
  x = st_read_db(conn, table = "public.meuse")
  print(st_crs(x)) # SRID resolved by the database, not by GDAL!
  dbDisconnect(conn)
 }
# }

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