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readOGR {rgdal}

Read OGR vector maps into Spatial objects
Package: 
rgdal
Version: 
0.8-16

Description

The function reads an OGR data source and layer into a suitable Spatial vector object. It can only handle layers with conformable geometry features (not mixtures of points, lines, or polygons in a single layer). It will set the spatial reference system if the layer has such metadata.

If reading a shapefile, the data source name (dsn= argument) is the folder (directory) where the shapefile is, and the layer is the name of the shapefile (without the .shp extension). For example to read bounds.shp from C:/Maps, do map <- readOGR(dsn="C:/Maps", layer="bounds"). The logic behind this is that typically one keeps all the shapefiles for a project in one folder (directory).

As noted below, for other file type drivers, the dsn= argument is interpreted differently, and may be the file name itself, as for example with the GPX driver for reading GPS data as layer="tracks" lines or layer="track_points" points.

Usage

readOGR(dsn, layer, verbose = TRUE, p4s=NULL, 
 stringsAsFactors=default.stringsAsFactors(), 
 drop_unsupported_fields=FALSE, input_field_name_encoding=NULL,
 pointDropZ=FALSE, dropNULLGeometries=TRUE,
 useC=TRUE, disambiguateFIDs=FALSE, addCommentsToPolygons=TRUE,
 encoding=NULL, use_iconv=NULL)
ogrInfo(dsn, layer, encoding=NULL, input_field_name_encoding=NULL,
 use_iconv=NULL)
ogrFIDs(dsn, layer)
ogrDrivers()
OGRSpatialRef(dsn, layer)
ogrListLayers(dsn)
 
## S3 method for class 'ogrinfo':
print((x, ...))

Arguments

dsn
data source name (interpretation varies by driver --- for some drivers, dsn is a file name, but may also be a folder)
layer
layer name (varies by driver, may be a file name without extension)
verbose
report progress
p4s
PROJ4 string defining CRS, if default NULL, the value is read from the OGR data set
stringsAsFactors
logical: should character vectors be converted to factors? The ‘factory-fresh’ default is TRUE, but this can be changed by setting options(stringsAsFactors = FALSE) (see link[base]{data.frame}).
drop\_unsupported\_fields
default FALSE, if TRUE skip fields other than String, Integer, and Real; Date, Time and DateTime are converted to String
input\_field\_name\_encoding
default NULL; if not NULL, should be a valid encoding name for the source platform of the data source and layer, see iconv
pointDropZ
default FALSE, if TRUE, discard third coordinates for point geometries; third coordinates are alway discarded for line and polygon geometries
dropNULLGeometries
default TRUE, drop both declared NULL geometries, and empty geometries with no coordinates; if FALSE, return a data frame with the attribute values of the NULL and empty geometries
useC
default TRUE, if FALSE use original interpreted code in a loop
disambiguateFIDs
default FALSE, if TRUE, and FID values are not unique, they will be set to unique values 1:N for N features; problem observed in GML files
addCommentsToPolygons
default TRUE, may be set FALSE for legacy behaviour; used to indicate which interior rings are holes in which exterior rings in conformance with OGC SFS specifications
encoding
default NULL, if set to a character string, and the driver is “ESRI Shapefile”, it is passed to the CPL Option “SHAPE_ENCODING” immediately before reading the DBF of a shapefile. The empty character string “” turns off OGR's encoding functionality, and may be used when GDAL is not built with iconv support.
use_iconv
default NULL, in which case the use of iconv is controlled by the GDAL version (TRUE < 1.9). If set to a logical value, this test will be overridden.
x
ogrinfo object
...
other arguments to print method

Details

The drivers available will depend on the installation of GDAL/OGR, and can vary; the ogrDrivers() function shows which are available, and which may be written (but all are assumed to be readable). Note that stray files in data source directories (such as *.dbf) may lead to suprious errors that accompanying *.shp are missing.

Values

A Spatial object is returned suiting the vector data source, either a SpatialPointsDataFrame (using an AttributeList for its data slot directly), a SpatialLinesDataFrame, or a SpatialPolygonsDataFrame.

Note

The bases for this implementation are taken from functions in Barry Rowlingson's draft Rmap package, and from Radim Blazek's v.in.ogr program in GRASS.

Please note that the OGR drivers used may not handle missing data gracefully, and be prepared to have to correct for this manually. From rgdal 0.5-27, missing value handling has been improved, and OGR unset field values are set to NA in R, but drivers and external files may vary in their representations of missing values.

In addition, from 0.6-9 date and time fields are read as strings rather than being treated as unsupported; NULL geometries are identified and dropped. There are differences in the reporting of NULL geometries between ogrInfo and readOGR - in ogrInfo, only declared NULL geometries are reported, but in readOGR, any line or polygon geometries with no coordinates are assigned NULL geometry status as well. An attempt is made to close unclosed rings in polygon geometries.

For reading GPX files, refer to the OGR GPX format documentation for the use of layer tags: “waypoints”, “tracks”, “routes”, “track_points” and “route_points” - reading GPX files requires a build of GDAL/OGR with the expat XML library.

From 0.6-10, attempts are made to detect deleted features still present in the layer, but not read. Apparently features deleted in Qgis are only marked as deleted, but are still in the layer. These are not NULL geometries, but still need to be handled. An attempt is made to check the FID values, and ogrFIDs now returns attributes permitting this oddity to be detected. Such deleted features were seen as NULL in 0.6-9, but are not such.

From 0.7-24, if the layer has no fields, a single field containing the FID values is placed in the data slot of the returned object.

From 0.7-24, attempts are begun to provide users with arguments to control reading from OGR/shapefile driver when the encoding is inappropriate (especially the setting of LDID in shapefile DBFs, and the SHAPE_ENCODING environment variable).

Because of the representation of DateTime data in OGR, decimal seconds in input data are rounded to integer seconds, see: http://trac.osgeo.org/gdal/ticket/2680.

See Also

SpatialPointsDataFrame-class, SpatialLinesDataFrame-class, SpatialPolygonsDataFrame-class, readShapePoly, iconv

Examples

ogrDrivers()
dsn <- system.file("vectors", package = "rgdal")[1]
ogrListLayers(dsn)
ogrInfo(dsn=dsn, layer="cities")
owd <- getwd()
setwd(dsn)
ogrInfo(dsn="cities.shp", layer="cities")
setwd(owd)
cities <- readOGR(dsn=dsn, layer="cities")
summary(cities)
ogrInfo(dsn=dsn, layer="kiritimati_primary_roads")
OGRSpatialRef(dsn=dsn, layer="kiritimati_primary_roads")
kiritimati_primary_roads <- readOGR(dsn=dsn, layer="kiritimati_primary_roads")
summary(kiritimati_primary_roads)
ogrInfo(dsn=dsn, layer="scot_BNG")
OGRSpatialRef(dsn=dsn, layer="scot_BNG")
scot_BNG <- readOGR(dsn=dsn, layer="scot_BNG")
summary(scot_BNG)
if ("GML" %in% ogrDrivers()$name) {
  dsn <- system.file("vectors/airports.gml", package = "rgdal")[1]
  airports <- try(readOGR(dsn=dsn, layer="airports"))
  if (class(airports) != "try-error") summary(airports)
}
dsn <- system.file("vectors/ps_cant_31.MIF", package = "rgdal")[1]
ogrInfo(dsn=dsn, layer="ps_cant_31")
ps_cant_31 <- readOGR(dsn=dsn, layer="ps_cant_31")
summary(ps_cant_31)
sapply(as(ps_cant_31, "data.frame"), class)
ps_cant_31 <- readOGR(dsn=dsn, layer="ps_cant_31", stringsAsFactors=FALSE)
summary(ps_cant_31)
sapply(as(ps_cant_31, "data.frame"), class)
dsn <- system.file("vectors/Up.tab", package = "rgdal")[1]
ogrInfo(dsn=dsn, layer="Up")
Up <- readOGR(dsn=dsn, layer="Up")
summary(Up)
dsn <- system.file("vectors/test_trk2.gpx", package = "rgdal")[1]
test_trk2 <- try(readOGR(dsn=dsn, layer="tracks"))
if (class(test_trk2) != "try-error") summary(test_trk2)
test_trk2pts <- try(readOGR(dsn=dsn, layer="track_points"))
if (class(test_trk2pts) != "try-error") summary(test_trk2pts)
dsn <- system.file("vectors", package = "rgdal")[1]
ogrInfo(dsn=dsn, layer="trin_inca_pl03")
birds <- readOGR(dsn=dsn, layer="trin_inca_pl03")
summary(birds)

Author(s)

Roger Bivand

Documentation reproduced from package rgdal, version 0.8-16. License: GPL (>= 2)