Read OGR vector maps into Spatial objects

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,
 pointDropZ=FALSE, dropNULLGeometries=TRUE,
 useC=TRUE, disambiguateFIDs=FALSE, addCommentsToPolygons=TRUE,
 encoding=NULL, use_iconv=FALSE, swapAxisOrder=FALSE, require_geomType = NULL,
 integer64="allow.loss")
ogrInfo(dsn, layer, encoding=NULL,
 use_iconv=FALSE, swapAxisOrder=FALSE, require_geomType = NULL)
ogrFIDs(dsn, layer)
ogrDrivers()
OGRSpatialRef(dsn, layer)
ogrListLayers(dsn)
## S3 method for class 'ogrinfo'
print(x, ...)

Arguments

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.

Value

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).

While there is no certainty, newer drivers such as KML, GML, SQLite and Geopackage (GPKG) may encode string fields as UTF-8. Users are advised to explore this on a case to case basis using Encoding on string fields of input objects.

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.

Because some drivers support reading string, integer and real list fields, support has been introduced into ogrInfo from version 0.9-1 to report their presence and the maximum counts of list items. This may lead to the introduction of the -splitlistfields facility from the command line utility ogrinfo. In addition, ogrInfo reports that there are no features when counting FIDs in a while loop over features in ogrFIDs never enters the loop, despite the layer feature count reporting at least one feature.

Author(s)

Roger Bivand

References

http://www.gdal.org/ogr_formats.html, http://examples.oreilly.com/webmapping/

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)
table(Encoding(as.character(cities$NAME)))
ogrInfo(dsn=dsn, layer="kiritimati_primary_roads")
OGRSpatialRef(dsn=dsn, layer="kiritimati_primary_roads")
kiritimati_primary_roads <- readOGR(dsn=dsn, layer="kiritimati_primary_roads")
summary(kiritimati_primary_roads)
ogrInfo(dsn=dsn, layer="scot_BNG")
OGRSpatialRef(dsn=dsn, layer="scot_BNG")
scot_BNG <- readOGR(dsn=dsn, layer="scot_BNG")
summary(scot_BNG)
if ("GML" %in% ogrDrivers()$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)
dsn <- system.file("vectors/PacoursIKA2.TAB", package = "rgdal")[1]
try(ogrInfo(dsn, "PacoursIKA2"))
ogrInfo(dsn, "PacoursIKA2", require_geomType="wkbPoint")
plot(readOGR(dsn, "PacoursIKA2", require_geomType="wkbLineString"), col="red")
plot(readOGR(dsn, "PacoursIKA2", require_geomType="wkbPoint"), add=TRUE)
odir <- getwd()
setwd(system.file("vectors", package = "rgdal")[1])
ow <- options("warn")$warn
options("warn"=1)
ogrInfo("test64.vrt", "test64")
str(readOGR("test64.vrt", "test64", verbose=FALSE, integer64="allow.loss")$val)
str(readOGR("test64.vrt", "test64", verbose=FALSE, integer64="warn.loss")$val)
str(readOGR("test64.vrt", "test64", verbose=FALSE, integer64="no.loss")$val)
str(readOGR("test64.vrt", "test64", verbose=FALSE, stringsAsFactors=FALSE,
 integer64="no.loss")$val)
options("warn"=ow)
setwd(odir)

Results