OGR feature class definition for vector data

This topic contains documentation and helper functions for defining an OGR feature class. ogr_def_field() creates an attribute field definition, a list containing the field data type and potentially other optional field properties. ogr_def_geom_field() similarly creates a geometry field definition. A list containing zero or more attribute field definitions, along with one or more geometry field definitions, comprise an OGR feature class definition (a.k.a. layer definition). ogr_def_layer() initializes such a list with a geometry field. Attribute fields can be added to a feature class definition with calls to ogr_def_field() as in the examples.

Usage

ogr_def_field(
  fld_type,
  fld_subtype = NULL,
  fld_width = NULL,
  fld_precision = NULL,
  is_nullable = NULL,
  is_unique = NULL,
  default_value = NULL
)

ogr_def_geom_field(geom_type, srs = NULL, is_nullable = NULL)

ogr_def_layer(geom_type, geom_fld_name = "geometry", srs = NULL)

Arguments

fld_type: Character string containing the name of a field data type (e.g., OFTInteger, OFTReal, OFTString).
fld_subtype: Character string containing the name of a field subtype. One of OFSTNone (the default), OFSTBoolean, OFSTInt16, OFSTFloat32, OFSTJSON, OFSTUUID.
fld_width: Optional integer scalar specifying max number of characters.
fld_precision: Optional integer scalar specifying number of digits after the decimal point.
is_nullable: Optional NOT NULL field constraint (logical scalar). Defaults to TRUE.
is_unique: Optional UNIQUE constraint on the field (logical scalar). Defaults to FALSE.
default_value: Optional default value for the field as a character string.
geom_type: Character string specifying a geometry type (see Details).
srs: Character string containing a spatial reference system definition as OGC WKT or other well-known format (e.g., the input formats usable with srs_to_wkt()).
geom_fld_name: Character string specifying a geometry field name Defaults to "geometry".

Details

All features in an OGR Layer share a common schema (feature class), modeled in GDAL as OGR Feature Definition. The feature class definition includes the set of attribute fields and their data types and the geometry field(s). In R, a feature class definition is represented as a list, having as names the attribute/geometry field names, with each list element holding a field definition.

An attribute field definition is a list with named elements:

$type       : OGR Field Type ("OFTReal", "OFTString" etc.)
$subtype    : optional ("OFSTBoolean", ...)
$width      : optional max number of characters
$precision  : optional number of digits after the decimal point
$is_nullable: optional NOT NULL constraint (logical scalar)
$is_unique  : optional UNIQUE constraint (logical scalar)
$default    : optional default value as character string
$is_geom    : FALSE (the default) for attribute fields

An OGR field type is specified as a character string with possible values: OFTInteger, OFTIntegerList, OFTReal, OFTRealList, OFTString, OFTStringList, OFTBinary, OFTDate, OFTTime, OFTDateTime, OFTInteger64, OFTInteger64List.

An optional field subtype is specified as a character string with possible values: OFSTNone, OFSTBoolean, OFSTInt16, OFSTFloat32, OFSTJSON, OFSTUUID.

By default, fields are nullable, have no unique constraint, and are not ignored (i.e., not omitted when fetching features). Not-null and unique constraints are not supported by all format drivers.

A default field value is taken into account by format drivers (generally those with a SQL interface) that support it at field creation time. If given in the field definition, $default must be a character string. The accepted values are "NULL", a numeric value (e.g., "0"), a literal value enclosed between single quote characters (e.g., "'a default value'", with any inner single quote characters escaped by repetition of the single quote character), "CURRENT_TIMESTAMP", "CURRENT_TIME", "CURRENT_DATE" or a driver-specific expression (that might be ignored by other drivers). For a datetime literal value, format should be "'YYYY/MM/DD HH:MM:SS[.sss]'" (considered as UTC time).

A geometry field definition is a list with named elements:

$type       : geom type ("Point", "Polygon", etc.)
$srs        : optional spatial reference as WKT string
$is_nullable: optional NOT NULL constraint (logical scalar)
$is_geom    : TRUE (required) for geometry fields

Typically, there is one geometry field on a layer, but some formats support more than one geometry column per table (e.g., PostGIS).

Geometry types are specified as a character string containing OGC WKT. Common types include: Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon. See the GDAL documentation for a list of all supported geometry types:
https://gdal.org/api/vector_c_api.html#_CPPv418OGRwkbGeometryType

Format drivers may or may not support not-null constraints on attribute and geometry fields. If they support creating fields with not-null constraints, this is generally before creating any features to the layer. In some cases, a not-null constraint may be available as a layer creation option. For example, GeoPackage format has a layer creation option GEOMETRY_NULLABLE=[YES/NO].

Note

The feature id (FID) is a special property of a feature and not treated as an attribute of the feature. Additional information is given in the GDAL documentation for the OGR SQL and SQLite SQL dialects. Implications for SQL statements and result sets may depend on the dialect used.

Some vector formats do not support schema definition prior to creating features. For example, with GeoJSON only the Feature object has a member with name properties. The specification does not require all Feature objects in a collection to have the same schema of properties, nor does it require all Feature objects in a collection to have geometry of the same type (https://geojson.org/).

Examples

dsn <- file.path(tempdir(), "test.sqlite")
opt <- NULL
if (has_spatialite())
  opt <- "SPATIALITE=YES"
ogr_ds_create("SQLite", dsn, dsco = opt)
#> [1] TRUE

# define a layer
defn <- ogr_def_layer("Point", srs = epsg_to_wkt(4326))
defn$fld1_int64 <- ogr_def_field("OFTInteger64")
defn$fld2_string <- ogr_def_field("OFTString")

if (ogr_ds_test_cap(dsn)$CreateLayer)
  ogr_layer_create(dsn, "layer1", layer_defn = defn)
#> [1] TRUE

ogr_ds_layer_names(dsn)
#> [1] "layer1"
ogr_layer_field_names(dsn, "layer1")
#> [1] "fld1_int64"  "fld2_string" "GEOMETRY"   

deleteDataset(dsn)
#> [1] TRUE

Usage

Arguments

Details

Note

See also

Examples