g_unary_op: Unary operations on WKB or WKT geometries

In gdalraster: Bindings to 'GDAL'

Unary operations on WKB or WKT geometries

Description

These functions implement algorithms that operate on one input geometry for which a new output geometry is generated. The input geometries may be given as a single raw vector of WKB, a list of WKB raw vectors, or a character vector containing one or more WKT strings.

Usage

g_buffer(
  geom,
  dist,
  quad_segs = 30L,
  as_wkb = TRUE,
  as_iso = FALSE,
  byte_order = "LSB",
  quiet = FALSE
)

g_boundary(
  geom,
  as_wkb = TRUE,
  as_iso = FALSE,
  byte_order = "LSB",
  quiet = FALSE
)

g_convex_hull(
  geom,
  as_wkb = TRUE,
  as_iso = FALSE,
  byte_order = "LSB",
  quiet = FALSE
)

g_delaunay_triangulation(
  geom,
  tolerance = 0,
  only_edges = FALSE,
  as_wkb = TRUE,
  as_iso = FALSE,
  byte_order = "LSB",
  quiet = FALSE
)

g_simplify(
  geom,
  tolerance,
  preserve_topology = TRUE,
  as_wkb = TRUE,
  as_iso = FALSE,
  byte_order = "LSB",
  quiet = FALSE
)

Arguments

geom	Either a raw vector of WKB or list of raw vectors, or a character vector containing one or more WKT strings.
dist	Numeric buffer distance in units of the input geom.
quad_segs	Integer number of segments used to define a 90 degree curve (quadrant of a circle). Large values result in large numbers of vertices in the resulting buffer geometry while small numbers reduce the accuracy of the result.
as_wkb	Logical value, TRUE to return the output geometry in WKB format (the default), or FALSE to return as WKT.
as_iso	Logical value, TRUE to export as ISO WKB/WKT (ISO 13249 SQL/MM Part 3), or FALSE (the default) to export as "Extended WKB/WKT".
byte_order	Character string specifying the byte order when output is WKB. One of "LSB" (the default) or "MSB" (uncommon).
quiet	Logical value, TRUE to suppress warnings. Defaults to FALSE.
tolerance	Numeric value. For g_simplify(), the simplification tolerance as distance in units of the input geom. Simplification removes vertices which are within the tolerance distance of the simplified linework (as long as topology is preserved when preserve_topology = TRUE). For g_delaunay_triangulation(), an optional snapping tolerance to use for improved robustness.
only_edges	Logical value. If TRUE, g_delaunay_triangulation() will return a MULTILINESTRING, otherwise it will return a GEOMETRYCOLLECTION containing triangular POLYGONs (the default).
preserve_topology	Logical value, TRUE to simplify geometries while preserving topology (the default). Setting to FALSE simplifies geometries using the standard Douglas-Peucker algorithm which is significantly faster (see Note).

Details

These functions use the GEOS library via GDAL headers.

g_boundary() computes the boundary of a geometry. Wrapper of OGR_G_Boundary() in the GDAL Geometry API.

g_buffer() builds a new geometry containing the buffer region around the geometry on which it is invoked. The buffer is a polygon containing the region within the buffer distance of the original geometry. Wrapper of OGR_G_Buffer() in the GDAL API.

g_convex_hull() computes a convex hull, the smallest convex geometry that contains all the points in the input geometry. Wrapper of OGR_G_ConvexHull() in the GDAL API.

g_delaunay_triangulation() returns a Delaunay triangulation of the vertices of the input geometry. Wrapper of OGR_G_DelaunayTriangulation() in the GDAL API. Requires GEOS >= 3.4.

g_simplify() computes a simplified geometry. By default, it simplifies the input geometries while preserving topology (see Note). Wrapper of OGR_G_Simplify() / OGR_G_SimplifyPreserveTopology() in the GDAL API.

Value

A geometry as WKB raw vector or WKT string, or a list/character vector of geometries as WKB/WKT with length equal to the number of input geometries. NULL (as_wkb = TRUE) / NA (as_wkb = FALSE) is returned with a warning if WKB input cannot be converted into an OGR geometry object, or if an error occurs in the call to the underlying OGR API.

Note

Definitions of these operations are given in the GEOS documentation (https://libgeos.org/doxygen/, GEOS 3.14.0dev), some of which is copied here.

g_boundary() computes the "boundary" as defined by the DE9IM (https://en.wikipedia.org/wiki/DE-9IM):

• the boundary of a Polygon is the set of linear rings dividing the exterior from the interior

• the boundary of a LineString is the two end points

• the boundary of a Point/MultiPoint is defined as empty

g_buffer() always returns a polygonal result. The negative or zero-distance buffer of lines and points is always an empty Polygon.

g_convex_hull() uses the Graham Scan algorithm.

g_simplify():

• With preserve_topology = TRUE (the default):
  Simplifies a geometry, ensuring that the result is a valid geometry having the same dimension and number of components as the input. The simplification uses a maximum distance difference algorithm similar to the one used in the Douglas-Peucker algorithm. In particular, if the input is an areal geometry (Polygon or MultiPolygon), the result has the same number of shells and holes (rings) as the input, in the same order. The result rings touch at no more than the number of touching point in the input (although they may touch at fewer points).

• With preserve_topology = FALSE:
  Simplifies a geometry using the standard Douglas-Peucker algorithm. Ensures that any polygonal geometries returned are valid. Simple lines are not guaranteed to remain simple after simplification. Note that in general D-P does not preserve topology - e.g. polygons can be split, collapse to lines or disappear, holes can be created or disappear, and lines can cross. To simplify geometry while preserving topology use TopologyPreservingSimplifier. (However, using D-P is significantly faster).

N.B., preserve_topology = TRUE does not preserve boundaries shared between polygons.

Examples

g1 <- "POLYGON((0 0,1 1,1 0,0 0))"
g_boundary(g1, as_wkb = FALSE)

g2 <- "POINT (0 0)"
g_buffer(g2, dist = 10, as_wkb = FALSE)

g3 <- "GEOMETRYCOLLECTION(POINT(0 1), POINT(0 0), POINT(1 0), POINT(1 1))"
g_convex_hull(g3, as_wkb = FALSE)

g4 <- "MULTIPOINT(0 0,0 1,1 1,1 0)"
g_delaunay_triangulation(g4, as_wkb = FALSE)

g5 <- "LINESTRING(0 0,1 1,10 0)"
g_simplify(g5, tolerance = 5, as_wkb = FALSE)

gdalraster documentation built on Aug. 29, 2025, 5:15 p.m.