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Simplify WKB/WKT geometries optionally preserving topology

Source: R/geom.R
g_simplify.Rd

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

Usage

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.

tolerance

Numeric value of 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).

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 Details).

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.

Value

A polygon as WKB raw vector or WKT string, or a list/character vector of polygons as WKB/WKT with length equal to the number of input geometries. NA 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.

Details

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

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).

Note

preserve_topology = TRUE does not preserve boundaries shared between polygons.

Examples

g <- "LINESTRING(0 0,1 1,10 0)"
g_simplify(g, tolerance = 5, as_wkb = FALSE)
#> [1] "LINESTRING (0 0,10 0)"