# thinnedSpatialPoly: Douglas-Peuker line generalization for Spatial Polygons

## Description

The function applies the implementation of the Douglas-Peuker algorithm for line generalization or simplification (originally from shapefiles) to objects inheriting from Spatial Polygons. It does not preserve topology, so is suitable for visualisation, but not for the subsequent analysis of the polygon boundaries, as artefacts may be created, and boundaries of neighbouring entities may be generalized differently. If the rgeos package is available, `thinnedSpatialPolyGEOS`

will be used with partial topology preservation instead of the R implementation here by passing arguments through.

## Usage

thinnedSpatialPoly(SP, tolerance, minarea=0, topologyPreserve = FALSE,
avoidGEOS = FALSE)

## Arguments

SP

an object inheriting from the SpatialPolygons class

tolerance

the tolerance value in the metric of the input object

minarea

the smallest area of Polygon objects to be retained, ignored if rgeos used

topologyPreserve

choose between two rgeos options: logical determining if the algorithm should attempt to preserve the topology (nodes not complete edges) of the original geometry

avoidGEOS

use R DP code even if rgeos is available

## Value

An object of the same class as the input object

## References

Douglas, D. and Peucker, T. (1973). Algorithms for
the reduction of the number of points required to
represent a digitized line or its caricature.
*The Canadian Cartographer* 10(2). 112-122.

## Examples

# NOT RUN {
xx <- readShapeSpatial(system.file("shapes/sids.shp", package="maptools")[1],
IDvar="FIPSNO", proj4string=CRS("+proj=longlat +ellps=clrk66"))
object.size(as(xx, "SpatialPolygons"))
xxx <- thinnedSpatialPoly(xx, tolerance=0.05, minarea=0.001)
object.size(as(xxx, "SpatialPolygons"))
par(mfrow=c(2,1))
plot(xx)
plot(xxx)
par(mfrow=c(1,1))
# }