joinPolys: Join One or Two PolySets using a Logic Operation

In PBSmapping: Mapping Fisheries Data and Spatial Analysis Tools

Join One or Two PolySets using a Logic Operation

Description

Join one or two PolySets using a logic operation.

Usage

joinPolys(polysA, polysB=NULL, operation="INT")

Arguments

polysA	PolySet to join.
polysB	optional second PolySet with which to join.
operation	one of "DIFF", "INT", "UNION", or "XOR", representing difference, intersection, union, and exclusive-or, respectively.

Details

This function interfaces with the Clipper library, specifically version 6.2.1 released 2014-10-31, developed by Angus Johnson. Prior to 2013-03-23, 'joinPolys' used the General Polygon Clipper library by Alan Murta at the University of Manchester. We keep this historic reference to GPC because 'joinPolys' remains faithful to Murta's definition of a generic polygon, which we describe below.

Murta (2004) defines a generic polygon (or polygon set) as zero or more disjoint boundaries of arbitrary configuration. He relates a boundary to a contour, where each may be convex, concave or self-intersecting. In a PolySet, the polygons associated with each unique PID loosely correspond to a generic polygon, as they can represent both inner and outer boundaries. Our use of the term generic polygon includes the restrictions imposed by a PolySet. For example, the polygons for a given PID cannot be arranged arbitrarily.

If 'polysB' is NULL, this function sequentially applies the logic 'operation' between the generic polygons in 'polysA'. For example, suppose 'polysA' contains three generic polygons (A, B, C). The function outputs the PolySet containing ((A op B) op C).

If 'polysB' is not NULL, this function applies the logic 'operation' between each generic polygon in 'polysA' and each one in 'polysB'. For example, suppose 'polysA' contains two generic polygons (A, B) and 'polysB' contains two generic polygons (C, D). The function's output is the concatenation of A op C, B op C, A op D, B op D, with PIDs 1 to 4, respectively. Generally there are n times m comparisons, where n = number of polygons in 'polysA' and m = number of polygons in 'polysB'. If 'polysB' contains only one generic polygon, the function maintains the PIDs from 'polysA'. It also maintains them when 'polysA' contains only one generic polygon and the 'operation' is "DIFF" (difference). Otherwise, if 'polysA' contains only one generic polygon, it maintains the PIDs from 'polysB'.

Value

If 'polysB' is NULL, the resulting PolySet contains a single generic polygon (one PID), possibly with several components (SIDs). The function recalculates the PID and SID columns.

If 'polysB' is not NULL, the resulting PolySet contains one or more generic polygons (PIDs), each with possibly several components (SIDs). The function recalculates the SID column, and depending on the input, it may recalculate the PID column.

Author(s)

C code: Angus Johnson, Computer Programmer

Implementation: Nicholas M. Boers, Staff Software Engineer
Jobber, Edmonton AB

Maintainer: Rowan Haigh, Program Head – Offshore Rockfish
Pacific Biological Station (PBS), Fisheries & Oceans Canada (DFO), Nanaimo BC
locus opus: Offsite, Vancouver BC
Last modified Rd: 2023-10-30

References

Murta, A. (2004) A General Polygon Clipping Library. Accessed: Jul 29, 2004.

Johnson, A. (2014) Clipper – an open source freeware library for clipping and offsetting lines and polygons. Accessed: Oct 31, 2014.

See Also

In package PBSmapping:
addPolys, appendPolys, clipPolys, closePolys, fixBound, fixPOS, locatePolys, plotMap, plotPoints, thickenPolys, thinPolys

Examples

local(envir=.PBSmapEnv,expr={
  oldpar = par(no.readonly=TRUE)
  #--- load the data (if using R)
  if (!is.null(version$language) && (version$language=="R"))
    data(nepacLL,envir=.PBSmapEnv)

  ### Example 1. Cut a triangle out of Vancouver Island
  par(mfrow=c(1,1))
  #--- create a triangle to use in clipping
  polysB <- data.frame(PID=rep(1, 3), POS=1:3,
    X=c(-127.5, -124.5, -125.6), Y = c(49.2, 50.3, 48.6))
  #--- intersect nepacLL with the single polygon, and plot the result
  plotMap(joinPolys(nepacLL, polysB), col="cyan")
  #--- add nepacLL in a different line type to emphasize the intersection
  addPolys(nepacLL, border="purple", lty=3, density=0)
  box()

  ### Example 2. Cut Texada and Lasqueti Islands out of Boxes
  xlim = list(box1=c(-124.8,-124),box2=c(-124,-123.9))
  ylim = list(box1=c(49.4,49.85), box2=c(49.85,49.9))
  Xlim = extendrange(xlim); Ylim=extendrange(ylim)
  polyA = as.PolySet(data.frame(
    PID = rep(1:2,each=4), POS = rep(1:4,2),
    X = as.vector(sapply(xlim,function(x){x[c(1,1,2,2)]})),
    Y = as.vector(sapply(ylim,function(x){x[c(1,2,2,1)]}))
    ), projection="LL")
  data(nepacLLhigh,envir=.PBSmapEnv)
  polyB = nepacLLhigh[is.element(nepacLLhigh$PID,c(736,1912)),]
  polyC = joinPolys(polyA, polyB, "DIFF")
  par(mfrow=c(2,2),cex=1,mgp=c(2,0.5,0))
  plotMap(polyA,col="lightblue",xlim=Xlim,ylim=Ylim)
  addPolys(polyB,col="gold");
  text(mean(Xlim)-0.05,Ylim-0.04,"Boxes (A,B)  and  Isles (C,D)")
  labs = calcCentroid(polyA)
  labs[1,c("X","Y")] = labs[2,c("X","Y")]+c(-0.1,-0.05)
  text(labs[,"X"],labs[,"Y"],c("A","B"),font=2)
  plotMap(polyC[is.element(polyC$PID,1),],col="pink",xlim=Xlim,ylim=Ylim)
  text(mean(Xlim)-0.05,Ylim-0.04,"Box A  \"DIFF\"  Isle C")
  plotMap(polyC[is.element(polyC$PID,3),],col="green",xlim=Xlim,ylim=Ylim)
  text(mean(Xlim)-0.05,Ylim-0.04,"Box A  \"DIFF\"  Isle D")
  plotMap(polyC[is.element(polyC$PID,c(1,3)),],col="cyan",xlim=Xlim,ylim=Ylim)
  text(mean(Xlim)-0.05,Ylim-0.04,"Box A  \"DIFF\"  Isles (C,D)")
  par(oldpar)
})

PBSmapping documentation built on Nov. 4, 2023, 9:08 a.m.