abuff.R

abuff <- function(spgeom, ratio = 1.1, precision = 0.00001, maxsteps = 20){
    require(rgeos)
    spgeom.area <- gArea(spgeom)
    spgeom.perimeter <- gLength(spgeom)
    buffer.width <- (ratio - 1) * spgeom.area / spgeom.perimeter
    buffered.spgeom <- gBuffer(spgeom, width = buffer.width)
    achieved.precision <- gArea(buffered.spgeom) / spgeom.area - ratio
    
    steps <- 1
    
    while(abs(achieved.precision) > precision & steps < maxsteps){
      buffered.perimeter <- gLength(buffered.spgeom)
      buffer.width <- buffer.width - achieved.precision * spgeom.area / buffered.perimeter
      buffered.spgeom <- gBuffer(spgeom, width = buffer.width)
      achieved.precision <- gArea(buffered.spgeom) / spgeom.area - ratio
      steps <- steps + 1
    }
    
    if(steps == maxsteps & abs(achieved.precision) > precision){
      warning("Required precision not reached after ", maxsteps, " steps. Achieved precision = ", achieved.precision)
    }
    
    return(buffered.spgeom)
}