Merges polygons

Merge sf data table, such as StratumPolygon

Usage

mergePolygons(shape, mergeCol, tolerance = 0, snapProjection = "EPSG:3857")

Arguments

shape

sf data.table with polygons to convert

mergeCol

name of column that should be used for merging, all polygons with the same value in this column will be merged into one.

tolerance

parameter passed to st_snap, a value of 0 correspond to no snapping

snapProjection

projection to perform snapping

Value

sf with polygons merged

Details

All columns must have the same value for all polygons that are to be merged. If columns are not consistent in this regard, an error is raised.

In order to deal with any inaccuracies in polygon definitions, polygons are snapped to a common grid before merging This has to be done in planar coordinates, the projection used is controlled by 'snapProjection' The resolution of the grid is controlled by the parameter 'tolerance', which can be either a units object, or a numerical value passed to st_snap. The interpretation of tolerance as a numerical value is defined in sf, but visual inspection of results is always adviced.

Examples

  #merge ICES area-definitions by division
  library(sf)
  division <- RstoxFDA::ICESareas
  division$StratumName <- paste(division$Major_FA, 
      division$SubArea, division$Division, sep=".")
  merged <- mergePolygons(division["StratumName"], "StratumName", tolerance=.001)
  #compare original area definition with merged
  RstoxFDA::plotAreaComparison(RstoxFDA::ICESareas, merged, ylim=c(30,80), areaLabels2 = TRUE)