ms_filter_islands function

Remove small detached polygons (islands)

Remove small detached polygons, keeping those with a minimum area and/or a minimum number of vertices. Optionally remove null geometries.

ms_filter_islands(
  input,
  min_area = NULL,
  min_vertices = NULL,
  drop_null_geometries = TRUE,
  ...
)

Arguments

• input: spatial object to filter. One of:

  • geo_json or character polygons;
  • SpatialPolygons*;
  • sf or sfc polygons object

• min_area: minimum area of polygons to retain. Area is calculated using planar geometry, except for the area of unprojected polygons, which is calculated using spherical geometry in units of square meters.

• min_vertices: minimum number of vertices in polygons to retain.

• drop_null_geometries: should features with empty geometries be dropped? Default TRUE. Ignored for SpatialPolyons*, as it is always TRUE.

• ...: Arguments passed on to apply_mapshaper_commands

  • force_FC: should the output be forced to be a FeatureCollection (or sf object or Spatial*DataFrame) even if there are no attributes? Default TRUE. If FALSE and there are no attributes associated with the geometries, a GeometryCollection (or Spatial object with no dataframe, or sfc) will be output.
  • sys: Should the system mapshaper be used instead of the bundled mapshaper? Gives better performance on large files. Requires the mapshaper node package to be installed and on the PATH.
  • sys_mem: How much memory (in GB) should be allocated if using the system mapshaper (sys = TRUE)? Default 8. Ignored if sys = FALSE. This can also be set globally with the option "mapshaper.sys_mem"
  • quiet: If sys = TRUE, should the mapshaper messages be silenced? Default FALSE. This can also be set globally with the option "mapshaper.sys_quiet"

Returns

object with only specified features retained, in the same class as the input

Examples

library(geojsonsf)
library(sf)

poly <- structure("{\"type\":\"FeatureCollection\",
           \"features\":[{\"type\":\"Feature\",\"properties\":{},
           \"geometry\":{\"type\":\"Polygon\",
           \"coordinates\":[[[102,2],[102,4],[104,4],[104,2],[102,2]]]}},
           {\"type\":\"Feature\",\"properties\":{},
           \"geometry\":{\"type\":\"Polygon\",
           \"coordinates\":[[[100,2],[98,4],[101.5,4],[100,2]]]}},
           {\"type\":\"Feature\",\"properties\":{},
           \"geometry\":{\"type\":\"Polygon\",
           \"coordinates\":[[[100,0],[100,1],[101,1],[101,0],[100,0]]]}}]}",
           class = c("geojson", "json"))

poly <- geojson_sf(poly)
plot(poly)

out <- ms_filter_islands(poly, min_area = 12391399903)
plot(out)