Topologically-aware geometry simplification. — ms

Uses mapshaper to simplify polygons.

Usage

ms_simplify(
  input,
  keep = 0.05,
  method = NULL,
  weighting = 0.7,
  keep_shapes = FALSE,
  no_repair = FALSE,
  snap = TRUE,
  explode = FALSE,
  drop_null_geometries = TRUE,
  snap_interval = NULL,
  ...
)

Arguments

input

spatial object to simplify. One of:

geo_json or character polygons or lines;
SpatialPolygons* or SpatialLines*;
sf or sfc polygons or lines object

keep

proportion of points to retain (0-1; default 0.05)

method

simplification method to use: "vis" for Visvalingam algorithm, or "dp" for Douglas-Peuker algorithm. If left as NULL (default), uses Visvalingam simplification but modifies the area metric by underweighting the effective area of points at the vertex of more acute angles, resulting in a smoother appearance. See this https://github.com/mbloch/mapshaper/wiki/Simplification-Tipslink for more information.

weighting

Coefficient for weighting Visvalingam simplification (default is 0.7). Higher values produce smoother output. weighting=0 is equivalent to unweighted Visvalingam simplification.

keep_shapes

Prevent small polygon features from disappearing at high simplification (default FALSE)

no_repair

disable intersection repair after simplification (default FALSE).

snap

Snap together vertices within a small distance threshold to fix small coordinate misalignment in adjacent polygons. Default TRUE.

explode

Should multipart polygons be converted to singlepart polygons? This prevents small shapes from disappearing during simplification if keep_shapes = TRUE. Default FALSE

drop_null_geometries

should Features with null geometries be dropped? Ignored for Spatial* objects, as it is always TRUE.

snap_interval

Specify snapping distance in source units, must be a numeric. Default NULL

...

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"
gj2008: Generate output that is consistent with the pre-RFC 7946 GeoJSON spec (dating to 2008). Polygon rings are CW and holes are CCW, which is the opposite of the default RFC 7946-compatible output. This should be rarely needed, though may be useful when preparing data for D3-based data visualizations (such as plotly::plot_ly()). Default FALSE

Value

a simplified representation of the geometry in the same class as the input

Examples

# With a simple geojson object
poly <- structure('{
 "type": "Feature",
 "properties": {},
 "geometry": {
   "type": "Polygon",
   "coordinates": [[
     [-70.603637, -33.399918],
     [-70.614624, -33.395332],
     [-70.639343, -33.392466],
     [-70.659942, -33.394759],
     [-70.683975, -33.404504],
     [-70.697021, -33.419406],
     [-70.701141, -33.434306],
     [-70.700454, -33.446339],
     [-70.694274, -33.458369],
     [-70.682601, -33.465816],
     [-70.668869, -33.472117],
     [-70.646209, -33.473835],
     [-70.624923, -33.472117],
     [-70.609817, -33.468107],
     [-70.595397, -33.458369],
     [-70.587158, -33.442901],
     [-70.587158, -33.426283],
     [-70.590591, -33.414248],
     [-70.594711, -33.406224],
     [-70.603637, -33.399918]
   ]]
 }
}', class = c("geojson", "json"))

ms_simplify(poly, keep = 0.1)
#> {"type":"FeatureCollection", "features": [
#> {"type":"Feature","geometry":{"type":"Polygon","coordinates":[[[-70.603637,-33.399918],[-70.697021,-33.419406],[-70.668869,-33.472117],[-70.609817,-33.468107],[-70.603637,-33.399918]]]},"properties":null}
#> ]} 

# With an sf object

poly_sf <- geojsonsf::geojson_sf(poly)
ms_simplify(poly_sf, keep = 0.5)
#> Simple feature collection with 1 feature and 0 fields
#> Geometry type: POLYGON
#> Dimension:     XY
#> Bounding box:  xmin: -70.70045 ymin: -33.47212 xmax: -70.58716 ymax: -33.39247
#> Geodetic CRS:  WGS 84
#>                         geometry
#> 1 POLYGON ((-70.60364 -33.399...