[GIS] GeoJSON convert Polygons and MultiPolygons to LineStrings and MultiLineStrings

geojsonlinestringpolygonrasterio

Is there a gdal or python based tool for converting Polygons and MultiPolygons inside of GeoJSON to LineStrings and MultiLineStrings? I'm using rasterio / rio rasterize or gdal_rasterize to rasterize GeoTIFF w/ GeoJSON.

I need to read in GeoJSON files and be able to convert any (Multi)Polygons into LineStrings and MultiLineStrings so I can rasterize just the boundary or outline of the (Multi)Polygons onto GeoTIFFs (this includes exterior and interior polygons) and not have it fill in the entire shape.

Any ideas?

Best Answer

With ogr2ogr you can do it with the SQLite dialect and the ExteriorRing function from SpatiaLite http://www.gaia-gis.it/gaia-sins/spatialite-sql-latest.html

Test with ogrinfo and shapefile

ogrinfo polygons.shp -dialect SQLite -sql "select exteriorring(geometry) from polygons limit
 1"
INFO: Open of `polygons.shp'
      using driver `ESRI Shapefile' successful.

Layer name: SELECT
Geometry: Unknown (any)
Feature Count: 1
Extent: (453.000000, 636.000000) - (598.000000, 736.000000)
Layer SRS WKT:
(unknown)
Geometry Column = exteriorring(geometry)
OGRFeature(SELECT):0
  LINESTRING (453 683,598 736,564 636,453 683)

This should work for you:

ogr2ogr -f GeoJSON -dialect SQLite -sql "select exteriorring(geometry) from my_layer" output.json my_file.json

Check first with ogrinfo my_file.json what is the layer name in your source data.

I believe that you can use the same SQL of SQlite dialect directly as an input for gdal_rasterize http://www.gdal.org/gdal_rasterize.html without creating an interim file.

Related Solutions

[GIS] intersection between two multipolygons yielding anomalous GeometryCollection object full of LineString’s & Polygon’s (trying to get intersect area)

Solution for now: ignore the LineStrings, these are probably produced by an "st_touches" type spatial join where only the perimeters of the multipolygons are touching

RGeo allows you to break out the pieces of a Geometry Collection so I wrote some code to catch the intersect shape if its a GeometryCollection type, sending it to a different area to pluck out the polygons, sum up their size, mark that as the intersect size, and move on

    x = shape.intersection(p.proj_shape_2264)
    if x.geometry_type.to_s == "GeometryCollection"
      area = 0
      (0..(x.count - 1)).to_a.each do |k| 
        collection_shape = x.geometry_n(k)
        next if collection_shape.geometry_type.to_s == "LineString"
        area  += collection_shape.area            
      end
      z.intersect_size = area
    else

Convert Multiple Polygons and MultiPolygons into Single MultiPolygon in GeoJSON

You could do this using Javascript Topology Suite which will work with Node.js. Start with an empty MultiPolygon (or the first geometry in your collection) and union this with each (Multi)Polygon in your collection. You can only have one format for the whole collection, obviously, as properties are one to one with the geometry in the GeoJSON. Here are some jsts/node.js examples.

There is also a GeoJSON parser in jsts, which will return a jsts geometry. So putting it all together, you would do something along the lines of:

var jsts = require("jsts");

var reader = new jsts.io.GeoJSONParser();

//read your geometries
var geoms = reader.read(geojson);

//grab the first one
var multipolygon = geoms[0];

//union with all the others
for (var x=1; x < geoms.length ; x++){
    multipolygon = multipolygon.union(geoms[x]);
}

This is untested, but the general idea should work. You would have to recreate the GeoJSON with properties afterwards, but this should be easy with the built in JSON methods in Javascript.

Best Answer

Related Solutions

[GIS] intersection between two multipolygons yielding anomalous GeometryCollection object full of LineString’s & Polygon’s (trying to get intersect area)

Convert Multiple Polygons and MultiPolygons into Single MultiPolygon in GeoJSON

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