Getting error while finding difference between a multipolygon and a polygon

[amishas157]

I am getting following error while finding the difference betweeen a polygon and a multipolygon.

/Users/AmishaSingla/Documents/osm-adiff-compare/node_modules/@mapbox/osm-compare/node_modules/@turf/difference/node_modules/jsts/dist/jsts.min.js:14
ln.checkValid(this.edgeList.getEdges()),this.graph.addEdges(this.edgeList.getEdges()),this.computeLabelling(),this.labelIncompleteNodes(),this.findResultAreaEdges(t),this.cancelDuplicateResultEdges();var n=new Sn(this.geomFact);n.add(this.graph),this.resultPolyList=n.getPolygons();var i=new wn(this,this.geomFact,this.ptLocator);this.resultLineList=i.build(t);var r=new Ln(this,this.geomFact,this.ptLocator);this.resultPointList=r.build(t),this.resultGeom=this.computeGeometry(this.resultPointList,this.resultLineList,this.resultPolyList,t)},labelIncompleteNode:function(t,e){var n=this.ptLocator.locate(t.getCoordinate(),this.arg[e].getGeometry());t.getLabel().setLocation(e,n)},copyPoints:function(t){for(var e=this.arg[t].getNodeIterator();e.hasNext();){var n=e.next(),i=this.graph.addNode(n.getCoordinate());i.setLabel(t,n.getLabel().getLocation(t))}},findResultAreaEd

The polygon and multipolygon are following:

• http://bl.ocks.org/d/44385641e432c52cdfd548218454fd42
• http://bl.ocks.org/d/28768027d930ce93f0798fa6f6463fad

Source Code:
https://jsfiddle.net/tboq1cv5/

Not sure if the input to difference method is right or not.

cc @rowanwins @DenisCarriere

[rowanwins]

Hi @amishas157

This is likely due to the same issue previously around jsts not supporting multipolygons very well.

Also as an FYI - in jsfiddle you can add an external resource like the minified version of turf with the url https://npmcdn.com/@turf/turf/turf.min.js. then rather the require failing you can just say turf.difference(feature, feature2). Check out this example and open the console to see the result, In this case I just took one part of your multipoly and it works fine, so yeah Im pretty sure its the multipoly thing.

Cheers

[amishas157]

Whoops cool. Thank for the turfy information. : ) Will try it sure.

[DenisCarriere]

Looks like @turf/difference doesn't support MultiPolygon, however now that we've got a spiffy flattenEach method we could iterate over each flattened Polygon and apply the same jsts difference operations. This shouldn't be to hard to include, @amishas157 let me look into this.

[DenisCarriere]

@amishas157 Looks like the 2nd Multi-Polygon is really a LineString (seems like the issue is coming from jsts).

Solution (quick fix):

We could remove empty polygons prior to performing the difference operations.

[amishas157]

@DenisCarriere Little confused, do we need to use flattenEach method or do any preprocessing before applying difference operation?

[DenisCarriere]

No, the issue wasn't the MultiPolygon, it was related to empty polygons.
In the next release you won't have to change anything in your code, the fix excludes those empty polygon geometries.

[DenisCarriere]

@amishas157 Just released a patch release v4.2.1 @turf/difference, try it out now and let me know if this fixed the issue for you.

Note: This won't be fixed in the main TurfJS library, changes only apply in @turf/difference.