US-SEC zone: merge before converting all zones to TopoJSON

for electricitymaps#1713

web/generate-geometries.js

@@ -641,7 +641,7 @@ const zoneDefinitions = [
 ];
 
 const getDataForZone = (zone, mergeStates) => {
-  /* for a specifi zone, defined by an Object having at least `zoneName` and
+  /* for a specific zone, defined by an Object having at least `zoneName` and
    * `type` as properties, call the corresponding function to get the data */
   if (zone.type === 'country'){
     return getCountry(zone.id)
@@ -735,6 +735,15 @@ zoneDefinitions.forEach(zone => {
   }
 });
 
+// merge contiguous Florida counties in US-SEC so that we only see the outer
+// region boundary line(s), not the interior county boundary lines.
+// Example: https://bl.ocks.org/mbostock/5416405
+// Background: https://github.com/tmrowco/electricitymap-contrib/issues/1713#issuecomment-517704023
+const topojson = require('topojson');
+const sec = topojson.topology({'US-SEC': zones['US-SEC']})
+const secMerged = topojson.mergeArcs(sec, [sec.objects['US-SEC']]);
+zones['US-SEC'] = topojson.feature(sec, secMerged)
+
 // create zonesMoreDetails by getting zone having moreDetails===true
 let zonesMoreDetails = {};
 zoneDefinitions.forEach(zone => {
@@ -768,15 +777,8 @@ zoneFeatures = toListOfFeatures(zoneFeaturesInline);
 fs.writeFileSync('public/dist/zonegeometries.json', zoneFeatures.map(JSON.stringify).join('\n'));
 
 // Convert to TopoJSON
-const topojson = require('topojson');
 let topo = topojson.topology(zones);
 
-// merge contiguous Florida counties in US-SEC so that we only see the outer
-// region boundary line(s), not the interior county boundary lines.
-// Example: https://bl.ocks.org/mbostock/5416405
-// Background: https://github.com/tmrowco/electricitymap-contrib/issues/1713#issuecomment-517704023
-topo.objects['US-SEC'] = topojson.mergeArcs(topo, [topo.objects['US-SEC']]);
-
 // Simplify all countries
 topo = topojson.presimplify(topo);
 topo = topojson.simplify(topo, 0.01);

web/package.json

@@ -48,6 +48,7 @@
     "extract-text-webpack-plugin": "^4.0.0-beta.0",
     "ndjson-cli": "^0.3.1",
     "nodemon": "^1.17.1",
+    "polygon-clipping": "^0.14.2",
     "readline-sync": "^1.4.9",
     "shapefile": "^0.6.2",
     "style-loader": "^0.20.2",