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• Spatialized.io (Elasticsearch & Google Maps consulting)
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Use Case

Given a full-text query of "New York City", I want to cluster the selected locations into buckets that I can render on a map. On top of that, I want to retrieve the bounds of the viewport that my map will "fly to".

The points of interest below are marked red:

Base map courtesy of geojson.io

Approach

We'll be utilizing:

• a [match query](<https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-match-query.html>) for the full text search,
• a [geohash_grid](<https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-geohashgrid-aggregation.html>) to group locations on the geohash grid
  • followed by a [geo_centroid](<https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-geocentroid-aggregation.html>) sub-aggregation to determine the positions of the weighted centroids → the clusters,
• and a [geo_bounds](<https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-geobounds-aggregation.html>) aggregation to calculate the viewport bounds.

Here's the pseudo-code:

{
  "query": MATCH city
  "aggs": {
    "clusters": {
      AGGREGATE on geohash grid,
        COMPUTE weighted centroid
      }
    },
    "bounds": {
		  COMPUTE viewport bounds
    }
  }
}

It's tempting to use a geo_centroid aggregation without the parent geohash_grid but geo_centroid does not allow for adjusting the zoom-based cluster density. But of course, UI clustering only makes sense when we provide the map's current zoom level (→ "precision"):

Now, ES implements the 12 geohash precision levels.

<aside> 🔗 Geo-hashing deserves a chapter of its own but for now, check out this interactive map to gain an intuitive understanding of the geohash mechanism.

</aside>

Back to the precision levels — these don't directly correspond to the zoom levels you know from Google Maps, Mapbox, or Leaflet, so an adjustment will be needed. Here's the formula that I use to translate a GoogleMaps zoom into the precision parameter:

// only integer values are allowed -- mapbox & leaflet support floats too so keep that in mind
let precision = int(map_zoom - 8);

if (precision < 1) {
  // Default to the highest "zoom-out".
  // Consider the whole earth & expect 1 - 3 clusters in total
	precision = 1; 
} else if (precision > 7) {
	// Don't go beyond 7 because we'll get too many 1-member
  // clusters which could be actual markers/pins instead
	precision = 7;
}

// pass the `precision` into the ES request body