GeoHash Flutter

Firebase Solutions Geoqueries

Geohash is a system for encoding a (latitude, longitude) pair into a single Base32 string. In the Geohash system the world is divided into a rectangular grid. Each character of a Geohash string specifies one of 32 subdivisions of the prefix hash. For example the Geohash abcd is one of 32 four-character hashes fully contained within the larger Geohash abc.

The longer the shared prefix between two hashes, the closer they are to each other. For example abcdef is closer to abcdeg than abcdff. However the converse is not true! Two areas may be very close to each other while having very different Geohashes:

// Compute the GeoHash for a lat/lng point
double lat = 51.5074;
double lng = 0.1278;
MyGeoHash myGeoHash = MyGeoHash();

String hash = geofire.geohashForLocation(GeoPoint(lat, lng));

// Add the hash and the lat/lng to the document. We will use the hash
// for queries and the lat/lng for distance comparisons.
CollectionReference londonRef = FirebaseFirestore.instance.collection('cities').doc('LON');
londonRef.update({
  'geohash': hash,
  'lat': lat,
  'lng': lng
}).then((){
  // ...
});

// Find cities within 50km of London
GeoPoint center = GeoPoint(51.5074, 0.1278);
double radiusInM = 50 * 1000;

// Each item in 'bounds' represents a startAt/endAt pair. We have to issue
// a separate query for each pair. There can be up to 9 pairs of bounds
// depending on overlap, but in most cases there are 4.
List<List<String>> bounds = geofire.geohashQueryBounds(center, radiusInM);
List<Future> futures = [];
for (List<String> b of bounds) {
  var q = FirebaseFirestore.instance.collection('cities')
    .orderBy('geohash')
    .startAt([b[0]])
    .endAt([b[1]]);
  futures.add(q.get());
}

// Collect all the query results together into a single list
await Future.wait(futures).then((snapshots){
  var matchingDocs = [];

  for (var snap of snapshots) {
    for (var doc of snap.docs) {
      var lat = doc['lat'];
      var lng = doc['lng'];

      // We have to filter out a few false positives due to GeoHash
      // accuracy, but most will match
      double distanceInKm = myGeoHash.distanceBetween(GeoPoint(lat, lng), center);
      double distanceInM = distanceInKm * 1000;
      if (distanceInM <= radiusInM) {
        matchingDocs.add(doc);
      }
    }
  }
  return matchingDocs;
}).then((matchingDocs){
  // Process the matching documents
  // ...
});

Limitations

Using Geohashes for querying locations gives us new capabilities, but comes with its own set of limitations:

False Positives - querying by Geohash is not exact, and you have to filter out false-positive results on the client side. These extra reads add cost and latency to your app.

Edge Cases - this query method relies on estimating the distance between lines of longitude/latitude. The accuracy of this estimate decreases as points get closer to the North or South Pole which means Geohash queries have more false positives at extreme latitudes.