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Geo queries

Many apps have documents that are indexed by physical locations. For example, your app might allow users to browse stores near their current location.

Solution: Geohashes

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:

Geohashes far apart

We can use Geohashes to store and query documents by position in Cloud Firestore with reasonable efficiency while only requiring a single indexed field.

Install helper library

Creating and parsing Geohashes involves some tricky math, so we created helper libraries to abstract the most difficult parts on Android, Apple, and Web:

Web

// Install from NPM. If you prefer to use a static .js file visit
// https://github.com/firebase/geofire-js/releases and download
// geofire-common.min.js from the latest version
npm install --save geofire-common

Web

// Install from NPM. If you prefer to use a static .js file visit
// https://github.com/firebase/geofire-js/releases and download
// geofire-common.min.js from the latest version
npm install --save geofire-common

Swift

Note: This product is not available on watchOS and App Clip targets.

// Add this to your Podfile pod 'GeoFire/Utils'

Kotlin

// Add this to your app/build.gradle
implementation 'com.firebase:geofire-android-common:3.2.0'

Java

// Add this to your app/build.gradle
implementation 'com.firebase:geofire-android-common:3.1.0'

Store Geohashes

For each document you want to index by location, you will need to store a Geohash field:

Web

import { doc, updateDoc } from 'firebase/firestore';

// Compute the GeoHash for a lat/lng point
const lat = 51.5074;
const lng = 0.1278;
const hash = geofire.geohashForLocation([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.
const londonRef = doc(db, 'cities', 'LON');
await updateDoc(londonRef, {
  geohash: hash,
  lat: lat,
  lng: lng
});fs_geo_add_hash.js

Web

// Compute the GeoHash for a lat/lng point
const lat = 51.5074;
const lng = 0.1278;
const hash = geofire.geohashForLocation([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.
const londonRef = db.collection('cities').doc('LON');
londonRef.update({
  geohash: hash,
  lat: lat,
  lng: lng
}).then(() => {
  // ...
});test.solution-geoqueries.js

Swift

Note: This product is not available on watchOS and App Clip targets.

// Compute the GeoHash for a lat/lng point
let latitude = 51.5074
let longitude = 0.12780
let location = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)

let hash = GFUtils.geoHash(forLocation: location)

// Add the hash and the lat/lng to the document. We will use the hash
// for queries and the lat/lng for distance comparisons.
let documentData: [String: Any] = [
  "geohash": hash,
  "lat": latitude,
  "lng": longitude
]

let londonRef = db.collection("cities").document("LON")
londonRef.updateData(documentData) { error in
  // ...
}SolutionGeoPointViewController.swift

Kotlin

// Compute the GeoHash for a lat/lng point
val lat = 51.5074
val lng = 0.1278
val hash = GeoFireUtils.getGeoHashForLocation(GeoLocation(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.
val updates: MutableMap<String, Any> = mutableMapOf(
    "geohash" to hash,
    "lat" to lat,
    "lng" to lng,
)
val londonRef = db.collection("cities").document("LON")
londonRef.update(updates)
    .addOnCompleteListener {
        // ...
    }SolutionGeoqueries.kt

Java

// Compute the GeoHash for a lat/lng point
double lat = 51.5074;
double lng = 0.1278;
String hash = GeoFireUtils.getGeoHashForLocation(new GeoLocation(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.
Map<String, Object> updates = new HashMap<>();
updates.put("geohash", hash);
updates.put("lat", lat);
updates.put("lng", lng);

DocumentReference londonRef = db.collection("cities").document("LON");
londonRef.update(updates)
        .addOnCompleteListener(new OnCompleteListener<Void>() {
            @Override
            public void onComplete(@NonNull Task<Void> task) {
                // ...
            }
        });SolutionGeoqueries.java

Query Geohashes

Geohashes allow us to approximate area queries by joining a set of queries on the Geohash field and then filtering out some false positives:

Web

import { collection, query, orderBy, startAt, endAt, getDocs } from 'firebase/firestore';

// Find cities within 50km of London
const center = [51.5074, 0.1278];
const 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.
const bounds = geofire.geohashQueryBounds(center, radiusInM);
const promises = [];
for (const b of bounds) {
  const q = query(
    collection(db, 'cities'), 
    orderBy('geohash'), 
    startAt(b[0]), 
    endAt(b[1]));

  promises.push(getDocs(q));
}

// Collect all the query results together into a single list
const snapshots = await Promise.all(promises);

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

    // We have to filter out a few false positives due to GeoHash
    // accuracy, but most will match
    const distanceInKm = geofire.distanceBetween([lat, lng], center);
    const distanceInM = distanceInKm * 1000;
    if (distanceInM <= radiusInM) {
      matchingDocs.push(doc);
    }
  }
}fs_geo_query_hashes.js

Web

// Find cities within 50km of London
const center = [51.5074, 0.1278];
const 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.
const bounds = geofire.geohashQueryBounds(center, radiusInM);
const promises = [];
for (const b of bounds) {
  const q = db.collection('cities')
    .orderBy('geohash')
    .startAt(b[0])
    .endAt(b[1]);

  promises.push(q.get());
}

// Collect all the query results together into a single list
Promise.all(promises).then((snapshots) => {
  const matchingDocs = [];

  for (const snap of snapshots) {
    for (const doc of snap.docs) {
      const lat = doc.get('lat');
      const lng = doc.get('lng');

      // We have to filter out a few false positives due to GeoHash
      // accuracy, but most will match
      const distanceInKm = geofire.distanceBetween([lat, lng], center);
      const distanceInM = distanceInKm * 1000;
      if (distanceInM <= radiusInM) {
        matchingDocs.push(doc);
      }
    }
  }

  return matchingDocs;
}).then((matchingDocs) => {
  // Process the matching documents
  // ...
});
test.solution-geoqueries.js

Swift

Note: This product is not available on watchOS and App Clip targets.

// Find cities within 50km of London
let center = CLLocationCoordinate2D(latitude: 51.5074, longitude: 0.1278)
let radiusInM: Double = 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.
let queryBounds = GFUtils.queryBounds(forLocation: center,
                                      withRadius: radiusInM)
let queries = queryBounds.map { bound -> Query in
  return db.collection("cities")
    .order(by: "geohash")
    .start(at: [bound.startValue])
    .end(at: [bound.endValue])
}

@Sendable func fetchMatchingDocs(from query: Query,
                       center: CLLocationCoordinate2D,
                       radiusInMeters: Double) async throws -> [QueryDocumentSnapshot] {
  let snapshot = try await query.getDocuments()
  // Collect all the query results together into a single list
  return snapshot.documents.filter { document in
    let lat = document.data()["lat"] as? Double ?? 0
    let lng = document.data()["lng"] as? Double ?? 0
    let coordinates = CLLocation(latitude: lat, longitude: lng)
    let centerPoint = CLLocation(latitude: center.latitude, longitude: center.longitude)

    // We have to filter out a few false positives due to GeoHash accuracy, but
    // most will match
    let distance = GFUtils.distance(from: centerPoint, to: coordinates)
    return distance <= radiusInM
  }
}

// After all callbacks have executed, matchingDocs contains the result. Note that this code
// executes all queries serially, which may not be optimal for performance.
do {
  let matchingDocs = try await withThrowingTaskGroup(of: [QueryDocumentSnapshot].self) { group -> [QueryDocumentSnapshot] in
    for query in queries {
      group.addTask {
        try await fetchMatchingDocs(from: query, center: center, radiusInMeters: radiusInM)
      }
    }
    var matchingDocs = [QueryDocumentSnapshot]()
    for try await documents in group {
      matchingDocs.append(contentsOf: documents)
    }
    return matchingDocs
  }

  print("Docs matching geoquery: \(matchingDocs)")
} catch {
  print("Unable to fetch snapshot data. \(error)")
}SolutionGeoPointViewController.swift

Kotlin

// Find cities within 50km of London
val center = GeoLocation(51.5074, 0.1278)
val radiusInM = 50.0 * 1000.0

// 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.
val bounds = GeoFireUtils.getGeoHashQueryBounds(center, radiusInM)
val tasks: MutableList<Task<QuerySnapshot>> = ArrayList()
for (b in bounds) {
    val q = db.collection("cities")
        .orderBy("geohash")
        .startAt(b.startHash)
        .endAt(b.endHash)
    tasks.add(q.get())
}

// Collect all the query results together into a single list
Tasks.whenAllComplete(tasks)
    .addOnCompleteListener {
        val matchingDocs: MutableList<DocumentSnapshot> = ArrayList()
        for (task in tasks) {
            val snap = task.result
            for (doc in snap!!.documents) {
                val lat = doc.getDouble("lat")!!
                val lng = doc.getDouble("lng")!!

                // We have to filter out a few false positives due to GeoHash
                // accuracy, but most will match
                val docLocation = GeoLocation(lat, lng)
                val distanceInM = GeoFireUtils.getDistanceBetween(docLocation, center)
                if (distanceInM <= radiusInM) {
                    matchingDocs.add(doc)
                }
            }
        }

        // matchingDocs contains the results
        // ...
    }SolutionGeoqueries.kt

Java

// Find cities within 50km of London
final GeoLocation center = new GeoLocation(51.5074, 0.1278);
final 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<GeoQueryBounds> bounds = GeoFireUtils.getGeoHashQueryBounds(center, radiusInM);
final List<Task<QuerySnapshot>> tasks = new ArrayList<>();
for (GeoQueryBounds b : bounds) {
    Query q = db.collection("cities")
            .orderBy("geohash")
            .startAt(b.startHash)
            .endAt(b.endHash);

    tasks.add(q.get());
}

// Collect all the query results together into a single list
Tasks.whenAllComplete(tasks)
        .addOnCompleteListener(new OnCompleteListener<List<Task<?>>>() {
            @Override
            public void onComplete(@NonNull Task<List<Task<?>>> t) {
                List<DocumentSnapshot> matchingDocs = new ArrayList<>();

                for (Task<QuerySnapshot> task : tasks) {
                    QuerySnapshot snap = task.getResult();
                    for (DocumentSnapshot doc : snap.getDocuments()) {
                        double lat = doc.getDouble("lat");
                        double lng = doc.getDouble("lng");

                        // We have to filter out a few false positives due to GeoHash
                        // accuracy, but most will match
                        GeoLocation docLocation = new GeoLocation(lat, lng);
                        double distanceInM = GeoFireUtils.getDistanceBetween(docLocation, center);
                        if (distanceInM <= radiusInM) {
                            matchingDocs.add(doc);
                        }
                    }
                }

                // matchingDocs contains the results
                // ...
            }
        });SolutionGeoqueries.java

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.