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Using Uber H3 cells with Swift/iOS

What is H3?

Uber maintains an open source library called H3 which they use in their mobile apps. H3 is based on the concept of dividing the surface of the planet into a grid of hexigons or 'cells'. Each call has a unique identifier and a fixed size and location. This type of grid system is much more powerful than a simple grid of latitude/longitude which produces a set of 'rectangles'. The H3 code library can be used in various ways:

  • Given a latitude/longitude point, find the index of the containing H3 cell
  • Given an H3 index, find the latitude/longitude cell center
  • Given an H3 index, determine the cell boundary in latitude/longitude coordinates
  • Find the coordinates of the 6 points for any cell
  • Find the distance from a point to the cell edge

Furthermore, H3 defines more than one grid size for the planet. In fact H3 defines 16 distince 'resolutions' ranging from 0.0000009 km^2 up to 4,250,546.8477000 km^2. There is a parent/child relationship between two cells at adjacent resolutions.

Read more about the H3 grid system here: https://h3geo.org

Installation via Cocoapods

Create a Podfile with the following specification:

pod 'H3', :git => '[email protected]:ehmjaysee/h3-ios.git'

Usage with Swift

import H3

func testH3() {
    let latitude = 40.1234
    let longitude = 98.5432
    let point = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
    let index = point.h3CellIndex(resolution: resolution)
    let hex = String(index, radix: 16, uppercase: true)
    print(hex)
    
    let neighbors = point.h3Neighbors(resolution: 6, ringLevel: 1)
    for item in neighbors {  
        print(String(item, radix: 16, uppercase: true)) 
    }
}

Usage with C interface

The C librarary has a rich set of APIs documented here: https://h3geo.org/docs/api/indexing You can access the entire API from your Swift based project. Here are some of the main functions:

H3Index geoToH3(const GeoCoord *g, int res);
void h3ToGeo(H3Index h3, GeoCoord *g);
void h3ToGeoBoundary(H3Index h3, GeoBoundary *gp);
void kRing(H3Index origin, int k, H3Index* out);
int maxKringSize(int k);
h3Line

Example showing how to take a set of 2D coordinates and generate the set of cells sourrounding that position.

// Determine the set of nearby H3 cells based on location
let latitude = degsToRads( 41.3343 )   // MUST convert to radians
let longitude = degsToRads( 101.1188 )
var location = GeoCoord(lat: latitude, lon: longitude)
let index = geoToH3(&location, 6)
let count = Int(maxKringSize(1))
var neighbors = Array(repeating: H3Index(), count: count)
kRing(index, 1, &neighbors);