Trecho de código utilizado no Hands-on React-native do DevPleno

arquivo.js

    regionFrom = (lat, lon, distance) => {
        distance = distance/2
        const circumference = 40075
        const oneDegreeOfLatitudeInMeters = 111.32 * 1000
        const angularDistance = distance/circumference

        const latitudeDelta = distance / oneDegreeOfLatitudeInMeters
        const longitudeDelta = Math.abs(Math.atan2(
                Math.sin(angularDistance)*Math.cos(lat),
                Math.cos(angularDistance) - Math.sin(lat) * Math.sin(lat)))

        return result = {
            latitude: lat,
            longitude: lon,
            latitudeDelta,
            longitudeDelta,
        }
    }