compare the distance calculate between django geo and go.geo.

distance.go

package main

import (
	"fmt"
	"github.com/paulmach/go.geo"
)

func main() {
	// django.geo: 1143 m (result from django geo)
	// go.geo: 1142.81 m (Mercator.Project + DistanceFrom)
	// go.geo: 977.26 m (GeoDistanceFrom)

	lat1, lng1 := 31.223440, 121.445300 // Jin'an Temple
	lat2, lng2 := 31.226290, 121.435590 // Leader Building
	p1, p2 := geo.NewPoint(lng1, lat1), geo.NewPoint(lng2, lat2)

	// calculates GeoDistance using math, result is inaccurate.
	fmt.Println(p1, p2)
	fmt.Printf("%f\n", p2.GeoDistanceFrom(p1))

	// point value has converted to mercator projection
	geo.Mercator.Project(p1)
	geo.Mercator.Project(p2)

	// should calculate euclidian distance
	fmt.Println(p1, p2)
	fmt.Printf("%f\n", p2.DistanceFrom(p1))]
        
        // raw sql
        // SELECT ST_Distance(ST_Transform(ST_GeomFromText('POINT(121.445300 31.223440)', 4326), 3857), ST_Transform(ST_GeomFromText('POINT(121.435590 31.226290)', 4326), 3857))
}