Calculate the bounds of geo circle

bboxbounds.go

package bboxbounds

import "math"

func BBoxBounds(lat, lon, meters float64) (latMin, lonMin, latMax, lonMax float64) {
	// see http://janmatuschek.de/LatitudeLongitudeBoundingCoordinates#Latitude
	lat = lat * (math.Pi / 180)
	lon = lon * (math.Pi / 180)

	r := meters / 6371e3 // angular radius

	latMin = lat - r
	latMax = lat + r

	latT := math.Asin(math.Sin(lat) / math.Cos(r))
	lonΔ := math.Acos((math.Cos(r) - math.Sin(latT)*math.Sin(lat)) / (math.Cos(latT) * math.Cos(lat)))

	lonMin = lon - lonΔ
	lonMax = lon + lonΔ

	// Adjust for north poll
	if latMax > math.Pi/2 {
		lonMin = -math.Pi
		latMax = math.Pi / 2
		lonMax = math.Pi
	}

	// Adjust for south poll
	if latMin < -math.Pi/2 {
		latMin = -math.Pi / 2
		lonMin = -math.Pi
		lonMax = math.Pi
	}

	// Adjust for wraparound. Remove this if the commented-out condition below this block is added.
	if lonMin < -math.Pi || lonMax > math.Pi {
		lonMin = -math.Pi
		lonMax = math.Pi
	}

	lonMin = math.Mod(lonMin+3*math.Pi, 2*math.Pi) - math.Pi // normalise to -180..+180°
	lonMax = math.Mod(lonMax+3*math.Pi, 2*math.Pi) - math.Pi

	return latMin * (180 / math.Pi), lonMin * (180 / math.Pi),
		latMax * (180 / math.Pi), lonMax * (180 / math.Pi)
}