Using Golang with Proj4 to convert between OSGB and WGS84 (both to and from)

osgb.go

package main

/*
#cgo LDFLAGS: -lproj
#include <string.h>
#include <proj_api.h>
*/
import "C"
import (
	"fmt"
	"os"
	"strconv"
	"strings"
	"unsafe"
)

type Projection string

// Proj4 defs
const (
	EPSG_27700 Projection = "+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs +nadgrids=/usr/local/share/proj/OSTN02_NTv2.gsb"
	EPSG_4326  Projection = "+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs"
)

// radians <-> degrees
const (
	radians float64 = 0.017453292519943295
	degrees float64 = 57.29577951308232
)

// Holds a LatLng pair
type LatLng struct {
	Lat, Lng float64
}

func (ll *LatLng) ToDegrees() *LatLng {

	return &LatLng{
		Lat: ll.Lat * degrees,
		Lng: ll.Lng * degrees,
	}
}

func (ll *LatLng) ToRadians() *LatLng {

	return &LatLng{
		Lat: ll.Lat * radians,
		Lng: ll.Lng * radians,
	}
}

type Proj4Error struct {
	retval int
}

func (e *Proj4Error) Error() string {
	return fmt.Sprintf("Proj4 Error: %d", e.retval)
}

// Converts the given LatLng coordinate pair from the source projection to destination projection
func Convert(ll *LatLng, Source, Destination Projection) (*LatLng, error) {

	fromProj := C.CString(string(Source))
	toProj := C.CString(string(Destination))

	defer func() {
		C.free(unsafe.Pointer(fromProj))
		C.free(unsafe.Pointer(toProj))
	}()

	fpj := C.pj_init_plus(fromProj)
	tpj := C.pj_init_plus(toProj)

	lat := C.double(ll.Lat)
	lng := C.double(ll.Lng)

	retval := int(C.pj_transform(fpj, tpj, 1, 1, &lng, &lat, nil))

	C.pj_free(fpj)
	C.pj_free(tpj)

	if retval != 0 {
		return nil, &Proj4Error{retval}
	}

	return &LatLng{
		Lat: float64(lat),
		Lng: float64(lng),
	}, nil

}

func main() {
	if len(os.Args) != 4 {
		fmt.Println("usage: osgb [from|to] <x> <y>\n\n\tUsed to convert between WGS84 and OSGB using Proj4 in Go.")
		os.Exit(0)
	}

	cmd := strings.ToLower(os.Args[1])

	x, err := strconv.ParseFloat(os.Args[2], 64)

	if err != nil {
		fmt.Printf("Failed to parse X: %s", err)
		os.Exit(-1)
	}

	y, err := strconv.ParseFloat(os.Args[3], 64)

	if err != nil {
		fmt.Printf("Failed to parse Y: %s", err)
		os.Exit(-1)
	}

	latlng := &LatLng{y, x}
	var ll *LatLng

	if cmd == "from" {
		ll, err = Convert(latlng, EPSG_27700, EPSG_4326)
	} else {
		ll, err = Convert(latlng.ToRadians(), EPSG_4326, EPSG_27700)
	}

	if err != nil {
		fmt.Println(err)

		if pe, ok := err.(*Proj4Error); ok {
			os.Exit(pe.retval)
		}
	}

	//ll = ll.ToDegrees()
	if cmd == "from" {
		ll = ll.ToDegrees()
	}

	fmt.Printf("%.15f %.15f", ll.Lng, ll.Lat)
}

https://gist.github.com/wroge/b7cd3c9dda9973b7085a10b09360ea00