A couple of functions to download tiles from OpenStreetMaps and concatenate them into one single image. NB! Not recommended for more than 5 in zoom level!

OpenStreetMapTilesDownloader.go

package main

import (
	"fmt"
	"os"
	"os/exec"
)

func main() {
  	var zoom = 2
	numberOfTiles := 1 << zoom
	for j := 0; j < numberOfTiles; j++ {
		for i := 0; i < numberOfTiles; i++ {
			path := fmt.Sprintf("http://b.tile.openstreetmap.org/%d/%d/%d.png", zoom, i, j)
			curl(path, fmt.Sprintf("./%d/%d_%d.png", zoom, i, j))
		}
	}

	for i := 0; i < numberOfTiles; i++ {
		concatColumns(zoom, i, numberOfTiles)
	}
	concatRows(zoom, numberOfTiles)
}

func curl(url, filepath string) error {
	cmd := exec.Command("curl", url, "--output", filepath)
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	return cmd.Run()
}

func concatColumns(zoom int, index int, numberOfTiles int) error {
	images := []string{"-mode", "concatenate", "-tile", "1x"}
	for i := 0; i < numberOfTiles; i++ {
		images = append(images, fmt.Sprintf("./%d/%d_%d.png", zoom, index, i))
	}
	images = append(images, fmt.Sprintf("./%d/out_%d.png", zoom, index))
	cmd := exec.Command("montage", images...)
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	return cmd.Run()
}

func concatRows(zoom int, x int) error {
	images := []string{"-mode", "concatenate", "-tile", "x1"}
	for i := 0; i < x; i++ {
		images = append(images, fmt.Sprintf("./%d/out_%d.png", zoom, i))
	}
	images = append(images, fmt.Sprintf("./zoom_%d.png", zoom))
	cmd := exec.Command("montage", images...)
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	return cmd.Run()
}

This tool requires to have ImageMagick to concatenate the images into one big single map.