text: /**
   * Mapbox image tile layer with topo4 map tiles
   * 
   * https://github.com/sverres/veiblikk
   * 
   * sverre.stikbakke 27.11.2017
   */
  
  map.on('load', () => {
  
    const minZoomThreshold = 15;
  
    map.addLayer({
      'id': 'topo2',
      'type': 'raster',
      'minzoom': minZoomThreshold,
      'source': {
        'type': 'raster',
        'tiles': [
          'https://opencache.statkart.no/gatekeeper/gk/gk.open_wmts?'
          + 'Service=WMTS&'
          + 'Version=1.0.0&'
          + 'Request=GetTile&'
          + 'Format=image/png&'
          + 'Style=default&'
          + 'Layer=topo4graatone&'
          + 'TileMatrixSet=EPSG:3857&'
          + 'TileMatrix=EPSG:3857:{z}&'
          + 'TileCol={x}&'
          + 'TileRow={y}'
        ],
        'tileSize': 256
      },
      'paint': {}
    });
  });