db.pre('w', function (ch, add) {
  for (i = 0; i < ch.value.nd.length; i++ ) {
    db.get('n' + ch.value.nd[i].$.ref, function (err, value) {
      if (err) return
      var val = JSON.parse(value);
      var coord = {
        lat: parseFloat(val.$.lat),
        lon: parseFloat(val.$.lon)
      };
      var precision = 16;
      var qtree = quadtree.encode(coord, precision);
      // Index tiles down to zoom level 16
      if (ch.key.match(/w/)) console.log(ch.key);
      add({
        key: ''+qtree+'~'+ch.key,
        value: ch.value,
        type: 'put',
        prefix: qt,
        valueEncoding: 'json'
      });
    });
  }