map.data.loadGeoJson(
  'data/trails2018.geojson', {},
  function(features) {
    console.log("loadGeoJson callback "+features.length);
    map.data.forEach(function(feature) {
      var skill = feature.getProperty('skill_leve');
        if (skill == 'ADVANCED'){
            feature.setMap(null);
        }
    });
  });

map.data.setStyle(function(feature) {
    var visible = false;
    if (feature.getProperty('skill_level')) {
      var skill = feature.getProperty('skill_level');
      if (skill == 'ADVANCED') {
        visible = true;
      }
    }
    return /** @type {google.maps.Data.StyleOptions} */ ({
      strokeColor: "blue",
      visible: visible
    });
  });