code: function(item) {
      if ('values' in item && layerName in item.values && item.values[layerName]) {
        return parseFloat(item.values[layerName]);
      }
      throw new Error('Unexpected');
    }
docs: Generic GMF extractor for the 'given' value in 'values' in profileData.
  @param {Object} item The item.
  @return {number} The elevation.
  @private