protected function loadDefinition($id)
    {
        $filename = $this->definitionPath . $id . '.json';
        $definition = @file_get_contents($filename);
        if (empty($definition)) {
            throw new UnknownZoneException($id);
        }
        $definition = json_decode($definition, true);
        $definition['id'] = $id;

        return $definition;
    }