private function getInstancesIdsFromExternalsCoveragesIds(\Traversable $perimeters)
    {
        $userPerimeterNavitiaIds = array();
        $instances = $this->getInstances();

        foreach ($perimeters as $perimeter) {
            $userPerimeterNavitiaId = array_search($perimeter->getExternalCoverageId(), $instances);
            if ($userPerimeterNavitiaId) {
                $userPerimeterNavitiaIds[] = array_search($perimeter->getExternalCoverageId(), $instances);
            } else {
                throw new NotFoundHttpException("Coverage not found: " . $perimeter->getExternalCoverageId() . " in tyr api (" . $this->tyrUrl . ") ");
            }
        }
        return $userPerimeterNavitiaIds;
    }