MODX aliasMap get resourceId

<? $resourceId = $modx->aliasMap['404.html'];?>

 [aliasMap] => Array
                        (
                            [index.html] => 1
                            [404.html] => 2
                            [sitemap.xml] => 3
                            [robots.txt] => 4
                            [о-нас/] => 8
                            [меню/] => 10
                            [thank-you.html] => 17
                            [rb/] => 19
                            [о-нас/контакты.html] => 9
                            [о-нас/test.html] => 18
                            [меню/главная.html] => 11
                            [меню/features.html] => 12
                            [меню/about-us.html] => 13
                            [меню/our-work.html] => 14
                            [меню/services.html] => 15
                            [меню/contact.html] => 16
                            [rb/fully-responsive.html] => 20
                            [rb/clean-design.html] => 21
                        )

                    [resourceMap] => Array
                        (
                            [0] => Array
                                (
                                    [0] => 1
                                    [1] => 2
                                    [2] => 3
                                    [3] => 4
                                    [4] => 8
                                    [5] => 10
                                    [6] => 17
                                    [7] => 19
                                )

                            [8] => Array
                                (
                                    [0] => 9
                                    [1] => 18
                                )

                            [10] => Array
                                (
                                    [0] => 11
                                    [1] => 12
                                    [2] => 13
                                    [3] => 14
                                    [4] => 15
                                    [5] => 16
                                )

                            [19] => Array
                                (
                                    [0] => 20
                                    [1] => 21
                                )

                        )