GTA V coordinates transformation to WGS84 [duplicate]

Question

I'd like to use GTA V coordinates in leaflet to show some markers on the map.

How to do a transformation between the (unknown) coordinate system of GTA V to WGS84 (leaflet)?

I already tried to write a function for that. Sometimes the coordinates are correct but not always. I'm not sure what I did wrong. Is there a more simple way to achieve this? Perhaps with leaflet's Transformation system?

function convertCoords(x, y) {
    var game_1_x = 1972.606;     // random lng point (GTA)
    var game_1_y = 3817.044;     // random lat point (GTA)
    var map_1_lat = 80.751791;   // same lng point (WGS84)
    var map_1_lng = -121.717529; // same lat point (WGS84)

    var game_2_x = -1154.11;    // random lng point (GTA)
    var game_2_y = -2715.203;   // random lat point (GTA)
    var map_2_lat = 71.3602;    // same lng point (WGS84)
    var map_2_lng = -144.30542; // same lat point (WGS84)

    var rLat = map_1_lat + (y - game_1_y) * (map_1_lat - map_2_lat) / (game_1_y - game_2_y);
    var rLng = map_1_lng + (x - game_1_x) * (map_1_lng - map_2_lng) / (game_1_x - game_2_x);

    return L.latLng(rLat, rLng);
}

Here is a gridded map of GTA V: https://drive.google.com/file/d/0B-zvE86DVcv2MXhVSHZnc01QWm8/view