Problem with Terrain.SampleHeight

I am trying to spawn grass on a terrain as it is randomly generated, but the grass doesn't spawn at a consistent height. here is the code:

 private const int DETAIL_RESOLUTION = 1024;
             private const int DETAIL_PER_PATCH = 8;
             private const int size = 500;
             
             // initialization terrain
             void Start () {
                     
                     gameObject.GetComponent<Terrain>().terrainData.SetDetailResolution(DETAIL_RESOLUTION,DETAIL_PER_PATCH);
                     
                     
                     int[,] newMap = new int[DETAIL_RESOLUTION,DETAIL_RESOLUTION];
                     
                     for(int i = 0; i < size; i++){
                 
                             for(int j = 0; j < size; j++){
                     
                                 
                                 
                             float height = gameObject.GetComponent<Terrain>().SampleHeight(new Vector3(i + gameObject.transform.position.x, 0, j + gameObject.transform.position.z)); 
                                 
                                 if( height > 11 + transform.position.y){
                                     
                                     newMap[i,j] = Random.Range(1, 2);
                                     }
                                 }
                             }
                     
                     
                     gameObject.GetComponent<Terrain>().terrainData.SetDetailLayer(0,0,0,newMap);     
                     gameObject.GetComponent<Terrain>().terrainData.SetDetailLayer(0,0,1,newMap); 
                     gameObject.GetComponent<Terrain>().terrainData.SetDetailLayer(0,0,2,newMap); 
             
             }

Thanks in advance ;)