How to export Heightmap(Terrain) Raw.(Script)

     public byte[] ToByteArray(float[,] nmbs)
     {
         byte[] nmbsBytes = new byte[nmbs.GetLength(0) * nmbs.GetLength(1) * 4];
         int k = 0;
         for (int i = 0; i < nmbs.GetLength(0); i++)
         {
             for (int j = 0; j < nmbs.GetLength(1); j++)
             {
                 byte[] array = System.BitConverter.GetBytes(nmbs[i, j]);
                 for (int m = 0; m < array.Length; m++)
                 {
                     nmbsBytes[k++] = array[m];
                 }
             }
         }
         return nmbsBytes;
     }
 
     public float[,] ImportRaw(string path)
     {
         //reading file
         System.IO.FileInfo fileInfo = new System.IO.FileInfo(path);
         System.IO.FileStream stream = fileInfo.Open(System.IO.FileMode.Open, System.IO.FileAccess.Read);
 
         int size = (int)Mathf.Sqrt(stream.Length / 2);
         byte[] vals = new byte[size * size * 2];
         float[,] rawHeights = new float[size, size];
 
         stream.Read(vals, 0, vals.Length);
         stream.Close();
 
         //setting matrix
         Rect rect = new Rect(0, 0, size, size);
         int i = 0;
         for (int z = size - 1; z >= 0; z--)
             for (int x = 0; x < size; x++)
             {
                 rawHeights[x,z] = (vals[i + 1] * 256f + vals[i]) / 65535f;
                 i += 2;
             }
         return rawHeights;
     }
 
     public void ExportTerrainData(string path)
     {
         if (path != null && path.Length != 0)
         {
             path = path.Replace(Application.dataPath, "Assets");
 
             Terrain terrain = GetComponent<Terrain>();
             TerrainData data = terrain.terrainData;
 
             int h = data.heightmapHeight;
             int w = data.heightmapWidth;
             
             float[,] rawHeights = data.GetHeights(0, 0, w, h);
 
             //reading file
             System.IO.FileInfo fileInfo = new System.IO.FileInfo(path);
             System.IO.FileStream stream = fileInfo.Create();
             
             byte[] bytes = ToByteArray(rawHeights);
             stream.Write(bytes, 0, bytes.Length);
             stream.Close();
         }
     }

How to export Heightmap(Terrain) Raw.

I can easily export / import the HeightMap through the editor GUI buttons supported by unity.

What I want to do is to automate export / import of HeightMap to black and white raw files on 16bit channel.

When I scripted them, Import works fine. But the export is not working well.

Plz, help me.