First, subtract the terrain gameobject's world position, then divide by your terrain's size (which you can access in terrainData.size). This will give you a position with values somewhere in the range of zero to one.

You can then multiply this by your heightmap resolution (using the terrainData heightmapWidth and heightmapHeight variables), and then convert to integers to find the correct heightmap array position to feed into the GetHeights and SetHeights functions.

You could also do a terrain.collider.Raycast:

RaycastHit intersection = new RaycastHit();
Ray originUp = new Ray(new Vector3(0,0,0), Vector3.up);
terrain.collider.Raycast(originUp, out intersection, 100000.0f);
if (intersection.collider != null)
{ ball.transform.position = intersection.point;
ball.rigidbody.position = intersection.point;
}


private Vector3 ConvertWordCor2TerrCor(Vector3 wordCor)
{
    Vector3 vecRet = new Vector3();
    Terrain ter = Terrain.activeTerrain;
    Vector3 terPosition =  ter.transform.position;
    vecRet.x = ((wordCor.x - terPosition.x) / ter.terrainData.size.x) * ter.terrainData.alphamapWidth;
    vecRet.z = ((wordCor.y - terPosition.z) / ter.terrainData.size.z) * ter.terrainData.alphamapHeight;
    return vecRet;
}

Is this right? or i should change it like this: vecRet.x *= ((wordCor.x - terPosition.x) / ter.terrainData.size.x) * ter.terrainData.alphamapWidth; vecRet.z *= ((wordCor.y - terPosition.z) / ter.terrainData.size.z) * ter.terrainData.alphamapHeight;