Interesting question, let me answer this with some logic too:
- soil should be moist in the area of earthing pit.
- If possible earth plates or pipes are located near water tap, water drain or rain water pipe.
- Earth wire and earth plate must be of the same metal.
- Charcoal and salt must be filled in the earth pit around earthing pipe or plate.
- The position of the earth plate or pipe when fixed should be clear from all building foundations.
So one can plan the earthing pit in East or North, people usually add some water in this direction, though it may not be compulsory but its good if we add water regularly.
Corner of Northeast is not recommended, because electricity which represent fire in general is not good in this direction.
Direction of Northwest or Southeast is good, if there is no such place available, than South or West is also a good location.
Southwest is not that good location for earthing pit but if that's the only place you can have it than put a heavy slab/item on top of this pit. Water sump or septic tanks may be dangerous at Southwest as per vastu, but earth wire pit is not such a serious one but not recommended.