The criteria for planning the left and right banks of the river in ancient times:
The shady side of the mountain is the place where the sun cannot shine, that is, the north.
The sunny side is where the sun can shine directly, that is, the south.
The opposite is true for rivers, because the sun can shine on the north side of the river bank but not on the south side.
So "Heyin" is the south and "Heyang" is the north.
The standard for planning the left bank and the right bank of a river in modern times:
Facing the downstream direction of the water flow, the left bank of a person is the left bank and the right bank is the right bank of the person.