My understanding is that when you have a floor sloping down you need a second vanishing point.  In this attached simplistic study it is easy to determine the second vanishing point using the outer left and right lines that slope down.  But how do you do this in nature?  Attached is a reference photo from a plein air session and the drawing from the plein air.  It seems easy to create an upwards slope but rather difficult to create a downward looking slope.  Any suggestions on how to find the second vanishing point to make the slope look like it is going downwards?