Path Distance calculation limited to downhill only

Question

I am trying to calculate a path between two points with a relative elevation difference of about 100m and 3km separation. The points represent the start and end of a gravity fed waste water pipe. Essentially the path should follow the contours and have a consistent gradual fall.

I have run the Esri Path Distance tool using Slope and DEM as cost and input surface (respectively) with the DEM as the vertical input as well using default values. The resultant path minimises slope but I also want it to only travel downhill.

Can someone explain how to control the vertical factor parameters to only calculate a downhill path. The Esri help has not really described this.

Answer 1

I would derive an Aspect surface from the DEM and use it as conditional surface for the path calculation. Aspect is the direction of the slope. The cell values to exclude would depend on the relative locations of the start and end points. For example if your start point is exactly North of your end point, you would exclude the cells with a aspect value less than 90° or greater than 270° (0° being the North and aspect values measured clockwise). Flat cells which have an aspect value of -1 should also be taken into account. I've never done this but I'm curious to see if it works.

Answer 2

I would suggest you to check the Hydrology tools such as Flow Direction (like the mentioned Aspect) and maybe Flow Accumulation (or its inverse).