I've successfully calculated anisotropic least cost paths using vertical factor tables (Tobler's Hiking Function Towards and Away created by Nicholas Tripcevich, Ph.D., University of California, Berkeley (2009), developed from Tobler. Original Webpage: <http://mapaspects.org/node/3744>) - the fastest routes.
And, for comparison, I've calculated isotropic least cost paths using slope only (least slope) - the easiest routes.
I'd now like to calculate several least cost paths within a group of 11 points (sites) that restrict travel direction - no travel to the east - and use least slope. Travel would be from one point - the easternmost in the group - to all the others which lie west. Presumably I would need to use the horizontal factor parameters, but not exactly sure how to begin - also wondering if simply restricting the processing extent in environments would provide the same or similar results.
PS-Using least slope only as the only factor with these particular points and geographic region (with river and lake valleys and mountains) produces LCPs that are nonsensical - traveling eastward and northward following the lowest-lying river valleys hundreds of miles out of the way; thus the rationale for restricting travel east.