I’m working on a network analysis of an urban archaeological site where I’m running a closest facility analysis on some 2000 structures to specific features within the city. I’ve segmented the road layer and calculated travel times for each segment based on Tobler’s Hiking Function, which I then enter as the time cost to calculate the quickest routes from each house to the closest feature (in terms of time). The problem I’ve run into is how to calculate these as round trips. I can’t simply double the total time of each route because the time values are anisotropic based on slope and Tobler. I’ve tried running the analysis backwards (from Facility to Incident, instead of Incident to Facility), but this often results in different matches and routes, again because of anisotropy. I’ve also tried summing the From-To and To-From time values for the segments and using those as a kind of "round trip" cost, but this again results in different Incident-Facility matches from the original Incident-Facility analysis using the anisotropic times. I’m at a loss as to how to do this. I thought if there was a way to produce a list of network layer segments for each route, I could simply sum the opposite time values (TF vs FT), but I can’t find a way to produce this, plus I’m sure that each route would consist of both TF and FT values for each segment. Does anyone have a solution or have I reach a limit of current ArcGIS capabilities?
UPDATE: I’ve found the "Copy Traversed Source Features)" tool that will export a table with all of the segment values, but the process of recalculating the accumulated inverse time values is daunting, to say the least. I have to link the segment IDs from each route to the original network layer, then somehow note if each segment’s time value was TF or FT (via separate fields with 0 or 1 values) and replace it with its inverse. Even worse, sometimes segments are joined in the middle (not at a junction) and the time values have to be proportionally shortened.
I feel like there must be a way to write a script or something to automate this, rather than attempt to do it manually, but I have no idea how to do that.
At GeoNet Can I run a Closest Facility analysis with anisotropic round trips? came up with a much easier solution than NetworkX.
All you have to do is run the Closest Facility analysis in reverse (Facility-Incident instead of Incident-Facility) and set the number of facilities to find to some higher number than 1 as I was doing (I used 3). This way the reverse route will be at least one of the 3 possible matches, if not the highest in the ranking. To join the two attribute tables so that I could sum the two total times, I created a new field in each layer and concatenated the Incident and Facility IDs to create a unique identifier for each route. The final tally is a round trip taking account of Tobler's Hiking Function in each direction.
Answered by Matt on November 14, 2021
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