March 17th, 2010

Network Analyst is a fun package to explore. It allows the calculation of much more meaningful distances than a buffer can accomplish. Of course with more power and potential comes more difficulty/buggyness and errors to be encountered. I was using ArcGIS’s Network Analyst (NA) extension to calculate the distance from residential blocks to various amenities using the ‘Find Closest Facility’ method. I was encountering significantly high failure rates – 90 percent of the blocks failed to find a facility.

I suspected that it was a topological error, that the points representing the access point of the blocks were slightly shifted from the road network and failing to find the route. This still shouldn’t have been the case for failure for various reasons. First off NA will simply traverse open spaces to reach a road – that in itself should solve the problem. Secondly, when setting up the street network I allowed points to be within a large threshold from the road network effectively ignoring the slight topological disconnection from the network. Neither of these factors seemed to function as they should. Because I was using an ArcView package I only had a limited set of topological testing available. These were deceiving as well as they confirmed correct topology.

I eventually found the Integrate Toolbox in the Data Management Toolbox. It fixes topology of all the layers you provide by snapping/adjusting features. My data was straight-forward so I didn’t encounter any collapsing intersections or anything strange that could possibly occur when automating the merging of multiple data sets. All my points where nudged to be exactly on the road network. I rebuilt my road network and the Find Closest Facility function worked flawlessly.