I found something interesting happening with the search tolerance in Network Analyst (the default Closest 5000 metres one) and would be grateful if anyone could confirm that what I'm doing is correct.
Basically, I have several thousand post codes (origins) and a list of shops (destinations) and want to count the number of shops within 800 metres of each postcode (using the Origin Destination or ODCost Matrix method).
First I ran the analysis and set the search tolerance to the Closest 50 metres and saved the output (lines and distances).
Secondly, I run the analysis again on the remaining postcodes that did not 'snap' onto the network the first time by setting the search tolerance to First Road Link within 100 metres.
Thirdly I used the default Closest 5000 metres search tolerance to capture any remaining postcodes and shops.
Although this takes a bit longer i.e. 3 separate runs, all of this process was successful and is really the preferred methodological way to do it where I work i.e. lower search tolerance at first to capture most features then increasing the search tolerance step by step until all the remainder of the data is captured.
Well, what was interesting was that when I ran this OD Cost Matrix analysis again using the default Closest 5000 metres first time only, I found that the results were exactly the same as when I ran the 3 separate analyses with different search tolerances. It wasn't that it was close to the previous results - they were precisely the same results down to a millimetre!
To me this is great news and saves a lot of time but my manager seems to think that the first approach (starting low then increasing search tolerance) is better.
Please could anyone with a bit of Network Analyst experience confirm that it is indeed ok to just use the default 5000 metre search tolerance (in most cases) rather than 3 separate runs with all the usual joining files etc. assocoated with it. I know this depends on your data but I just mean generally.
The fact that the results were exactly the same to a tee (and this was thousands of measurements) really makes me believe that the best approach is to use the default search tolerance where possible because it locates the feature onto its closest or first part of the network anyway.
My manager needs convincing even though I showed her that the results were the same and would be grateful if anyone else could confirm they use the same method.
I hope this makes sense and that the information is of some use to some Network Analyst users - it could save time.
Thanks again and any comments are welcome.