Invalid topology error when running BetterBusBuffers

DavisTurner

I'm relatively new to GIS and am trying to do a frequency analysis of Bay Area transit services with BetterBusBuffers. I have been using the "Count Trips in Polygon Buffers around Stops" tool, and created a network dataset solely with a street feature class to use the tool (previously I had build errors with my network dataset when I was using GTFS stop data feature classes, so I removed all but the streets feature class and that resolved the build errors). I also had network dataset issues dealing with empty geometry, but fixed it by removing my hierarchy attribute. After re-running the tool, it failed after 8 hours with the error 160196, "invalid topography." Any suggestions for how to fix it? My buffer size was set to 20 minutes of walking, and I used a network restriction for walking as well. My preprocessed GTFS data contains GTFS for five different agencies which I think is causing the long processing time. I have already checked and repaired geometry, and the streets feature class was copied into the geodatabase in which my network dataset is based.  

Melinda Morang

Hello @DavisTurner. First of all, you did the right thing in removing your GTFS data from the network dataset. BetterBusBuffers is about measuring access to public transit, not about measuring access to other facilities by public transit. Thus, you don't want your network dataset to model travelers taking public transit, you just want to model them walking.

 

Second, the size of your GTFS data is probably not the issue.  The Preprocess GTFS tool does the heavy lifting and reads in all the data from the different agencies. Once it's passed that point, constructing the Service Area polygon buffers becomes the difficult part. It will definitely take longer if you have more transit stops, but anyway, the slowness is in accounting for overlapping polygons and trying to "flatten" them.

 

Regarding your error: Are you getting this error on the "Step 1 – Preprocess Buffers" tool? That seems more likely, but please let me know if my assumption is incorrect.

 

This tool can often be finicky, and it's because it's doing some fairly complex geometric procedures to handle the overlapping polygons. Something strange is happening under the hood, and it's kind of impossible to know why. The only thing to do is to try again by altering some of the settings to change the geometry of the resulting polygons slightly and hoping that the geometry error will be avoided. Here are a few things you can try changing to see if it avoids the problem:

• Change the Polygon Trim setting to a different value.
• Update your impedance attribute slightly, like changing the assumed pedestrian walk speed by a small amount.
• Changing the Buffer Size from 20 minutes to some other number (I know, not ideal...)
• This last suggestion applies only if you are using ArcMap and not ArcGIS Pro: Calculate a Service Area Index on your network dataset. Basically, we introduced a newer, better algorithm for creating Service Area polygons which fixed a lot of weird geometry problems and made all-around better polygons. This is the default in ArcGIS Pro, but if you want to use it in ArcMap, you have to explicitly choose to calculate a Service Area Index in your network dataset settings. You just have to check on the check-box and run Build Network again.

 

Note: If you are not using ArcGIS Pro, I recommend switching to Pro anyway to get overall better functionality. The tool might take less time to run or at the very least be less likely to run out of memory.

 

Hope this helps!