City GIS data is usually built in a format where streets/avenues/boulevards/etc are modeled as lines that are geographically located in the middle of the streets/avenues/boulevards/etc. Is that sufficiently precise for big transit operators to manage their buses (real-time bus management and real-time information to users) or do they create their own set of GIS data that gives a more accurate model of bus routes?
If public (or city) GIS data is not sufficient, and given that more accuracy creates larger data sets, where do you draw the line, figuratively and literally? Do you take the 'middle line' from the public GIS data and just apply an offset on each side to represent both directions of a bus line? And if so, is that offset variable between different segments (since an avenue is wider than a street)? Or do you create 'from scratch' a line that is as close to the actual bus route (and one line per direction)?
Not exactly an expert in this field, so I hope my explanation is clear!
@ Jason Baker and dassouki :
I agree that centerlines are sufficient when you are planning a bus route, my question is more from the operating side. More and more transit operators (and in all but the smallest towns, the operator is a corporation separate from the city itself, albeit with close ties to the city administration) are using real-time bus management systems. Buses are equipped with systems to continuously broadcast their position to the control center which takes that information and projects it on a city map. Controllers can then follow every bus in service and make adjustments in real-time as they are needed. The management system can also send to users real-time information on ETAs at every bus stop in the city.
From my understanding of those types of bus management, they take the coordinates of the bus and they match it to the GIS data they have. Obviously, buses don't follow centerlines. On narrow streets, the delta between the centerline and the actual bus position is probably not significant, but most buses take wider boulevards or avenues, where the difference is significant, or at least I expect it is. I'm expecting that difference could create various accuracy issues. For example, when the management system tries to determine if the bus is on-time, late or early, or what information it should send the users about ETA at a bus stop. I'm guessing that transit operators refine the city GIS data to create a new dataset that is more accurate and better answers their needs. I see two ways of refining the data (see original post), I'm wondering which one is most widely used, or if there are other ways to address the accuracy issue of city GIS data.
Do not hesitate to point out something that isn't quite clear in my explanation, English is not my native tongue. ;-)
@ Brad Nesom :
You're correct as far as displaying the information. I'm looking at it from the management system's algorithm level: any information the system calculates and/or provides logically stems from the delta between the actual bus position, given by an onboard GPS, and the expected position. Now, if the expected bus position (the planned bus route if you will) is created by using basic city GIS data (i.e. the planned bus route follows the centerlines of streets/boulevards/avenues/etc) then the delta with the actual bus location can be quite large, which would, I expect, create accuracy issues with the information the management system calculates and/or provides. If I'm correct, how are those accuracy issues addressed? Do transit operator/agencies refine the city GIS data to create a more precise dataset that then allows them to have planned bus routes that are closer to the actual path a real bus would follow in the real world?
The more I explain myself (and thanks to all for making me do so), the more I get the feeling that only a GIS expert within a transit agency could give me the answer I'm looking for. But all views and answers are appreciated !