On The Bridge: One Step Forward, Two Steps Back

Talk about hedonic adaptation! A week or so ago I was sure I was far from ever being able to route on my web map using pgRouting, then a triumphant breakthrough, and now here I am, annoyed that it’s not perfect…

My first problem is a data issue, and a recurring one in my mapping and routing life: dealing with bridges. Once I got the routing to work, I started to customize it with a separate “get_cost” function, which deals with ascent and descent (other criteria are coming), and that worked fine. Then I noticed that the routes seemed a little off, like they were avoiding what I thought would be the optimal routes, the main one being that it would do a lot to avoid a certain section of Broad Street. That’s when I remembered: there are several bridges on that section, and rather than following the elevation of the bridge’s road surface, my elevation data followed the contours of the ground below it, leading to large ascents and descents along that section.

To solve the immediate problem, I changed the ascent and descent to be zero for the section containing the bridge — close enough to the truth, for that short a span on a mostly flat road. That made the routes in the vicinity more sensible, but what to do about other bridges?

I think I have three options: I can either find the actual elevation data for the top of the road surface (using a “digital surface model” rather than “digital elevation model” and probably using LIDAR rather than satellite radar data), or I can assume that the bridge has a mostly constant slope, and calculate the slope from elevations where it attaches to the ground, or I can save myself a lot of work and just say “they’re flat, or flat enough to make no difference,” and make all ascents and descents be zero for bridges. I am still thinking about this…

The second problem is a little harder to figure out, since it involves the PostGIS routing function I got off the Internet: when the beginning and end points of the route are on the same segment of road (ie there are no intersections between them), the function fails. I don’t know enough about Postgres functions to be able to solve this, so I may have to either live with it for a while, and contact the person who wrote the function for some help.

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