Diagnostic Tools

pgRouting is a large black box to most users. When things are working nobody has a problem, but when it is not working as expected what do you do? One of the most common problems is issues with the routing data. I built the following tools that aew easy to implement and use and can reveal a lot about your data. I assume that you are working with OpenLayers? as you map viewer and we will build two visualization tools.

1. View just the streets in the routing database
2. View intersections and detect deadends

You can see these tools in action at  http://imaptools.com/leaddog/routing/dd.html Zoom into a city, open the layerswitcher and select "Just the Streets" and "Dead Ends"

1. View just the streets in the routing database

Create a mapfile that displays the street segments from the routing database and add this as an optional base layer. You might want to add color oneway streets a different color, but in general keep the display simple and probably single pixel lines. You might label the segments with their UID so you can go back to the database and query them in detail.

  LAYER
    NAME "Streets"
    STATUS DEFAULT
    TYPE LINE
    CONNECTIONTYPE postgis
    CONNECTION "user=postgres dbname=routing host=centos port=5432"
    DATA "the_geom from (select gid, the_geom from st) as foo
          using SRID=4326 using unique gid"
    LABELITEM "gid"
    MAXLABELSCALEDENOM 25000
    CLASS
      STYLE
        COLOR 87 87 87
        WIDTH 1
      END
      LABEL
        FONT "arial"
        TYPE TRUETYPE
        ANGLE AUTO
        SIZE 7
        COLOR 1 1 1
      END
    END
  END

  symbol
    name "one_way_from"
    type truetype
    font arial-bold
    character ">"
    gap -40
  end

  symbol
    name "one_way_to"
    type truetype
    font arial-bold
    character "<"
    gap -40
  end


  LAYER
    NAME "One_Way_Arrows"
    STATUS DEFAULT
    TYPE LINE
    MAXSCALEDENOM 25000
    CONNECTIONTYPE postgis
    CONNECTION "user=postgres dbname=routing host=centos port=5432"
    DATA "the_geom from (select gid, one_way, the_geom from st
          where one_way is not null and length(one_way)>0) as foo
          using SRID=4326 using unique gid"
    CLASSITEM 'one_way'
    CLASS
      NAME 'From'
      EXPRESSION "FT"
      STYLE
        SYMBOL "one_way_from"
        COLOR 80 80 80
        SIZE 8
      END
    END
    CLASS
      NAME 'To'
      EXPRESSION "TF"
      STYLE
        SYMBOL "one_way_to"
        COLOR 80 80 80
        SIZE 8
      END
    END
  END

2. View intersections and detect deadends

Add a column cnt integer to the vertices_tmp table and update it with the count of segments that reference that node. Maybe something like this:

alter table vertices_tmp add column cnt integer;
update vertices_tmp set cnt=0;
update vertices_tmp set cnt=cnt+1 where streets.source=id;
update vertices_tmp set cnt=cnt+1 where streets.target=id;

Now create a mapfile layer for points and display the vertices_tmp

LAYER
   NAME "deadends"
   TYPE POINT
   CONNECTIONTYPE "postgis"
   CONNECTION ...
   DATA ...
   CLASSEXPRESSION "cnt"
   CLASS
     EXPRESSION "cnt=1"
     STYLE
       SYMBOL "circle"
       SIZE 5
       COLOR 255 0 0
     END
   END
   CLASS
     EXPRESSION "cnt>2"
     STYLE
       SYMBOL "circle"
       SIZE 3
       COLOR 0 255 0
     END
   END
END

This will display red dots at dead ends and green dots as good connections be segments. If you have a lot of red dots between segments that should be joined, you probably need to rebuild your vertices_tmp table with a larger tolerance.

It is very hard to debug a large black box like a router without having some tools that can give you some better insight as to what is going on.