[mkgmap-dev] [PATCH v1] head the through_route relation when adjusting turn headings

Sat Feb 6 00:35:52 GMT 2010

This new relation (type through_route) specifies which 2 ways are the
"through route" at a junction. The relation requires 2 ways (no role
required) and a node with role = junction. It should only be used at
those junctions where the through route cannot be reliably inferred
from the ways' names and/or refs.

Here's an example from my local town:

     |   1
     |  /
     | /
     |/
     A
     |
     |
     |
     |
     B
    /|
   / |
  /  |
 2   |

The vertical road is the "through route", i.e. you can drive from one
end to the other without crossing any white lines. But, from point 1 to
point 2 is a B road so ways 1-A, A-B, and B-2 all have some ref,
say, B1234.

Without the through_route relation, mkgmap assumes (erroneously)
that the through route is 1-2 because all of the ways in that route
have the ref B1234.

By adding 2 through_route relations (one at A and the other at B),
mkgmap is informed of the real through route and will take that into
account when adjusting the turn headings. The end result is that the
quality of the routing directions is improved (we hope).

In this example, we need to have 2 relations because the B road
joins/leaves the through route at two places. 

So if you know of any junctions that would benefit from this relation,
please add it to the map data and try this patch to see if it improves
the routing instructions.

Mark

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