[mkgmap-dev] [PATCH] make gps report correct exit when routing through roundabouts

Wed Feb 11 10:24:31 GMT 2009

As discussed yesterday, the gps sometimes tells you to leave the roundabout
at an earlier exit than it should. I believe that whether this happens or
not is influenced by the angle at which roads are attached to the roundabout or
something related to the angles between the roundabout segments.

For example, a roundabout with 4 points, each point having a road, will probably
work OK if the roads "point" pretty much at the centre of the roundabout, like
this:

  |
-- at --
  |

But if the roads are more like this:

-- --
  @
-- --

the gps is likely to get the exit count wrong.

The attached patch "frigs" roundabouts by adding an extra point between each
pair of adjacent nodes. The extra points are positioned "outside" of
the line joining the two nodes. The effect is to make roundabouts
"rounder". Unfortunately, it also makes them ugly as the frigging is
being done by a dumb computer with little aesthetic capability.

However, the good news is that it appears to make the routing instructions
specify the correct exit now. YMMV

If interested, please try this out and report success/failure. In particular,
it would be good to know if it makes roundabouts that previously were OK, bad.

The patch is for the current "nod" branch.

Cheers,

Mark
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