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[mkgmap-dev] Routing Algorithm of Garmin

From Bernhard Hiller bhil at gmx.de on Fri Aug 15 18:19:11 BST 2014

When selecting "Minimize Distance", the result conforms with the 
expectations: the shortest route is selected by the Garmin Oregon 600.

But with "Minimize Time", things are different. It is not at all the 
route with the shortest time.
By setting a "Via Point", I can have it calculating my preferred route, 
which is calculated with a shorter time. I.e. despite the title of my 
selection, the device did not minimize time.

Example: Travel from Bayreuth to Erlangen.
The Oregon calculates a long route via motorways, 98.4 km, at 67 minutes.
With a Via Point west of Ebermannstadt, the distance is shortened to 
64.9 km, but also the time is now 60 minutes only.
The longer distance might be appropriate - but I find the longer time 
strange. It is 7 minutes / 10% longer by time!

Travelling along my preferred route, the device has to recalculate 
several times, with some more such results.
E.g. from Obernsees, it prefers 86.4 km at 56 minutes over a route of 
51.8 km at 46 minutes - 10 minutes or 20% longer!

 From east of Ebermannstadt, it prefers 39.8 km at 30 minutes over 29 km 
at 26 minutes.

All the preferred routes use long sections of motorways, and reduce the 
sections on roads with low road_class attribute(road_class=2; lower 
values are not present in my preferred route).

What are your experiences? How much is the "penalty" for low road_class 
Are still other factors important?

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