My girlfriend has only one thing in common with my ex, and that is their insistence on arguing with the directions given by Nokia Maps. I've even been told that "directions are different in the French version than the English version..." - I'm convinced that one day, Nokia Maps will be named as co-respondant in my divorce!
Nokia Maps: "After 250 metres, turn left."
Backseat driver: "No, don't go turn left, go straight up - it's quicker."
Driver: "But there's only 600m left to go"
Backseat driver: "I'm telling you, turn left."
Then Maps recalculates and shows a total remaining distance of 2.5km...
Here, then, are the basics of how GPS satellite navigation software works, and why it may not give you what you consider to the best route:
Route planning
It is natural for us to break down any route into stages in our minds, we go to a certain point, turn a certain way, follow the route for so long, then turn towards another direction et cetera, but sat-nav software cannot visualise a route this way, it plans the entire route from your current position to the destination at the start and will only recalculate that route in certain circumstances:
(1) If you lose and regain your GPS fix (such as when going through a tunnel).
(2) If you take a different turn than suggested, in which case a new route is planned from that point.
(3) If you have traffic information services activated, your route can be replanned due to jams at a certain place.
If the route is recalculated during travel, it will be a completely new route from the current place, and so it is perfectly possible that the remainder of your journey could be planned differently from that point than originally.
Fastest route:
If you are set to navigate by the fastest route, then the route planned will calculate the fastest route based on the stored information about speed limits on the roads concerned. Of course, planning the route on this basis cannot take into consideration traffic lights, poor road surfaces, adverse weather conditions or any of a thousand things that might influence the way you drive; it simply bases its calculations on data about the speed limits on particular routes, and the assumption that you will travel more-or-less constantly at those speeds.
Many people consider that Maps takes them too far, but with the fastest route selected in the settings, it is perfectly possible that it can calculate a route involving a stretch of motorway rather than an urban route, and as a result the route may be longer - quickest is not necessarily shortest. There is no intuition involved, the planning is pure mathematics.
Shortest route:
This is a simpler calculation, and is exactly as described - the route is chosen according to the least number of kilometres (or miles if you are English/American) covered between the current position and the destination. Again, the calculation is done mathematically and cannot consider factors on the way that might affect your driving.
Optimised route:
This options tries to combine both of the above in order to minimise both the time taken to travel to the destination and the distance covered.
Other factors:
In answer to the point raised by my girlfriend about French and English directions being different, the instructions are simply recorded sound files which are played in a particular sequence to make up the required sentences - the selected voice has no affect on the calculation of the route. It is possible, however, that the system may calculate the same route differently on two different occasions. If there is a slight difference in your start position (ie: you are parked some distance away from where you started last time), then the formulae used to calculate the route may detect that an aternative route is more advantages - the differnece could be by as little as a minute or 100 metres.
We always need to remember the message that we have to agree to when start the software for the first time: mapping software cannot be guaranteed to be 100% accurate. This is for two main reasons, firstly because there may be inaccuracies in the map data, or the map data may not be completely up-to-date; and secondly because individual users do not get the pinpoint accuracy enjoyed by the US military when it plans its bombing raids by GPS, commercial GPS services are accurate to within a number of metres, so the fixing of your position is liable to some variance.
In conclusion, Nokia Maps (and, for that matter, any other sat-nav software) gives you the fastest or the shortest route based on the data available to it, not necessarily the route that you expect. One might even argue that if you know the route is wrong, why do you need the GPS, but I usually have Maps running in the car even if I know where I'm going, because if an unforeseen circumstance takes me off my usual route, I don't have to think about an alternative - my phone does the thinking for me and lets me concentrate on driving. There have been times when I have been surprised by the route selected by Nokia Maps, but also sometimes it has shown me short cuts that I didn't know about. Some you win, some you lose...
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