How does a GPS calculate which route to display? Shortest? Fastest? Most ridiculous?
October 16, 2010 10:55 PM   Subscribe

How does my GPS decide which route to take? Fastest? Shortest? What?

Before our recent trip back to MI, we bought a Garmin Nuvi 260W because we were going to be driving across the state and up the west coast, in areas we weren't totally familiar with, but in some we were. In the areas we were familiar with, we left the GPS on for shits and giggles, as one does, and the Garmin was directing us down dirt roads and snowmobile tracks, and routes that were neither the fastest, shortest, or only way to get to our destination.

So, how on earth does the GPS decide which route to spit out? In some cases we would turn on to a major road (seriously, a 4 lane road that's been there for decades) that wasn't even "listed" on the GPS screen until we turned onto it and it recalculated. Clearly, the road is in the database or whatever, as it found it when it recalculated, and it was a road it had taken us down the day before for something else!

So, does it calculate the fastest, shortest, something elsest? I've read the online manual, and there does not seem to be a way to choose one of those options, as there was in the old days of MapQuest directions. Am I missing something completely?

When it's telling us to take a seasonal snowmobile trail instead of US-31, I really take issue. I mean, we get there, but seriously....
posted by misanthropicsarah to Travel & Transportation (8 answers total) 1 user marked this as a favorite
Best answer: Page 27 of the manual.

Changing the Navigation Settings
Touch Tools > Settings > navigation.

Route Preference—select a
preference for calculating your route:

Faster Time—to calculate routes
that are faster to drive but can be
longer in distance.

Shorter Distance—to calculate
routes that are shorter in distance
but can take more time to drive.

Off Road—to calculate point-to-
point routes (without roads).

Avoidances—select the road types
you want to avoid on your routes.

Restore—restore the original
navigation settings.
posted by sharkfu at 11:01 PM on October 16, 2010

...wasn't even "listed" on the GPS screen ... it found it when it recalculated ...

The road has been there for decades, but has the entrance/exit changed in location or setup or anything? I have had that foul up the GPS in some constantly changing cities (Houston, I'm looking at you).

The snowmobile trail suggestion is a new one on me. My GPS is a Garmin nuvi. Overall, I've been really happy with the GPS and find I can usually trust it if I also leave my brain on.
posted by whatzit at 2:31 AM on October 17, 2010

Best answer: In calculating a route for you, the GPS has to evaluate a whole bunch of different routes. It will not attempt to evaluate every single possible route between two points, but will try to make a "guess" at which routes are most promising, and look at the "cost" of each leg of the route, the "cost" here being time or distance. It can't calculate every single possibility, because that takes time - and here we're talking "the sun will have extinguished before completion of calculation" type of time.

Two common methods to calculating these kinds of problems are Dijkstra's algorithm and it's successor the A* search algorithm.

The GPS unit's guess will therefore often be slightly "off" compared to how a human with local knowledge will route.
posted by Harald74 at 3:29 AM on October 17, 2010

Further to sharkfu's contribution, it does sound like it was in off-road (pedestrian) mode which apparently turns off it's lock to normal automotive roads, allow for a more direct shorter distance for those not limited by where a street vehicle can go.
posted by hungrysquirrels at 10:46 AM on October 17, 2010

Response by poster: Was not in pedestrian mode.

Snowmobile tracks are very narrow almost-one-lane dirt roads that are not taken care of in any way what so ever (at least where my parents live). So for people with off roaders or dirt bikes, or you know, snowmobiles when there's 6' of snow on the ground, they're great. Not so much for people with hoity rental cars that can't handle getting over a downed tree.

The road has been there for decades, but has the entrance/exit changed in location or setup or anything?.
nope. no entrance or exit to speak of where we were, just a turn.
posted by misanthropicsarah at 11:35 AM on October 17, 2010

Had a very similar experience in Slovenia with a Garmin GPS (it wasn't mine so I can't recall the exact make/model) that took us on a fire trail over a mountain in the dead of night... a fire trail that was blocked numerous times by fallen logs that we had to get out and clear. At night. In the country we were tod had the highest percentage of wild wolves and bears.

Imagine my squeal when a rabbit jumped out of the darkness.
posted by Admira at 2:44 PM on October 17, 2010

Response by poster: i guess my conclusion shall be that a) i need to read the manual a bit better (thank you for the link sharkfu) and b) the gps is fucking with me just like the rest of the world is, and it was somehow hoping i would die on a forlorn snowmobile trail, so months from now a foraging bear could feast on my rotting flesh.

(harald74's answer was actually the type of thing i was looking for, so thanks for that!).
posted by misanthropicsarah at 3:22 PM on October 17, 2010

Had a very similar experience in Slovenia with a Garmin GPS (it wasn't mine so I can't recall the exact make/model) that took us on a fire trail over a mountain in the dead of night

Ahem. I can't emphasize this enough. A GPS is a tool which a prudent person uses to assist them in route planning. A GPS doesn't "take" someone anywhere. Anyone who blindly follows a route suggested by a GPS is failing to utilize the best tool which they have available to them, their brain. Never blindly follow a proposed route that takes up a mountain fire trail at night in an area infested with fallen logs, Werewolves and Grizzly Bears and Vampires.

I travel a lot on business, and often head through mountain passes in the winter. My approach is multi-pronged. The first thing I do before setting out is utilize Google maps to suggest possible routes. I then examine the proposed routes , and zoom in and out of the target area to see just what kind of roads and conditions I am likely to encounter. I also look to if there are other possible that might have been discounted or ignored for some reason. I then search the relevant travel advisory sites, and examine road conditions, any road cams, posted closures, construction and weather reports. I then print out out any tricky areas I might want to reference along the way and attach these maps to clipboard I carry which Velcros to the dash.

Having devised a plan, I then inform someone of my intended route and schedule, and let them know that I will advise them once I reach the destination. In most cases this my boss and/or my girlfriend. I then load my winter driving kit into the Jeep, add the cooler with food and water and en sure the batteries in my work and personal cell-phones are fully charged, and programmed with any phone numbers I might need along the way. I also verify that I have my in-car chargers for them.

I then program the GPS , and head off on my trip, confident that if I do get stuck or lost, I won't frikken die.

Last winter, about 20 miles north Of Quesnel I came around a corner and discovered that a logging truck had jackknifed and lost it's load across the highway, creating a long line of baked-up traffic. I watched with growing concern as the Mountie walking yp the road spoke the drivers and many them then turned aroung and headed back down the highway .

When the officer finally arrived at my vehicle I had a feeling that I knew what he was going to say. Sure enough, he informs me that the road was closed, it would be at least 4-5 hours and it might not be until morning till they got it cleared. He suggested that I turn around and head back to Quesnel. I told him that I couldn't do this, as I had an appointment in Prince George the next day that had taken me a year to arrange, and I've have to take my chances.

He looks over the Jeep, with the brand-new extreme-weather winter-rated tires, and asks me if " those are brand-new store tires?"

"Yep ", I say " bought 'em last week"

Then he points at the blaze-orange survival bag in the cargo area and asks " What's in there?"

"Oh" I say, "that's my emergency winter travel-kit."

"In that case" he says , " you can take that logging road right over there. It takes a big loop through that timber license, and pops back out onto the highway, apst the accident, about 15 miles north of here".

Somewhat surprised, I ask him why he didn't tell this to some of the other drivers I'd seen with 4-wheel drive vehicles ahead of me.

"Because you're not an idiot" he says.

So off I went, after calling my boss and telling her my plan.
posted by PareidoliaticBoy at 6:30 PM on October 17, 2010

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