Intersection geometry use in routing maneuvers

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Intersection geometry use in routing maneuvers

Postby george » Sat May 26, 2007 4:20 am

I recently contacted Garmin regarding what I perceive to be errors in maneuvers required to follow the route provided by my C550. The errors occur when approaching a fork in the road - one branch of which continues straight, while the other required you to bear left. In this case, two roads merge into one for a brief period of time, and then diverge.

Specifically, a County road remains straight though the area in question, while a State Highway intersects from the right at a signalized intersection, and then diverges to the left at the next signalized intersection.

While approaching the first signalized intersection on the State Highway, the geometry of the intersection requires you to turn right to continue on the State Highway. As you approach the second signalized intersection, the geometry requires that you bear left to remain on the State Highway. My C550 gives no indication that either of these maneuvers is required.

Similarly, when approaching the second signalized intersection, my C550 advises me to "bear right" to remain on the County road (obviously traveling a different route than in the first example).

Garmin's explanation for this is that the State Highway is a "higher classified" street, resulting in no maneuvers being indicated.

My question is: Is this typical of Garmin and/or other standalone units?

The reason I ask is that I have been using several navigation applications on my phone over the past 18 months, and this is the first I have encountered this problem at these intersections. In fact, every mapping site I could find, including Google Maps, Yahoo Maps, MapsOnUs, MapQuest, and Rand McNally all show the maneuvers I would expect.

I'm disappointed so far, and feel like I have to keep checking the map display to see what I should be doing.
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Postby ninosat » Sat May 26, 2007 4:26 am

Garmin and google use different routing software (i.e. "engine" if you want to call it that) but the same base data. garmins is optimized for the <300Mhz CPUs which are common in GPS unit but google uses an algorithm designed for 1+GHz CPUs on their servers. the base data is navteq so both of them should theoretically come up with the same route for the same set of data but individual algorithms using the same set of data will come up with some minor differences in the final product. for example, google maps makes significant errors in time predictions...nearly 4 hours off on my final ETA from google maps while my garmin was 15 mins off (on a 5 hour trip). ive no doubt that google maps is more "conservative" in terms of driving directions since it never knows your exact position, while garmin is built for a GPS and hence will prompt only if the GPS thinks its relevant to do so (i.e. you are off-track). garmins software is built for the minimum prompts in a given trip to reduce the annoyance factor while googles is designed for the passenger reading from a laser printed output in a moving vehicle with no re-routing so it gives every possible direction it can think off to make sure you never go off-track.
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