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396 question, I have another quick question then!
"karl gruber" wrote in message
... Why does the 396 in "Automotive" mode sometimes take me off an interstate off ramp, and then right back on the on ramp? Best, Karl ATP, CFI, ETC. World's most hangar queeny Skywagon http://temp.corvetteforum.net/c5/kgr...fterwaxjob.jpg "Travis Marlatte" wrote in message t... I work for NAVTEQ - although I am not directly involved in the data I can share some of the issues that I am familiar with. Navigation systems are not running on the most powerful computing platform. So, anything that they do is a tradeoff between response time and quality. To find the truly optimal path between two points on the map is computationally prohibitive. The Route Calculation algorithm wants to find the least cost path from point A to point B. Lease cost may be defined as quickest based on predicted travel speed or smallest distance - combined with other possiblities like avoid tollways, avoid highways, etc. Every road in the database has attributes, a length and a speed. Even the intersections have tables to indicate the "cost" to get from one branch to another through that intersection. Techniques are used to reduce the amount of searching that must be done but these techniques sometimes result in far from optimal results. One of the popular algorithms that is used searches from both the origin and from the destination. During the search, branches are pruned off that don't seem to be going in the right direction. This, of course, sometimes eliminates one of the good routes. The other result is that sometimes the two searches meet but because of the ordering of the search paths, the first meet is not the best but somehow wins. Of course, the other problem is personal preference. My sound-bite is "only use a navigation system when you don't know where you are going and you will be please with the result. Use it to get to the office and home every day and you will soon conclude that it couldn't route itself out of a paper bag." It's going to be a long time before navigation systems will cut down that side street to avoid that typically long light. To address your specific question of ramp jumping, fundamentally, the algorithm believes that getting off and back on is shorter (or faster) than staying on the highway. I have seen several causes: 1) the user has chosen to avoid highways but there is no other way to get there. But, while determining the path, the algorithm mistakenly is still trying to avoid highways wherever it can; 2) for some reason, the cost of staying on the highway is less than using the ramp. This certainly could be a mistake in the data but it could also be a bug in the algorithm. 3) In pruning, the algorithm eliminated the path that stayed on the highway but then was forced to make a connection and the ramps were used instead. Another problem is that ramps are typically posted at something like 45mph. I know that, for a while, the algorithm we used presumed that people would drive them faster than coded which basically made them as good as a highway. Prune a few branches. Ignore some data. And, presto, the ramp is better than the highway. If you happen to remember the highway junction where that has occured, I can check our data. ------------------------------- Travis Lake N3094P PWK |
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