How does the personal gps system work such as garmin, tom tom, etc.?

Question

I am familiar with the basic theory of the reception of the satallite signals to locate a spot on the earth. I am interested in knowing how the actual car equipment stores so much info and how the directions are created. In addition to the actual map routes being loaded, how does the system know if the road is 4 lane or 2 lane and where the crossovers are on a 4 lane. How does the system know where turns are not allowed, etc.

Answer 1

Its all programed in to the gps just like a computer is they have cars drive the country with gps locators on them and record the drive then they aply that to the program and then download to the gps

Answer 2

Garmin uses Navteq navigational software to produce maps and directions.  The map/roadway information is stored and the directions are calculated using this information.  The information is compiled using satellite imagery and highway information built into the software.  The software looks at current position, destination and available roadways between, applying logial analysis to determine the best route.  This is why the GPS units can recalculate on the fly, because they look from where they are, not where they were.