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Wingerr said:
It's true you normally would just want the end points to be specified, but stored routes can come in handy when you need to set up some via points to force routing to proceed a certain way. It doesn't actually store the information such as the ramps, but just the points that you pass through, so when it recalculates, it'll take the direction of travel into account.
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Actually, a couple of clarificatons:
You can store, manipulate, and transfer routes between the GPS and a PC. If you have maps with road information encoded for autorouting (as for the GPS V, SPIII and the new Streetpilots) you can control the routing using via points (manual re-routing, etc.). On a routing GPS it will display the proposed track rubber-banded only to logically identified roads. A non-routing GPS will simply show a straight line beteen any two via points.
The difference between the SPIII/etc. and the Etrex lines are that the SPIII/2xxx/GPSV use the "extra" data that comes with the map (road type, direction, limitations, intersections, freeway ramps, etc.). The Etrex, basically only has the "scanned" map information (it knows there is a street, the name, and the address ranges). It does not have the logical map idea of intersections, on ramps, etc...
For example, the routing type GPS units (or if you have a route-able database on your computer) know that you must take a freeway cloverleaf--whereas an Etrex would just say hang a left at the next overpass.
Of course, you can use a computer to make an Etrex route follow a cloverleaf..., but it only knows that there are several via points very close together--not that there is a logical road underneath.
And, the routing GPS does take into account the direction you are heading and the next intersections/U-Turn possibilities to regenerate a route if you go "off route".
A non-routing GPS will simply point to the last via/way point until you go back to it (or manually point to the next waypoint--some GPS's have logic in them to automatically select the next waypoint--for example in an aircraft, you probably will not hit a waypoint dead on so you still want to go to the next waypoint, and not circle back for another try).
I am sure that there are many variations available from different vendors--but the basic information should apply.
-Bill