FWIW representing maps on a screen is a bit of an odd science.
You might think that it is a simple matter of drawing a map and scanning it in. However this uses up a lot of memory.
The map is therefore simplified by using standardised symbols and using vector graphics, which draw lines on the screen.
Also a lot of the info has to be able to rotate. If you turn an OS map upside down all the place names are upside down, as are all of the symbols. Most GPS's will rotate a lot of the data so if you are heading South everything's still the right way up.
This results in an approximation rather than an accurate map. I live in Portsmouth and Microsoft Autoroute has me driving in the middle of the harbour when in fact I'm going along the M275!
For these reasons reliance on the GPS map for close-quarters navigation is not sensible.
Mike