My built in sat' nav' utilises the speed sensor and a gyro sensor which combined with satellite data makes for pretty accurate positioning, eg. under tunnel into Heathrow airport, the car icon remains pretty accurate to the real position... Certainly better than our 2007 vintage Garmin Nuvi 300 under trees, tunnels or between taller buildings.
Built in job just is not as feature rich as the Garmin portable. Cannot set a destination using Lat' and Long' this is always useful for places without an address, find on google earth then use the co-ords OR use the intersection function at a junction near the required destination. My in car kit doesn't have the option of 'rally roadbook' type of readout or graphical preview of forthcoming, but as yet unseen junctions - There are, generally, quite a few useful functions beyond merely setting an address or post code in all the better makes.
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