Routes on device show as straight lines between waypoints

Hi,

Background: Device Montana 650T, Software: Basecamp 3.3.2 on a Windows XP computer.

I create a route using waypoints on Basecamp in my computer, and then transfer the route to my Montana 650T using the same map for both the device and Basecamp.

On my computer the route looks fine following the roads between the waypoints. On the device, however, the route shows as a series of straight lines between the waypoints.

What am I doing wrong?

Thanks for your help.

Ian