Thanks for your help but I suspect that Garmin actively makes it difficult/impossible to use anything but their own maps in their gps. This seems very shortsighted. I often have to find the line of a proposed road or the limits of a planned housing development in wooded areas. It would be a fantastic help to be able to transfer the dwg file to the gps and check out where I am as I go. I think that I will have to try another approach, perhaps over google maps.
I wasn't able to find any help in the Garmin manual but as I say I don't think Garmin wanted to help out!
Thanks again, I appreciate your help and advice.