If you are aiming to accurately map a real world location, there are a handful of people here who do that for a living or are studying it. From the little that I've seen, professional cartographers use specialised software (e.g. Arc Gis), which is expensive and requires some study to learn how to use properly. There was a recent post here about an open source version of 'proper' mapping software called Grass. I'm sure the professional mappers here could advise you further.
Another place worth visiting if you are into real world cartography is CartoTalk which is a bbs for real world mappers and a very friendly place.
On this site, apart from one or two whizz kids who know how to use the complicated GIS stuff, we generally use software like GIMP, Photoshop, Inkscape etc....ie more artistic packages as we are generally more after look and feel than we are about accuracy - given that most of our worlds are fictional.
Take care and welcome to the Guild!