It's all pretty basic, I guess. Perhaps the main thing to achieve this "atlas" style is the colors. The drawing itself is just creating layer after layer for different heights / depths. In case of heights, I copied and merged those into another layer that I used to create the shaded relief (mostly by means of the 'bump map' filter). After that I added rivers, lakes, and marshes (the latter not shown on the version linked to above). There is no particular "technique" involved really, except the obvious use of lots of different layers in the Gimp (or Photoshop).

Next stage will be the human geography: places, regions, etc. but that requires writing an historical outline first, and that in turn depends on some research for which I didn't have sufficient time yet.
In case it wasn't clear from the map coordinates, this group of islands is supposed to be West of Australia. In reality this landmass is sunk; see the section on geological history here: http://www.geopoeia.net/wiki/Houtmanland