ahhh it's nice to be using Idrisi again. When it comes to multi-criteria analysis its far superior to ArcGIS. Idrisi has a handy tool called Fuzzy which allows you to use fuzzy logic to create countinuous boundaries around (in my case) roads and rivers. For my project I was thinking of creating a 5km buffer around roads as the Magellanic woodpecker prefers undisturbed environments but buffers aren't very clever. The buffer approach would assume that being 1cm from the road is the same as being 4.9 km from the road. The fuzzy tool in Idrisi allows me to create a rule based on a membership function stating for example that you will definitely not find a woodpecker within 100m of the road and that your chances would increase up to 5km after which the road would have no effect at all. I will also apply fuzzy logic to distance from rivers and elevation. The images below shows the buffer and fuzzy approach applied to the road network. Click for detail.
The image below shows the Idrisi Distance tool applied to the river network of the study area. Each pixel is assigned the value of its distance to the nearest river in metres.