Situationist Space

In his “Situationalist Space”, Thomas McDonough talks about Debord's The Naked City and it's relation with the Plan de Paris. McDounough argues that the Plan de Paris was a map which was the representation of the real city by mapping out the entire city with small details, including all the streets and places. On the other hand, The Naked City used only “spatializing actions/ derives”, a totally different approach from Plan de Paris. Therefore, The Naked City is not map, and McDonough argued that map is fragmented. He then connects this idea with Jameson, stating that the fragmentations of it enables the situationalist to vaster totality and ensemble the whole city's structure. Instead of using the space as context, The Naked City uses the space as part of “social practice”, which means the areas where social relations are produced and where people inhabit. I don't really understand the concept of “derives”. What I'm guessing is that derive is the people who are trying to figure out the meanings of life. In this sense, The Naked City is more vague in term of viewing at a map. I think this article was not hard to read, but since I didn't totally get the concept of derives, it is hard for me to understand some of the points McDonough is trying to make. What is derive? Why the Situationists don't want to locate cultural struggle within the city?