Maps and trail guides can only help so much on a hike. They don't always provide all the information I need. They show the location of a water source, for example, but can't reliably say if it is flowing. I can see the trail's elevation gain and loss, but there's no way to know if it is littered with large rocks or exposed roots.
Sometimes, I have to rely on my instincts. Experience and gut feelings are needed to make decisions.
I spent a lot of time using that yesterday when attempting to find a campsite. Comparing contour lines with the trail's path, I tried to deduce a spot where I might be able to pitch my tent. I also had to judge how close this was to the Blue Ridge Parkway. If I couldn't find a site outside the parkway's boundary, I needed it to be a stealth site (not visible from the road).