Here’s another field visualization from a while back. These folks made a directional antenna for WiFi and mapped each network’s intensity as a function of direction. The blue mist shows a WiFi signal coming through a door near the middle of the image, maybe reflecting off the wall on the left. It’s an image-based technique. It won’t tell you the WiFi strength at the red table farther back in the photo, but it can tell you how to point your antenna when you’re standing at the spot where the “image” was collected.
Thanks to Paul Faget for the link.