I did a WiFi survey of a basement today. I don't know how such surveys are typically done, but my method was to place a test AP at a location that would likely offer good coverage and then walk around rooms in spots where people will likely use WiFi and take dBm measurements, and if coverage is bad somewhere, move the AP and try again. By the 2nd attempt, I noticed a thick concrete wall, and when I took that into account, I found a good spot for the AP on the 3rd attempt.