iOS app to map Wi-Fi signals?
The Fing network scanner is the tool I'd use. You'll have to pay their upgrade to get syncing of the data and some advanced tools, but the free capabilities allow you to tap on the SSID and then geo-locate the device for the scanning and save the results for later review when you are elsewhere.



As for making colorful maps, that isn't something I've seen on an iOS map. You can collect that data for free using Airport Utility - but it's not geo-located and certainly not accurate over the ranges WiFi degrades.

You'd want to get a tool like AeroHive software and hardware to do enough measurements to map signal levels in a building or network area. Pretty much any enterprise managed WiFi solution has this.