This is essentially what Google already does for (used when GPS is off). However, Google does not make this IP-to-location mapping publicly available. Why?

Google has the largest dataset of Wi-Fi access points, GPS traces from Android devices, and Street View cars that have scanned millions of wireless networks. In theory, Google could build an extremely accurate IP-to-location mapping by correlating:

Aqui está um guia completo sobre como funciona a localização de IP, como utilizar o Google Maps para esse fim e as limitações técnicas envolvidas.