What I do is get a couple of Androids (preferably different versions, not even necessary different vendors. The difference / efficiency in WiFi & GPS implementations are enormous.), install Wiggle and off you go.

No messing with GPSD, no tuning, absolutely nothing just supply power and you are ready to rock.

Additional benefit: you also detect cell towers & bluetooth if you wish to do so.

For extra good results, drive... slow... I use a scooter that allows me to comfortably drive 20km/h (15 in Freedoms) with a plastic case on the back with a handful of phones and can easily get about 80 / 90% of the SSIDs.

I VERY seldomly wiggle, am in the 600 - 700 range.