Please find the documentation at http://www.nutch.org/cgi-bin/twiki/view/Main/GeoPosition
We have only indexed german pages.
Here are some example parameters for some German cities:
Berlin - position:n52.510e13.370r25
Hamburg - position:n53.550e9.590r25
Frankfurt - position:n50.118e8.558r25
Stuttgart - position:n48.780e9.180r25
Esslingen - position:n48.726e9.310r25
Winterbach - position:n48.800e9.483r25
You can test the plugin at our GeoPosition Test URL.