Geomapper module

The geomapper module is a quest item in Fallout 3.

Gameplay
The device downloads data about any world map markers you find from your Pip-Boy. This data can be downloaded by Reilly at the Ranger compound who in turn will pay 30 caps per location found. It will also count the locations you have already discovered before receiving it, and you don't have to carry it around - just get it out of your house locker when ready to visit Reilly again, and she'll pay for any new locations.

Location
The module is special item that can be received from Reilly after completing the Reilly's Rangers quest and taking the unmarked quest Geomapping with Reilly.