Geomapping with Reilly is a repeatable quest in Fallout 3.
After completing the quest Reilly's Rangers and offering to help Reilly in mapping the Capital Wasteland, she will give you a geomapper module. As long as you have the module in your inventory, it will record every new location you discover, for which Reilly will pay you 30 caps per location every time you return to the Ranger compound and talk to her. Don't forget to speak to her right away, after receiving the module. She will pay you for the locations which you already have on your map, even those that you have already discovered beforehand, making this quest lucrative very quickly.
- Despite not actually "discovering" the locations, Reilly will pay you for locations pointed out by the Explorer perk.
Occasionally, you won't get paid for locations you already have the first time you talk to Reilly after she gives you the geomapper module. She will say that the Rangers already know about all of the locations you have and will give you nothing. If this happens, try immediately talking to her again. If she still won't pay you for the locations you have already discovered, try leaving the Ranger compound and re-entering before speaking to her again. Another alternative is to simply seek out at least one new point of interest and then return. Sometimes, if you talk to Reilly and ask for the caps for the geomapping, she will say that she will give you the caps, but will make her way to her bed to sleep. When you wake her up, she will say that she already has all those locations and will not pay you. This can be fixed by reloading a previous save.