In the propaganda blitz that followed North Korea's missile launch last month, the country's state media released photos of leader Kim Jong Il visiting a hydroelectric dam and power station.
Images from the report showed two large pipes descending a hillside. That was enough to allow Curtis Melvin, a doctoral candidate at George Mason University in suburban Virginia, to pinpoint the installation on his online map of North Korea.
Mr. Melvin is at the center of a dozen or so citizen snoops who have spent the past two years filling in the blanks on the map of one of the world's most secretive countries. Seeking clues in photos, news reports and eyewitness accounts, they affix labels to North Korean structures and landscapes captured by Google Earth, an online service that stitches satellite pictures into a virtual globe. The result is an annotated North Korea of rocket-launch sites, prison camps and elite palaces on white-sand beaches.
"It's democratized intelligence," says Mr. Melvin...