TSA, Post: To get another view of how sprawling Top Secret America has become, just head west on the toll road toward Dulles International Airport.

As a Michaels craft store and a Books-A-Million give way to the military intelligence giants Northrop Grumman and Lockheed Martin, find the off-ramp and turn left. Those two shimmering-blue five-story ice cubes belong to the National Geospatial-Intelligence Agency, which analyzes images and mapping data of the Earth’s geography. A small sign obscured by a boxwood hedge says so.

Cryptome:

National Geospatial Intelligence Agency, 2207 Rockhill Road, Herndon, VA

If NGA is a tenant in the building there is no perimeter security customary for spy agency.
A sign at the entrance, cited by Priest above, is obscured in Google Street view, below, on Innovation Drive.
However, a rental advertisement states there is a SCIF in the facility.

Google Street View Source Source http://www.bing.com/maps/?v=2&cp=qgtk268jfg0r&lvl=18.820071359356223&dir=6.595352978997362&sty=b&where1=Dulles%20International%20Airport%2C%20VA&form=LMLTCC http://maps.google.com/maps?hl=en&ll=38.964022,-77.419882&spn=0.008017,0.013078&t=h&z=17&vpsrc=0 This is misnamed on Google as the George Mason University Executive Programs.

TSA, Post: Across the street, in the chocolate-brown blocks, is Carahsoft, an intelligence agency contractor specializing in mapping, speech analysis and data harvesting. Nearby is the government’s Underground Facility Analysis Center. It identifies overseas underground command centers associated with weapons of mass destruction and terrorist groups, and advises the military on how to destroy them.

Cryptome:http://www.bing.com/maps/?v=2&cp=38.95837031099434~-77.42316203297403&lvl=18&dir=0&sty=h&where1=Dulles%20International%20Airport%2C%20VA&form=LMLTCC