U.S. National Grid Simplifies Mapping

Ed. note: This article originally appeared in the September/October 2008 issue of Pathfinder.

In this era of GPS, homeland security and geospatial support to disaster relief operations, the civilian sector is beginning to realize what the military has known since World War I: for land navigation, the geographic coordinate system using latitude and longitude is not well-suited for referencing locations. The geospatial community, including NGA, has established and is working to implement a simple rectangular X, Y coordinate system for domestic location referencing, the U.S. National Grid (USNG).

The geographic coordinate system is essential for referencing large areas of the Earth, but for small areas, it has serious drawbacks - it uses angular units (degrees, minutes and seconds), which are cumbersome when expressing distance or converting coordinate values between map scales, and the spacing of latitude and longitude varies across the Earth, resulting in complicated calculations. USNG avoids these problems.

For example, rather than express distance in degrees, minutes and seconds, a gardener of a small backyard plot would more likely construct a simple X, Y grid, using linear units such as feet or meters. That way, the azalea hedge could be expressed as "60 feet long" instead of "0.6 seconds of latitude from end to end."

The same principle applies to land navigation, as the Allies realized during World War I. They abandoned geographic coordinates in favor of X, Y coordinates for guiding artillery fire and reporting positions, placing coordinates derived from rectangular grids on maps covering Allied areas of operation. This method was continued throughout World War II, but the global nature of that war made apparent the need for a uniform, worldwide system. In 1947, efforts by the U.S. Army Map Service and other Allied mapping organizations resulted in the development and adoption of the Universal Transverse Mercator (UTM) coordinate system, which uses a rectangular grid, and its associated location reporting method, the Military Grid Reference System (MGRS).

Implementing the U.S. National Grid, as depicted on this map of St. Louis, MO. will enable emergency responders to operate more effectively. Sample map provided by Kurt Schulz.

Half a century later, the civilian sector began to grapple with the issue of map grids for location reporting. With the rapid growth of GPS technology in the 1990s, anyone could use a handheld GPS receiver to quickly determine precise geographic positions. GPS soon became essential for emergency response, disaster relief, and search-and-rescue operations.

Unfortunately, this advance remained hampered by the geographic coordinate system and other inadequate position referencing systems. Street addresses offered only limited accuracy and range, and locally produced map grids each followed their own standards and definitions. The need for a standardized rectangular grid reference system for civilian use became obvious.

Recognizing this need, leading members of the U.S. mapping sciences community formed the Public XY Mapping Project in 1997. Representatives of the National Imagery and Mapping Agency, which would become NGA, soon became involved in the project. After considering several possibilities, the project committee concluded that a UTM-based, slightly modified version of MGRS would be the best choice for a standardized, national map grid for civilian use. In 2001, the U.S. National Grid became a standard for the interagency Federal Geographic Data Committee (FGDC), which was established in 1990 to coordinate geospatial data nationally. USNG is the preferred grid for applications of the National Spatial Data Infrastructure, the FGDC's data publishing effort.

The USNG is functionally equivalent to MGRS. In this system, the U.S. is divided into 6°x 8° grid zones, each with a three-character designator. These zones are further divided into 100-kilometer grid squares, each with a two-letter identifier. A location within a 100-kilometer grid square is referenced with an alphanumeric value composed of the grid zone designator, the grid square identifier, and X, Y coordinates measured, in meters, from the southwest corner of the grid square.

Currently, first responders and public safety officials have little training or experience with grid systems for location referencing, and many geographic information system (GIS) managers at the state and local level are unfamiliar with the USNG. For many, the response to Hurricane Katrina dramatically revealed the need to adopt a national standard. Trapped residents reported their addresses, but many street signs and house numbers were submerged or destroyed. Specialists converted addresses to coordinates, but many responders lacked the training and technology to relate coordinate values to actual locations.

Civilian agencies, local governments and private companies need to be educated, persuaded and encouraged in the use of the USNG to recognize its importance and initiate implementation under a spirit of cooperation, which is beginning to occur. Florida and North Carolina have recently adopted the USNG as their official standard for search-and-rescue and other emergency operations. GIS advisory councils in other states are also considering implementation.

Crucial to implementation is the placement of the USNG on geospatial products and technology. The challenge of establishing the USNG in the civilian sector presents a unique opportunity for NGA. With its years of experience administering MGRS and UTM and its leadership role in the geospatial community, the agency can pass along its expertise and knowledge of grid systems to mappers in the public safety field.

NGA's Office of the Americas, North America and Homeland Security Division, now includes the USNG on domestic geospatial intelligence (GEOINT) graphics, and the NGA Support Team serving the Department of Homeland Security portrays the USNG on the Ground Search Maps produced for the Federal Emergency Management Agency's (FEMA's) Urban Search and Rescue. The NGA Office of GEOINT Sciences, Coordinate Systems Analysis Branch has provided useful guidance to these efforts.

The wider geospatial community is also making progress toward implementing the USNG. The U.S. Geological Survey's National Map, its online interactive map service, includes the USNG, as does the Department of Transportation map of Washington, D.C. In addition, several state and local GIS managers have caught the vision, portraying the USNG on GIS-generated maps and providing USNG coordinates for land parcels.

The geospatial community, including NGA and other federal agencies, professional organizations and academic institutions, can all assist by adopting and promoting USNG. As the public becomes more familiar with the USNG and grid referencing, implementation will become easier.

As inclusion of the USNG on map products increases, so will acceptance of the USNG by the civilian community. NGA is poised to help all its customers take part in this important initiative. The work of NGA and the geospatial community to implement the USNG will continue to improve public safety and homeland security operations for a safer and more secure nation.