The United States National Grid (USNG) is functionally and operationally equivalent to the Military Grid Referencing System (MGRS), specified by the U.S. military for tactical (local/regional) operations. The USNG provides a multi-purpose location system for civilian use in the United States, especially to support emergency services. Like the MGRS, it provides a nationally consistent "language of location", based on Universal Transverse Mercator (UTM) coordinates, optimized for local applications, in a compact, user-friendly format. It is similar in design to the UTM-based national grid reference systems used in other countries.
The USNG was adopted as a national standard by the Federal Geographic Data Committee (FGDC) of the US Government in 2001. The MGRS is the foundation of the USNG and both should be considered technically synonymous for all practical purposes. Please refer to the Wikipedia MGRS page for more detailed background information on the technical origins of that grid system. Information provided here should be considered supplemental to that which is provided for the MGRS, with a focus on the United States and civilian applications. There are many other resources available related to “grid referencing” and “land navigation”, which typically use UTM-based systems, like MGRS or USNG, which are all interoperable.
Overview
While latitude and longitude are well suited to describing locations over large areas of the Earth's surface, most practical land navigation situations occur within much smaller, local areas. As such, they are often better served by a local Cartesian coordinate system, in which the coordinates represent actual distance units on the ground, using the same units of measurement from two perpendicular coordinate axes. This can improve human comprehension by providing reference of scale, as well as making actual distance computations more efficient.
Paper maps often are published with overlaid rectangular (as opposed to latitude/longitude) grids to provide a reference to identify locations. However, these grids, if non-standard or proprietary (such as so-called "bingo" grids with references such as "B-4"), are typically not interoperable with each other, nor can they usually be used with GPS.
The goal of the USNG is to provide a uniform, nationally consistent rectangular grid system that is interoperable across maps at different scales, as well as with GPS and other location based systems. It is intended to provide a frame of reference for describing and communicating locations that is easier to use than latitude/longitude for many practical applications, works across jurisdictional boundaries, and is simple to learn, teach, and use. It is also designed to be both flexible and scalable so that location references are as compact and concise as possible.
The USNG is intended to supplement—not to replace—other location systems such as street addresses. It can be applied to printed maps and to computer mapping and other Geographic Information System (GIS) applications. It has found increasing acceptance especially in emergency management, search and rescue, and other public safety applications; yet, its utility is by no means limited to those fields.
Description and functioning
The USNG is an alpha-numeric reference system that overlays the UTM coordinate system. A number of brief tutorial references explain the system in detail, with examples. . Briefly, an example of a full USNG spatial address (grid reference) is:18S UJ 23371 06519(This example used by the FGDC is the full one-meter grid reference of the Jefferson Pier in Washington DC.)
This full form (15 characters) uniquely identifies a single one-meter grid square out of the entire surface of the earth. It consists of three parts (each of which follows a "read-right-then-up" paradigm familiar with other "X,Y" coordinates):
Grid Zone Designation (GZD); for a world-wide unique address. This consists of up to 2 digits (6-degree longitude UTM zone) for West to East, followed by a letter (8-degree latitude band) from South to North; in this example, "18S". 100,000-meter (100 km) Square Identification; for regional areas. This consists of two letters, the first West to East, the second South to North; in this example, "UJ". Grid Coordinates; for local areas. This part consists of an even number of digits, in this example, 23371 06519, and specifies a location within the 100 km grid square, relative to its lower-left corner. Split in half, the first part (here 23371), called the "easting", gives the displacement east of the left edge of the square; the second part (here 06519), called the "northing"), gives a distance north of the bottom edge of the containing square. Users determine the required precision, so a grid reference is typically truncated to fewer than the full 10 digits when less precision is required. These values represent a point position (southwest corner) for an area of refinement: Ten digits..... 23371 06519 ..Locating a point within a 1 m square Eight digits..... 2337 0651 ...Locating a point within a 10 m square Six digits......... 233 065 .....Locating a point within a 100 m square Four digits......... 23 06 .......Locating a point within a 1000 m (1 km) square Two digits........... 2 0 .........Locating a point within a 10000 m (10 km) square Note that when going from a higher- to a lower-precision grid reference, it is important to truncate rather than round when removing the unneeded digits. Because one is always measuring from the lower-left corner of the 100 km square, this ensures that a lower-precision grid reference is a square that contains all of the higher-precision references contained within it.
In addition to truncating references (on the right) when less precision is required, another powerful feature of USNG is the ability to omit (on the left) the Grid Zone Designation, and possibly even the 100 km Square Identification, when one or both of these are unambiguously understood; that is, when operating within a known regional or local area. For example:
Full USNG: 18S UJ 23371 06519 (world-wide unique reference to 1 meter precision) Without Grid Zone Designation: UJ 2337 0651 (when regional area is understood; here to 10 meter precision) Without 100 km Square Identification: 233 065 (when local area is understood; here to 100 meter precision)
Thus in practical usage, USNG references are typically very succinct and compact, making them convenient (and less error prone) for communication.
History Rectangular, distance-based (Cartesian) coordinate systems have long been recognized for their practical utility for land measurement and geolocation over local areas. In the United States, the Public Land Survey System (PLSS), created in 1785 in order to survey land newly ceded to the nation, introduced a rectangular coordinate system to improve on the earlier metes-and-bounds survey basis used earlier in the original colonies. In the first half of the 20th century, State Plane Coordinate Systems (SPCS) brought the simplicity and convenience of Cartesian coordinates to state-level areas, providing high accuracy (low distortion) survey-grade coordinates for use primarily by state and local governments. (Both of these planar systems remain in use today for specialized purposes.)
Internationally, during the period between World Wars I and II, several European nations mapped their territory with national-scale grid systems optimized for the geography of each country, such as the Ordnance Survey National Grid (British National Grid). Near the end of World War II, the Universal Transverse Mercator (UTM) coordinate system extended this grid concept around the globe, dividing it into 60 zones of 6 degrees longitude each. Circa 1949, the US further refined UTM for ease of use (and combined it with the Universal Polar Stereographic system covering polar areas) to create the Military Grid Reference System (MGRS), which remains the geocoordinate standard used across the militaries of NATO counties.
In the 1990s, a US grass-roots citizen effort led to the Public X-Y Mapping Project, a not-for-profit organization created specifically to promote the acceptance of a national grid for the United States. The Public XY Mapping Project developed the idea, conducting informal tests and surveys to determine which coordinate reference system best met the requirements of national consistency and ease of human use. Based on its findings, a standard based on the MGRS was adopted and brought to the Federal Geographic Data Committee (FGDC) in 1998. After an iterative review process and public comment period, the USNG was adopted by the FGDC as standard FGDC-STD-011-2001 in December 2001.
Since then, the USNG has seen gradual but steadily increasing adoption both in formal standards and in practical use and applications, in public safety and in other fields.
Advantages over latitude/longitude Users encountering the USNG (or similar grid reference systems) sometimes question why they are used instead of latitude and longitude coordinates, with which they may be more familiar. Proponents note that, in contrast to latitude and longitude coordinates, the USNG provides:
Coordinate units that represent actual distances on the ground Equal distance units in both east–west and north–south directions An intuitive sense of scale and distance, across a local area Simpler distance calculation (by Pythagorean Theorem, rather than spherical trigonometry) A single unambiguous representation instead of the three (3) formats of latitude and longitude, each in widespread use, and each having punctuation sub-variants: degrees-minutes-seconds (DMS): N 38°53'23.3", W 077°02'11.6" degrees-minutes-decimal minutes (DMM or DDM): 38°53.388' N, 077°02.193' W decimal degrees (DDD or DD): 38.88980°, -077.03654°
This format ambiguity has led to confusion with potentially serious consequences, particularly in emergency situations.
References comprising only alphanumeric characters (letters and positive numbers). (Spaces have no significance but are allowed for readability.) No negative numbers, hemisphere indicators (+, -, N, S, E, W), decimal points (.), or special symbols (°, ′, ″, :). A familiar "read right then up" convention of XY Cartesian coordinates. An explicit convention for shortening references (at two levels) when the local or regional area is already unambiguously known. A reference to a definite grid square with variable, explicit precision (size), rather than to a point with (usually) unspecified precision implicit in number of decimal places.
All of the above also lead to USNG references being typically very succinct and compact, with flexibility to convey precise location information in a short sequence of characters that is easily relayed in writing or by voice.
Limitations