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.
… excerpt ends here. Continue reading the full article.
