|Implementing The National Grid In Minnesota
By Randy Knippel, Dakota County Office of GIS
The U.S. National Grid (USNG) is currently proposed as a Minnesota standard. It was established as a federal standard in 2001 (FGDC-STD-011-2001
) and describes specific details of the use and cartographic presentation of a grid system that is uniform for the entire United States. Several states have already adopted it as a standard, as well.
The primary benefit of the USNG is creation of an interoperable location referencing system across jurisdictions. By definition, the Grid defines a unique grid reference for any point. In a sense, a USNG reference at any degree of precision is a unique geographic identifier for an area.
Grid Zone Labeling System
The USNG, which is identical to the Military Grid Referencing System (MGRS) over all parts of the United States, divides UTM zones (6 degrees of longitude) into “Grid Zone Designations” (GZD) of 8 degrees of latitude, named alphabetically from south to north (Figure 1). Further, the 100Km grid lines form squares that are labeled alphabetically from west to east and south to north within each UTM zone (Figure 2).
This labeling is continuous across grid zone boundaries from south to north, repeating as necessary. It is also continuous from west to east, requiring only 8 characters per zone (at the equator). This means that the alphabetic designation for any given 100Km square is not repeated in two grid zones in any direction.
The grid can be further subdivided to describe smaller and smaller areas (Figure 3).
Figure 1 – Grid zones over Minnesota
Figure 2 – Grid zones and 100Km squares over Minnesota
Figure 3 - 10Km squares over Twin Cities metro area.
Efficient, Concise Locations
With a little training, emergency responders will intuitively know that “VK” means a 100Km square. They will also know that the numeric portion of a USNG reference will always contain an even number of digits, each representing greater precision within the 100Km square. Therefore, “VK85” intuitively means a 10Km square (as shown in Figure 3), “VK8354” a 1Km square, and so on until the full 10-digit, 1-meter designation (“VK 83405 53354” – spaces optional).
Conversely, when operations are contained by Minnesota (or even the north central part of the United States), simply saying “VK” is unambiguous and clearly means something in the southern part of the Twin Cities metro area. In this way, the National Grid provides for efficient, concise location references.
Easily Scaled Resolutions
The Grid also creates a convenient mechanism for tracking resources and status of a response and recovery effort. Based on the extents of the effort, grid cells can be easily selected at varying resolutions of 10,000, 1,000, 100, or even 10 meters and used to record activities that are later represented on maps to provide regular updates to everyone involved. High-resolution squares (10 meter) can be easily aggregated to lower resolution squares (1000 meter) to represent a broader perspective for high level planning (Figure 4).
Figure 4 - Tornado path with 100m grid squares for tracking status of response operations.
Grid Zone Boundaries in Minnesota
The National Grid presents some special cases along UTM zone boundaries. Since the USNG reference is a unique geographic identifier for an area, grid zones cannot be extended beyond the UTM zone boundary. This would create confusion and maps that were not interoperable with other maps. For example, in Minnesota, we cannot simply extend grid zone 15T to cover the entire state. (This is often done in statewide GIS datasets that show Minnesota in one digital file with UTM “extended Zone 15” coordinates rather than in separate files with Zones 14, 15 and 16 coordinates).
However, this means that along the grid zone boundary, partial grid cells exist at all levels of precision (figure 5). This creates some interesting cartographic challenges.
Figure 5 – Counties split by grid zone boundaries.
Essentially, grid lines need to be shown from both grid zones and any measuring related to the grid must be performed relative to the grid zone in which the point of interest is located. Maps based on the grid are presented using the UTM zone appropriate for the GZD.
While this may seem awkward, it preserves the integrity of those map products and grid references derived from them. Cartographically, this is accomplished by treating the grid as a series of polygon or line data layers, projected to the map display.
Printed Maps Remain Critical
Maps are the foundation of GIS. However, the tendency is to focus on high-tech interactive, web-based, and mobile applications. While these will continue to be important, the National Grid and lessons learned from recent disasters strengthen the need to also create high quality printed maps that are interoperable between jurisdictions at various scales to support both large and small events.
A map with a hole in it is still a precision instrument. A laptop with a hole in it is a paperweight.
For additional information on the U.S. National Grid, see: