GRG stands for Grid Reference Graphic. A GRG is a prepared map or image graphic of a particular area, carrying a reference grid and, where needed, labelled sectors or objects. Personnel isolated during military operations can use it to record their location on a shared depiction and report it intelligibly when contact is possible. The person and the recipient must use the same version of the graphic and the same identifiers. A GRG may contain geographic coordinates or provide a local reference whose meaning depends on the accompanying graphic.
Geographic grids and local identifiers
US armed forces use both MGRS grids and simple local divisions for shared reference graphics. MGRS identifies geographic areas through a defined combination of zone, square and numeric components. A local GRG may instead label sectors with letters and buildings within those sectors with numbers. A letter-number combination can then identify a sector and a particular object. Reading it as a column and row in a rectangular grid would identify a different place. The legend and labels determine whether an identifier denotes a cell, sector, object or geographic grid reference.
A local identifier cannot be converted into an unambiguous geographic position without its graphic. The same sector name or building number can occur again on a different area graphic. The graphic's designation and version therefore belong with a local position report. A geographic coordinate instead requires the complete coordinate format and the appropriate geodetic reference. A recipient with an ordinary map can interpret a complete, compatible MGRS reference; they cannot reconstruct a lone local object number from it.
A local division may follow a small area or dominant terrain features. Its grid need not have the same cell size everywhere or place north at the top of the sheet. Directional descriptions use the graphic's north arrow, and the extent of a cell comes from the depicted geometry. A grid cell covers an area. A building number identifies the building; it does not establish which side the person is on or how far away they are.
Finding your position on the graphic
The first task is to correlate the observed surroundings with the depiction. Several matching features are more useful than a single similar-looking building: for example, the arrangement of a junction, a nearby larger structure and the alignment of a bridge. The person checks whether their relative positions actually agree with the graphic. Alterations, destroyed buildings, vegetation or an older aerial image can make the comparison harder. An available positioning device can supplement the correlation if its coordinates and the graphic's geographic reference are compatible.
The appropriate identifier is read only after this correlation. The report must make clear whether the person's position is confirmed, estimated or merely the last position known with confidence. If the location can only be narrowed to two adjacent sectors, that uncertainty is reported. Clear sector labelling does not improve the underlying position determination. An apparently precise object number derived from an uncertain correlation can send the recipient to the wrong place.
A position has a time reference. After further movement, the last reliably recognized location describes earlier whereabouts rather than the present location. A planned destination must also be identified separately. Even without a connection, the person can record the graphic version, last confirmed location, its time and subsequent movement. This allows a later contact to distinguish an observed location from one estimated through movement.
Making and checking an intelligible position report
When contact is actually possible, a usable position reference includes the graphic and version, reference type, identifier and the person's position relative to the identified feature. It also includes the time reference and uncertainty. On a fictional training graphic with freely chosen identifiers, the information might read: “Training graphic, version B; sector K, at the building marked as object 7; outside on its north side; whereabouts confirmed now, no further movement.” The graphic in use and the agreed communication procedures determine the identifiers and the actual reporting format.
The recipient can return the reference they understood: the same graphic version, sector and object with the identified side. The person compares this response with their original information. If it names a different object or version, the position reference must be resolved before further arrangements are based on it. An additional description of a clearly recognizable feature helps reveal transposed numbers or different correlations. Agreement on the graphic version and identifier still leaves any reported position uncertainty in place.
Transmission follows the prepared communication and contact procedures. An open channel or visible signal can also draw an unauthorized observer's attention to the position. The GRG supplies the spatial meaning of the report; recognition and authentication during contact use their own designated procedures. Knowledge of a graphic or its identifiers does not establish identity. Recovery forces authenticate the person before recovery even when their location is known precisely.
Wrong versions, illegible printing and confused references
A new edition can change the area shown, its divisions or object identifiers. If only one party adopts the change, both may give the same identifier while meaning different places. Before deployment, the graphic designation and version are therefore compared with those held by the intended recipients, and the individual's copy is checked for completeness. During isolation, contradictory identifiers can be detected through a further version and feature comparison. An unknown new designation cannot be guessed from an older version.
A printout that is too small or blurred can make letters and numbers indistinguishable; cropping can remove marginal identifiers, the north arrow or the legend. Resizing also changes distances measured on paper. The printed numeric scale may then no longer apply; a scale bar resized together with the graphic can show the revised relationship. Distorted copies do not allow reliable distance measurement even with that bar. A cell identifier alone contains no fixed distance in metres. An identifier that remains legible may still be usable for position correlation, while a distance measured from the distorted graphic gives the wrong separation.
Using another interpretable reference after loss
If the GRG is lost or becomes illegible, reliable access to its local identifiers is lost with it. An available legible map or evasion chart may still provide a geographic reference; an actually available compatible positioning device may provide a coordinate. The report carries its coordinate format, geodetic reference, time, determination method and uncertainty. The recipient can then interpret the location with another suitable map product.
If this is also unavailable, the person describes a clearly recognized terrain feature and their position relative to it, or gives the last reliably determined location with its time. If several objects remain possible, that ambiguity belongs in the information. A local number recalled from memory does not replace the lost correlation. Previously lodged evasion intentions can separately show recovery forces which movement was planned; the actual current position must be determined from available observations and reports.