Grids - Locating a Point Using Grid Coordinates


4­5. LOCATING A POINT USING GRID COORDINATES
Based on the military principle for reading maps (RIGHT and UP), locations on the map can be determined by grid coordinates. The number of digits represents the degree of precision to which a point has been located and measured on a map--the more digits the more precise the measurement.

a. Without a Coordinate Scale. In order to determine grids without a coordinate scale, the reader simply refers to the north­south grid lines numbered at the bottom margin of any map. Then he reads RIGHT to the north­south grid line that precedes the desired point (this first set of two digits is the RIGHT reading). Then by referring to the east­west grid lines numbered at either side of the map, the map reader moves UP to the eastwest grid line that precedes the desired point (these two digits are the UP reading). Coordinates 1484 locate the 1,000­meter grid square in which point X is located, the next square to the right would be 1584; the next square up would be 1485, and so forth. To locate the point to the nearest 100 meters, use estimation. By mentally dividing the grid square in tenths, estimate the distance from the grid line to the point in the same order (RIGHT and UP). Give complete coordinate RIGHT, then complete coordinate UP. Point X is about two­tenths or 200 meters to the RIGHT into the grid square and about seven­tenths or 700 meters UP. The coordinates to the nearest 100 meters are 142847.


b. With a Coordinate Scale. In order to use the coordinate scale for determining grid coordinates, the map user has to make sure that the appropriate scale is being used on the corresponding map, and that the scale is right side up. To ensure the scale is correctly aligned, place it with the zero­zero point at the lower left corner of the grid square. Keeping the horizontal line of the scale directly on top of the east­west grid line, slide it to the right until the vertical line of the scale touches the point for which the coordinates are desired. When reading coordinates, examine the two sides of the coordinate scale to ensure that the horizontal line of the scale is aligned with the east­west grid line, and the vertical line of the scale is parallel with the north­south grid line. The scale is used when precision of more than 100 meters is required. To locate the point to the nearest 10 meters, measure the hundredths of a grid square RIGHT and UP from the grid lines to the point. Point X is about 17 hundredths or 170 meters RIGHT and 84 hundredths or 840 meters UP. The coordinates to the nearest 10 meters are 14178484.

c. Recording and Reporting Grid Coordinates. Coordinates are written as one continuous number without spaces, parentheses, dashes, or decimal points; they must always contain an even number of digits. Therefore, whoever is to use the written coordinates must know where to make the split between the RIGHT and UP readings. It is a military requirement that the 100,000 meter square identification letters be included in any point designation. Normally, grid coordinates are determined to the nearest 100 meters (six digits) for reporting locations. With practice, this can be done without using plotting scales. The location of targets and other point locations for fire support are determined to the nearest 10 meters (eight digits).

NOTE: Care should be exercised by the map reader using the coordinate scale when the desired point is located within the zero­zero point and the number 1 on the scale. Always prefix a zero if the hundredths reading is less than 10. the desired point should be reported as 14818407.

NOTE: Special care should be exercised when recording and reporting coordinates. Transposing numbers or making errors could be detrimental to military operations.