Land Nav Task 4 - Determine the Grid Coordinates of a Point on a Military Map

Standards: Determined the six-digit grid coordinates for the point on the map with a 100-meter tolerance. Recorded the grid coordinates with the correct two-letter 100,000-meter-square identifier.

Conditions:
Given a standard 1:50,000-scale military map in a field location, a 1:50,000 grid coordinate scale, a pencil, paper, and a point on the map for which coordinates must be determined.

Standards:
Determined the six-digit grid coordinates for the point on the map with a 100-meter tolerance. Recorded the grid coordinates with the correct two-letter 100,000-meter-square identifier.

Performance Steps

Note.

1. A military map can help you spot your location accurately. The map has vertical lines (top to bottom) and horizontal lines (left to right). These lines form small squares 1,000 meters on each side, called grid squares.
2. The lines that form grid squares are numbered along the outside edge of the map picture. No two grid squares have the same number.
3. The precision of a point location is shown by the number of digits in the coordinates; the more digits, the more precise the location. For example: 1996-A 1,000-meter grid square. 192961-To the nearest 100 meters.

1.Look at figure C-11. Your address is grid square 1181. To determine your address, start from the left and read right until you come to 11, the first half of your address. Then read up to 81, the other half. Your address is somewhere in grid square 1181.

Figure C-11. Grid square 1181

2. Determine your address to the nearest 100 meters. Grid square 1181 gives your general neighborhood, but there is a lot of ground inside that grid square. To make your address more accurate, just add another number to the first half and another number to the other half so your address has six numbers instead of four.

• a. To get these extra numbers, suppose that each grid square has 10 lines inside it running north and south, and another 10 running east and west. This makes 100 smaller squares. You can estimate where these imaginary lines are (figure C-12).

Figure C-12. Grid square 1181 divided

• b. To get these extra numbers, suppose that each grid square has 10 lines inside it running north and south, and another 10 running east and west. This makes 100 smaller squares. You can estimate where these imaginary lines are (figure C-12).Suppose you are halfway between grid line 11 and grid line 12. Then the next number is 5 and the first half of your address is 115. Now suppose you are also 3/10 of the way between grid line 81 and grid line 82. Then the second half of your address is 813. Your address would be 115813 (figure C-12). (If you are exactly on line 81, the second half would be 810.)a. To get these extra numbers, suppose that each grid square has 10 lines inside it running north and south, and another 10 running east and west. This makes 100 smaller squares. You can estimate where these imaginary lines are (figure C-12).

3. Use a coordinate scale. The most accurate way to determine the coordinates of a point on a map is to use a coordinate scale. You do not have to use imaginary lines because you can come up with the exact coordinates. This scale is on the coordinate scale and protractor (GTA 05-02-012) (figure C-13) or the plotting scale (figure C-14). Both of these devices include two coordinate scales, 1:25,000 and 1:50,000 meters. Make sure that when you use either of these devices, you use the correct scale.

Figure C-13. Coordinate scale and protractor

Figure C-14. Plotting scale

• a. Locate the grid square in which the point is located (for example, point A, figure C-15) (the point should already be plotted on the map).
• b. The number of the vertical grid line on the left (west) side of the grid square gives the first and second digits of the coordinate.
• c. The number of the horizontal grid line on the bottom (south) side of the grid square gives the fourth and fifth digits of the coordinate.
• d. Place a coordinate scale on the bottom horizontal grid line of the grid square containing point A to determine the third and sixth digits of the coordinate.
• e. Check to see that the zeros of the coordinate scale are in the lower left-hand (southwest) corner of the grid square where point A is located (figure C-15).

Figure C-15. Placement of the coordinate scale

• f. Slide the scale to the right, keeping the bottom of the scale on the bottom grid line until point A is under the vertical (right-hand) scale (figures C-16 and C-17). To determine the six-digit coordinate, the 100-meter mark on the bottom scale, which is nearest the vertical grid line, is the third digit of the number 115. The 100-meter mark on the vertical scale, which is nearest point A, is the sixth digit of the number 813. Putting these together, you have 115813.

Figure C-16. Aligning the coordinate scale

Figure C-17. Aligning the plotting scale

• g. To determine the correct two-letter 100,000-meter-square identifier, look at the grid reference box in the margin of the map (figure C-18).

Figure C-18. Grid reference box

• h. Place the 100,000-meter-square identifier in front of the coordinate, GL115813.

Evaluation Preparation:

Setup: Give the soldier a standard 1:50,000-scale military map in a field location, a 1:50,000 grid coordinate scale, a pencil, paper, and a point on a map for which coordinates must be determined.

Brief Soldier: Tell the soldier to write down the two-letter 100,000-meter-square identifier and the six-digit grid coordinates for one point and the two-letter 100,000-meter-square identifier.

Evaluation Guidance:
Score the soldier GO if all performance measures are passed. Score the soldier NO GO if any performance measure is failed. If the soldier scores NO GO, show the soldier what was done wrong and how to do it correctly.