Section 3 EO M390.03 – DETERMINE GRID REFERENCES (GRs)

Grid lines are used to convey a person’s location to others and to plot a route on a topographical map using GRs.

The grid system is a network of intersecting vertical and horizontal blue lines superimposed on a topographical map. Maps are normally printed so that north is at the top of the sheet. The lines of the grid system are drawn evenly spaced, one scale kilometre apart, so that one set of lines run north-south (vertically) and the second set of lines run east-west (horizontally). The lines are assigned a sequential number and count up from the bottom left corner. The numbers are written along the edges of the map and occasionally within it. The intersecting grid lines at the lower left corner designate a grid square.

Eastings. Similar to the X-axis in mathematical graphing, eastings are a series of vertical parallel lines plotted as an overlay to the map sheet, which are drawn from top to bottom and numbered, with two digits, sequentially from west to east. They run north-south, similar to lines of longitude.

It is important to note that while eastings run parallel to each other, lines of longitude do not. The spacing between lines of longitude is widest at the equator and come together at the north and south poles. It is because of this difference that a bearing taken from a topographical map is a grid bearing, not a true bearing.

Northings. Similar to the Y-axis in mathematical graphing, northings are a series of horizontal parallel lines plotted as an overlay to the map sheet, which are drawn from left to right and numbered, with two digits, sequentially from south to north. They run east-west, the same as lines of latitude.

A four-figure GR represents one grid square and is accurate within a 1 000 m square (1 km² or 1 000 000 m²).

A six-figure GR represents one one-hundredth of a grid square and is accurate within a 100 m square (0.01 km² or 10 000 m²).

Characteristics of a four-figure GR:

The grid lines that intersect in the bottom left corner of the grid square are used to identify that grid square.

Steps to determine a four-figure GR:

1.Confirm the correct grid square.

2.Place a finger at the bottom left corner of the map.

3.Move that finger along the bottom of the map (left to right) up to the grid line (easting) before the grid square.

4.Record the two-digit easting.

5.Place a finger at the bottom left corner of the map.

6.Move that finger along the left side of the map (bottom to top) up to the grid line (northing) before the grid square.

7.Record the two-digit northing after the two-digit easting to create the four-figure GR.

8.Confirm the four-figure GR.

In Figure 18-3-1 Building A is located at GR 7433 and Building B at GR 7632.

Canadian Forces, Maps, Field Sketching, Compasses and the Global Positioning System, Department of National Defence (p. 37)

Figure 1  Four-Figure Grid References

Steps to determine a grid square using a four-figure GR:

1.Confirm the four-figure GR.

2.Place a right-hand finger at the bottom left corner of the map.

3.Move that finger along the bottom of the map (left to right) up to the grid line (easting) numbered the same as the first two digits of the four-figure GR.

4.Place a left-hand finger at the bottom left corner of the map.

5.Move that finger along the left side of the map (bottom to top) up to the grid line (northing) numbered the same as the last two digits of the four-figure GR.

6.Move the right-hand finger up the grid line and the left-hand finger right along the grid line.

7.Where the two grid lines intersect is the bottom left corner of the grid square.

8.Confirm the correct grid square.

In Figure 18-3-1, GR 7532 represents the grid square southeast of Building A and west of Building B.

Have the cadets practice the skill either on a topographical map or the worksheet located at Annex A. If using Annex A, check the cadets’ answers using the answer key located at Annex D.

Using either the overhead or the handout created from Annex B, explain and demonstrate how to determine a six-figure GR. Care must be taken to ensure that all cadets understand each step before proceeding to the next step. For this skill lesson, it is recommended that instruction takes the following format: (1) Explain and demonstrate the complete skill while cadets observe. (2) Explain and demonstrate each step required to complete the skill. Monitor cadets as they imitate each step. (3) Monitor the cadets’ performance as they practice the complete skill. Note: Assistant instructors may be used to monitor cadet performance.

Estimate a six-figure GR by:

1.creating an imaginary grid system to divide a grid square into 100 equally sized smaller grid squares with 10 along the bottom edge and 10 along the left-side edge (as illustrated at Figure 18-3-2);

2.noting that six-figure GRs will have six numerical digits derived from the numbers assigned to the eastings and northings on the map sheet and their estimated tenths;

3.recognizing that the numbers are listed by recording the three-digit easting followed by the three-digit northing; and

4.remembering that the grid lines that intersect in the bottom left corner of the grid square are used to identify that imaginary grid square.
Steps to determine a six-figure GR:

Canadian Forces, Maps, Field Sketching, Compasses and the Global Positioning System, Department of National Defence (p. 38)

Figure 2  Six-Figure Grid References

Example 1: Determine the six-figure GR for the building west of the town of Moyerville.

1.Building west of the town of Moyerville is within GR 7632.

2.76 combined with 4 tenths creates ’764’.

3.32 combined with 1 tenth creates ’321’.

4.Building west of the town of Moyerville is located at GR 764321.

Example 2: Determine the six-figure GR for the Inn north-north-west of the town of Moyerville.

1.The Inn at the north part of the grid square at GR 7632.

2.76 combined with 5 tenths creates ’765’.

3.32 combined with 7 tenths creates ’327’.

4.The Inn at the north part of the grid square is located at GR 765327.

Have the cadets complete the worksheet located at Annex C.

Romer. A device used for measuring a point within a grid square to determine its six-figure GR.

Romers may be purchased or created. Purchased romers include compasses and protractors. Constructed romers use a small piece of paper and the scale bars of a topographical map.

Many compasses include romers already printed on the compass base plate. There are commonly two romers, for use with 1 : 25 000 and 1 : 50 000 scale topographical maps.

Director Cadets 3, 2008, Ottawa, ON: Department of National Defence

Figure 3  Compass

All protractors may be used to determine a bearing on a map, however, few have romers already printed on them. The Canadian Forces has created the C2 protractor (as illustrated in Figure 18-3-4) specifically designed for use on topographical maps.

Canadian Forces, Maps, Field Sketching, Compasses and the Global Positioning System, Department of National Defence (p. 41)

Figure 4  C2 Protractor

A constructed romer requires a piece of paper with at least one square corner and the scale bars of the topographical map. Using the scale bars of the topographical map, a romer can be constructed as illustrated in Figure 18-3-5.

Canadian Forces, Maps, Field Sketching, Compasses and the Global Positioning System, Department of National Defence (p. 41)

Figure 5  Constructed Romer

Accuracy must be stressed to the cadets when constructing a romer. Have the cadets ensure that their pencils are sharp or their pens are fine tipped. For this skill lesson, it is recommended that instruction take the following format: (1) Explain and demonstrate the complete skill while cadets observe. (2) Explain and demonstrate each step required to complete the skill. Monitor cadets as they imitate each step. (3) Monitor the cadets’ performance as they practice the complete skill. Note: Assistant instructors may be used to monitor cadet performance.

Construct a romer for determining six-figure GRs by:

1.obtaining a blank piece of paper with a square edge;

2.placing one side of the square edge along the 100-m scale bars;

3.marking off 100-m segments beginning at the corner of the paper and working outward;

4.numbering these markings from zero (at the corner of the paper) to ten; and

5.repeating Steps 2. to 4. on the adjacent edge (eg, completed romer as illustrated in Figure 18-3-5).

It is important to use the correct scale bar. The constructed romer’s markings should match the grid lines of the topographical map; the side of a grid square must be equal to ten 100-m marks on each of the romer’s two edges.

This TP uses all the skills from the previous TPs and it is essential that this TP not be covered until problems from the previous TPs have been corrected. Unlike TP 3, where the cadets used the very visible ’imaginary’ grid (eg, Figure 18-3-2) to determine a six-figure GR, the cadets will now be using their constructed romer from TP 5 to determine a six-figure GR and to locate objects with a six-figure GR. Much greater care and attention to detail must used by the cadets in order to ensure accuracy. For this skill lesson, it is recommended that instruction take the following format: (1) Explain and demonstrate the complete skill while cadets observe. (2) Explain and demonstrate each step required to complete the skill. Monitor cadets as they imitate each step. (3) Monitor the cadets’ performance as they practice the complete skill. Note: Assistant instructors may be used to monitor cadet performance.

Determine a six-figure GR using a constructed romer, by:

1.placing the corner of the constructed romer on the bottom left corner of the grid square, noting the four-figure GR;

2.moving the constructed romer to the right the number of tenths required to align the romer directly to or before (never past) the conventional sign or location for which the GR is being determined;

3.reading the value along the X-axis of the romer where it crosses the easting on the map sheet (the value at this intersection becomes the value for the third digit of the six-figure GR);

4.moving the constructed romer up the number of tenths required for the corner of the romer to be positioned on or before (never past) the conventional sign or location for which the GR is being determined;

5.reading the value along the Y-axis of the romer where it crosses the northing on the map sheet (the value at this intersection becomes the value for the sixth digit of the six-figure GR); and

6.combining the two sets of digits to create the six-figure GR.

Director Cadets 3, Royal Canadian Army Cadet Reference Book, Department of National Defence (p. 5-20)

Figure 6  Using a Constructed Romer

Determine what a six-figure GR represents using a constructed romer, by:

1.determining the four-figure GR, by removing the third and sixth digits from the six-figure GR, to identify and locate the correct grid square;

2.placing the corner of the constructed romer on the bottom left corner of the grid square;

3.moving the constructed romer to the right the number of tenths, as identified by the third digit;

4.moving the constructed romer up the number of tenths, as identified by the sixth digit; and

5.determining the object (that is up and to the right from the tip of the romer).

Examples used will be from Annex C.

Example 1:

From Figure 18C-1, determine the six-figure GR for the Post Office.

1.Grid square GR 7632.

2.Four tenths to the right.

3.76 combined with 4 tenths creates ‘764’.

4.Four tenths up.

5.32 combined with 4 tenths creates ‘324’.

6.The Post Office is located at GR 764324.

Example 2:

From Figure 18C-1, determine the object located at GR 766323.

1.Four-figure GR is 7632.

2.Place romer at the bottom left corner of grid square 7632.

3.Move the romer to the right six tenths.

4.Move the romer up three tenths.

5.GR 766323 identifies the Train Station.

Have the cadets practice using the lists created before the lesson, of objects for the cadets to determine six-figure GRs for, and of six-figure GRs for the cadets to determine what objects they represent, on the appropriate topographical map.