Map & Compass 101: Lesson 1 – Locate a four a six figure grid reference

GRID REFERENCE SYSTEMS
When a mapmaker has projected a zone, and divided it into sections, a rectangular grid is laid over top of the projection. These grid lines are shown in blue on a topo map. The grid lines are exactly parallel to each other. The vertical grid lines are printed parallel to the meridian of the zone, and the horizontal grid lines are parallel to the equator.

The largest of the grids are squares that are 100km x 100km. Each of these 100km squares is identified by a letter which is stated after the UTM zone number. In the example above, the Grid Zone Designation is “10 U.” Each large square is further divided into smaller squares of 10km, and then again into 1km squares. It is these 1km x 1km (1000m x 1000m) squares that you see on a 1:50 000 scale topo map.

EASTINGS
Each grid line in the 1000m grid is numbered. The vertical lines are numbered from an imaginary line 500 000 metres west of the zone’s centre meridian. Each zone then starts at zero in the west and each 1000m line is numbered going towards the east. In the bottom and top margins you will find each vertical grid line’s number, usually a two-digit number at the top and bottom ends of the line. In the bottom left corner you can see the full number represented with the letter E printed behind it. This tells you how many metres east of the start point you are. Because these lines are numbered from west towards the east, they’re called “Eastings.”

NORTHINGS
Each horizontal line is also numbered, this time starting with zero at the equator. In the left and right margin you will find the two-digit numbers at the ends of each horizontal line. In the bottom left you will find the full number of metres from the equator with the letter N printed behind it. Note that the most southerly part of Canada 4 620 000 metres from the equator. Because these lines are numbered from the equator towards the north, they are called “Northings.”

MILITARY GRID REFERENCE SYSTEM
The military traditionally identifies grid lines by stating the two-digit short form of the grid line numbers. Because these two-digit numbers repeat over a large area (every 100km), the military has established a letter code for each 100km x 100km square. The military grid codes is found in the right margin underneath the UTM Zone number. In the example above, the military “100 000m Square Identification” is “EK.” This code also appears on your map face.

FOUR-FIGURE GRID REFERENCES
The grid system on a map allows us to identify locations and communicate them to other people with an internationally accepted system. When you identify a location using the grid system it is called using a “grid reference.”
Military grid references use the two-digit grid line numbers to identify specific grid squares. For centuries, mathematicians have always stated the X coordinate (vertical) before the Y coordinate (horizontal), so map users have adopted that procedure. Eastings are stated before Northings. Every 1000m grid square is identified by listing the numbers of the grid lines that intersect at its bottom left corner.

For example: The post office circled is located in the grid square identified as 7433. The hospital is at grid reference 7632. Remember: a four-figure grid reference refers to the entire grid square. The easiest way to remember to list the eastings then northings is to say, “In the door, then up the stairs.”

SIX-FIGURE GRID REFERENCES
In wilderness navigation we often need to be more accurate with a grid reference for a location than a 1000m x 1000m square. In the illustration below you’ll notice that the grid square has more than one bridge, so communicating which bridge you are going to meet at would be impossible using a four-figure grid reference.

By creating an imaginary grid inside a grid square, we can use the same principles of the grid reference to make a more accurate statement of location. Each small easting and northing is numbered 1 to 9, from west to east and from south to north respectively. Then each smaller (100m x 100m) square can be identified listing all eastings, then northings. For example: Grid reference 761326 is given, the easting is 761 or 76 and 1/10, and the northing is 326 or 32 and 6/10. Locate your grid square at 7632 and then go in 1 and up 6.
a. There is a church at grid reference 764324; and
b. There is a T-junction in the road at 768327

Remember that a six-figure grid reference describes a square 100m x 100m.
This imaginary grid inside a square can be estimated, or you can measure accurately using a tool called a “romer.”

CONSTRUCTION AND USE OF A ROMER
A romer is a device used for measuring a point within a grid square rather than estimating. The left side of the metres scale bar is divided into100m segments. Use a blank corner of a piece of paper, place it along the scale, mark off the 100m segments and then number them, starting with zero at the point. Number both sides up to 10.

To use, place the corner of your romer on the grid square and move in the number of tenths and up the number of tenths. The grid reference for the building in the example below is 766327. Note: always round down.