Map reading and field study

GRID REFERENCE

Focus questions:

What are grids, grid lines and grid reference?

For what are grid lines used and what purpose do they serve? What are the two rectangular grid systems? What are the types of grid references? Topographic maps are divided into grids.

A grid is a system of horizontal and vertical lines that cross each other to form squares.

Grid lines are a series of numbered horizontal and perpendicular lines that divide a map into squares to form a grid by means of which any point may be located by a system of rectangular coordinates.

Map grids are drawn on maps to help mapmakers find the exact location of a place.

To simplify the use of maps and to avoid the inconvenience of pinpointing locations on curved reference lines, cartographers superimpose on the map a rectangular grid consisting of two sets of straight, parallel lines, which are uniformly spaced, and each set is perpendicular to the other.

This grid is designed so that any point on the map can be designated by its latitude and longitude or by its grid coordinates.

A reference in one system can be converted into a reference in another system.

Such grids are usually identified by the name of the particular projection for which they are designed.

Two rectangular grid systems are available on topographic maps for identifying the location of points.

1. UNIVERSAL TRANSVERSE MERCATOR GRID SYSTEM

This is a special grid for military use throughout the world. In this grid, the world is divided into 60 north-south zones, each covering a strip 6¡ wide in longitude.

2. MILITARY GRID REFERENCE SYSTEM

This is a simplified form of the Universal Transverse Mercator Grid System.

It provides a very quick and easy method of referencing a location on a topographic map.

It is superimposed on the surface of topographic maps which have a scale of 1:50,000 and larger.

It is a blue-coloured series of equally spaced horizontal and vertical lines.

Horizontal lines run from east to west. Vertical lines run from north to south.

An identifying number for each of these lines is found along the map’s margin.

Each identifying number consists of two digits which range from a value of 00 to 99. So, remember that:

Grid lines consist of vertical lines or horizontal lines that intercept each other to form a pattern. They are aligned parallel to each other. Vertical grid lines are numbered lines with numbers written at the top and bottom, increasing by 1 moving eastwards, and are called easting.

Horizontal grid lines are numbered lines with numbers increasing by 1 moving northwards, and are called northing. Please note the following:

Each individual square in the grid system represents a distance of some 1,000 by 1,000 metres and the total size of the grid is 100,000 by 100,000 metres.

When referring to a location with the Military Grid Reference System, identifying letters are always given before the horizontal and vertical coordinate numbers.

Each individual square in the Military Grid Reference System can be further divided into 100 smaller squares (10 by 10). This division allows us to calculate the location of an object to within 100 metres.

A set of digits forms a reference using the grid values of eastings and northings.

These define a location on a map and are referred to as grid references. Things to remember: Each grid square is 1km x 1km or 1km. When you give a grid reference, always give the easting first ... ‘Along the corridor and up the stairs’.

TYPES OF GRID REFERENCES

Four-figure grid references can be used to pinpoint a location within a square measuring 1 sq km. To find the number of the square:

1. Start at the left-hand side of the map and go east until you get to the easting crossing through the bottom-left-hand corner of the square you want. Write this number down.

2. Move north until you get to the northing crossing the bottom-left-hand corner of the square you want. Look at the number of this grid line and add it to the two-digit number you already have.

3. This is your four-figure grid reference.

A four-figure reference will point you to a square on the map.

A six-figure grid references gives the exact location of a place or feature on a map.

It is useful to understand how to use fourfigure grid references before continuing to look at six-figure grid references.

HOW TO WORK OUT SIX-FIGURE GRID REFERENCES

Let us first look at how the six-figure grid reference is made up. Let us take 216 322 as an example. The first three figures refer to the easting of the point (or the distance along the map), while the second three figures refer to the northing of the point (or the distance up the map).

Looking the number 216 322

1. Start by working out the basic easting and northing as you would for a four-figure grid reference, using only the first two easting values and the first two northing values.

2. Imagine that grid square divided into tenths. 216 is the easting. 21 is the grid line. 6 is the number of tenths along towards the next grid line. 322 is the northing 32 is the grid line. 2 is the number of tenths along towards the next grid line.

STEPS IN FINDING A SIX-FIGURE REFERENCE

First, find the four-figure grid reference, but leave a space after the first two digits.

When you get to the easting at the left-hand side of the grid square you want, keep moving east and estimate or measure how many tenths across your symbol lies. Write this number after the first two digits.

Next, move north from the bottom-left-hand corner of your grid square and estimate how many tenths your symbol is from this point. Put them together to create a six-figure grid reference. Next lesson, we will look at using scale to measure distance. Send questions and comments to kerry-ann.hepburn@gleanerjm.com