3. Keeping both tick marks together, place the

point of the pencil on the tick mark of the paper to hold

it in place. Pivot the paper until another straight portion

is aligned and make another mark on both map and

paper.

4. Continue in this manner until the measurement

is complete. Then place the paper on the graphic scale

and read the ground distance.

The RATIO-TYPE SCALE is simply a comparison

between a given distance measured on the map and on

the ground. It is independent of any unit of measure.

A scale of 1/25,000 means that one unit of measure on

the map is equal to 25,000 of the same units of

measure on the ground. The ground distance between

two points may be determined by measuring between

the points on the map and multiplying the map

measurement by the scale. For example, the distance

commonly used in artillery, tank, and mortar gunnery. It

between two bridges on a certain map is 15 inches.

is convenient for many practical uses because it is

The scale of the map is 1:50,000. Therefore, the actual

approximately one unit of length at a distance (range) of

distance on the ground is found by multiplying 15

one thousand units.

inches by 50,000 (15 times 50,000 equals 750,000

inches). If this is to mean anything to you, change it

to units that can be easily pictured in your mind. These

units might be feet, yards, meters, kilometers, or

In order to measure anything, there must always be

miles. To change the 750,000 inches to feet, you need

a starting point, or zero measurement. To express a

to divide by 12 (the number of inches in a foot); hence

direction as a unit of angular measure, there must be a

750,000 divided by 12 equals 62,500 feet. To change

starting point, or zero measure, and a point of reference.

the 62,500 feet to miles, divide again by 5,280 (the

These two points designate the base, or reference, line.

number of feet per mile); thus 62,500 divided by 5,280

There are three base linestrue north, magnetic north,

equals 11.8 miles.

and grid north. Those most commonly used are

By using either of the methods described above, you

magnetic and grid; the magnetic when working with a

can determine the distance between any two points.

compass, and the grid when working with a military

map.

TRUE NORTH. This is a line from any position on

the surface of the earth to the North Pole. All lines of

Directions are expressed in everyday life as right,

longitude are true north lines. True north is usually

left, straight ahead, and so forth. But the question arises,

symbolized by a star (fig. 5-21).

To the right of what? Military personnel require a

MAGNETIC NORTH. The direction to the

method of expressing a direction that is accurate,

magnetic North Pole is indicated by the north-seeking

adaptable for use in any area of the world, and has a

needle of a magnetic instrument. Magnetic north is

common unit of measure.

usually symbolized by a half arrowhead (fig. 5-21).

Directions are expressed as units of angular

GRID NORTH. This is the north established by the

measure, and there are several systems used.

vertical grid lines on the map. Grid north may be

1. The most commonly used unit of angular

symbolized by the letters *GN *(fig. 5-21).

measure is the degree with its subdivisions of minutes

and seconds.

2. Another unit, less frequently used, is the mil

(abbreviated m). For the U.S. military Purposes, a

The purpose of a topographic map is to permit you

complete circle is divided into 6,400 mils. The mil is

to visualize an area of the surface of the earth with