7-77. There are different ways of measuring distances. Several techniques are included in this section.
Graphic Scales
7-78. On most maps, there is another method of determining ground distance. It is by means of the graphic (bar) scales. A graphic scale is a ruler printed on the map on which distances on the map may be measured as actual ground distances. To the right of the zero (0), the scale is marked in full units of measure and is called the primary scale. The part to the left of zero (0) is divided into tenths of a unit and is called the extension scale. Most maps have three or more graphic scales, each of which measures distance in a different unit of measure (see figure 7-23).
Ground Distance—Straight Line
7-79. To determine a straight-line ground distance between two points on a map, lay a straight-edged piece of paper on the map so that the edge of the paper touches both points. Mark the straight edge of the paper at each point. Move the paper down to the graphic scale and read the ground distance between the points. Use the scale that measures in the unit of measure desired (see figure 7-24).
Ground Distance—Curved Line
7-80. To measure distance along a winding road, stream, or any other curved line, the straight edge of a piece of paper is used. Mark one end of the paper and place it at the point from which the curved line is to be measured. Align the edge of the paper along a straight portion and mark both the map and the paper at the end of the aligned portion. Keeping both marks together, place the point of the pencil on the mark on the paper to hold it in place. Pivot the paper until another approximately straight portion is aligned and again mark on the map and the paper. Continue in this manner until measurement is complete. Then place the paper on the graphic scale and read the ground distance (see figure 7-25).
Measuring Azimuth
7-81. Use the following steps to determine the grid azimuth of a line from one point to another on a map (for example, from point A to point B or point C to point D) (see figure 7-26 on page 7-26):
- Step 1. Determine the grid azimuth of a line and draw a line connecting the two points.
- Step 2. Place the index of the protractor at the point where the line crosses a vertical (north-south) grid line.
- Step 3. Keeping the index at this point, align the 0° to 180° line of the protractor on the vertical grid line.
- Step 4. Read the value of the angle from the scale; this is the grid azimuth to the point.
Plotting a Direct Line
7-82. Use the following steps to plot a direction line from a known point on a map (see figure 7-27, page 7-27):
- Step 1. Construct a north-south grid line through the known point.
- Step 2. Generally, align the 0° to 180° line of the protractor in a north-south direction through the known point.
- Step 3. Holding the 0° to 180° line of the protractor on the known point, slide the protractor in the north-south direction until the horizontal line of the protractor (connecting the protractor index and the 90° tick mark) is aligned on an east-west grid line and draw a line connecting 0°, the known point, and 180°.
- Step 4. Holding the 0° to 180° line on the north-south line, slide the protractor index to the known point.
- Step 5. Make a mark on the map at the required angle. (Do not mark on the map.)
- Step 6. Draw a line from the known point through the mark made on the map.
7-83. A pace is equal to the distance covered every time the same foot touches the ground. To measure distance, count the number of paces, estimate the distance traveled, and apply that to the map and route of movement. Distances measured this way are only approximate. Before using dead reckoning navigation, it is important for each person to establish the length of his or her average pace. This is done by pacing a measured course many times and computing the mean. In the field, an average pace must often be adjusted because of the following conditions—
- Slopes. The pace lengthens on a downgrade and shortens on an upgrade.
- Winds. A headwind shortens the pace while a tailwind increases it.
- Surfaces. Sand, gravel, mud, and similar surface materials tend to shorten the pace.
- Elements. Snow, rain, or ice reduces the length of the pace.
- Clothing. Excess weight of clothing shortens the pace while the type of shoes affects traction and therefore pace length.
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