Limitation of Chart Reading

CHAPTER ONE

LOW-LEVEL AND TACTICAL FORMATION

108. LIMITATION OF CHART READING

When used within its limitations, chart reading is the most dependable DR aid. If you have

unrestricted visual contact with the terrain and a reliable chart of the area, map reading is limited

only by your ability to read and interpret the chart. Other limitations can be grouped under one

heading, restrictions to visibility. These can be caused by weather phenomena, altitude,

darkness, or physical construction of the cockpit. When selecting suitable checkpoints, consider

these variables:

Weather Restrictions

Clouds. If they are below you, clouds can restrict visibility by obstructing checkpoints. If

they are above you, they will change the amount of light you have available to identify

checkpoints.

Haze and Smoke. When the fine dust, salt particles, or other impurities that are normally

dispersed in the atmosphere are trapped and concentrated in a limited layer, the resulting

restriction to visibility is called haze. The greatest restriction to visibility in haze usually occurs

when you look directly into the sun. The best visibility in haze is when you look straight down.

Smoke from industrial areas and forest fires often create smog; visibility in haze is similar to

smoke and smog.

3. Blowing Dust, Sand, and Snow. Blowing dust occurs in several regions when the air is

unstable and winds are relatively strong. The strong winds and vertical currents can spread dust

over wide areas and lift it to great heights. Surface, flight, and slant-range visibilities are greatly

reduced in blowing dust. Sandstorms are more local and occur where loose sand is found in

desert regions. Blowing sand is seldom lifted above 50 feet. When the wind is strong, blowing

snow reaching a few feet above the ground can be as troublesome as ground fog over snow-

covered regions.

Precipitation. Precipitation in the form of rain hinders visibility as it streaks over the

canopy. Drizzle and snow can reduce visibility even more.

Altitude. Your altitude poses a problem in determining how far and how well you can see.

At low altitudes, vertical differences in cultural and topographical features are more apparent

than general shapes viewed from above. Good low altitude checkpoints are tall buildings, grain

elevators, mountain peaks, radio towers, road crossings/bends and bridges. However, because

your range of vision is severely restricted at a very low altitude, your checkpoints must be close

together and close to course. This is something you should consider when initially planning a

low-level mission. You can use charted AGL tower heights to help determine your altitude

above the ground. For example, if you know you are flying abeam a 400-foot tower and the top

of the tower is even with the horizon, you know you are too low if you are supposed to be at

500 feet AGL.

1-30 LOW-LEVEL NAVIGATION