A diagram at the bottom of most USGS topographic maps shows three north arrows–true north, grid north, and magnetic north–and the angles between them. Some maps, especially very old maps, do not have this diagram.

A map legend or key is a visual explanation of the symbols used on the map. It typically includes a sample of each symbol (point, line, or area), and a short description of what the symbol means.

MAP READING. What is a map? A map is representation of selected natural and manmade features of the whole or part of the earth’s surface on a flat sheet of paper on a definite scale and in their correct relative geographic positions and elevations.

CALCULATING AREA ON A MAP

1. Measure length in cm and convert to m / km.
2. Measure width in cm and convert to m / km.
3. Apply formula – L x W.
4. Write the answer in km² / m².

6 There are two types of prismatic compass, the dry and liquid filled. Liquid type is easier to use though it is less sensitive.

Two main types of the compass are magnetic compass and gyro compass.

Compasses normally come in three different styles.

• Baseplate Compass. The most typical compass is normally referred to as a Baseplate Compass.
• Mirrored Baseplate Compass.
• Lensatic Compasses.

Large objects’such as power lines’can interfere with an accurate reading, and smaller sources’such as handheld electric devices’can also prove to be a problem if they’re in close proximity to the compass.

It is possible for a very powerful magnet to bend a compass needle, damaging the instrument. Very powerful rare earth magnets should be used with caution.

Removing Bubbles from a Handheld Compass Place the compass on a sunny windowsill if you can’t find any obvious signs of leaks. Leave it undisturbed for two to three hours. Remove your compass from the windowsill once the bubbles have disappeared. Warmth from sun exposure causes the liquid to expand and fill the bubble.

A compass should not have an air bubble under normal operating conditions. Bubbles form when the compass is exposed to high altitude and cold weather conditions. Small bubbles shouldn’t affect the compass performance, but there are ways to remove air bubbles.

Are compass bubbles a problem? Small bubbles have no influence on the accuracy of a compass and in most cases will disappear when the compass is returned to sea level and/or warmer temperatures. In fact, compasses are designed in such a way that the development of bubbles is actually both inevitable and desirable.

If a small bubble forms in the liquid-filled capsule, it has no influence on the accuracy of the compass. If a small bubble forms in the liquid-filled capsule, it has no influence on the accuracy of the compass. Its appearance and disappearance are due to changes in temperature and atmospheric pressure.

Usually, the bubble will disappear when the compass is returned to sea level and/or room temperature. If it doesn’t, place it in a warm spot-like a sunny windowsill-so the liquid can heat, expand, and return to it’s normal volume.

The freezing point of mineral spirits is -94 degrees Fahrenheit. So if the liquid in your compass is mineral spirits you’ll have no problem with it freezing.

Modern compasses usually use a magnetized needle or dial inside a capsule completely filled with a liquid (lamp oil, mineral oil, white spirits, purified kerosene, or ethyl alcohol are common).

It is important to note that this compass fluid may not be compatible with some compasses, user should decide on the usability of this product. DANGER: Harmful or Fatal if swallowed; Combustible. Keep from heat, sparks, open flame, or other ignition sources. Avoid breathing of vapor or contact with skin.

Here are the best compasses:

• Best overall: Suunto M-3 Leader compass.
• Best on a budget: Brunton TruArc 3 baseplate compass.
• Best mirrored: Suunto MC-2 mirrored compass.
• Best compact: Brunton Tag Along Zip compact compass.
• Best military-grade: Cammenga 3H tritium lensatic compass.