2.6: Map Reading Skills

Directions

Recall, cardinal directions are expressed as north (N), east (E), south (S), and west (W), with gradations in between, like northeast (NE), southwest (SW), north-northwest (NNW), etc. The most common way to describe direction is with a compass. The compass needle, which is magnetic, aligns itself with the Earth’s magnetic north pole. A common misconception is that if you follow the north needle on a compass you will end up at the North Pole (hi polar bears!). In reality, the magnetic Links to an external site. north pole Links to an external site. is offset from the geographic north pole Links to an external site. location, which we will discuss more in the plate tectonics chapter (Figure 2.6). On a topographic map there will always be a north arrow to indicate where north is. If there is not an arrow on the map, it is OK to assume north is to the top of the page.

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⚒️Can You Dig It? 

Earth acts like a very large bar magnet Links to an external site. with its south-seeking pole near the geographic North Pole. That is why the north needle of your compass is attracted toward the geographic north pole of Earth, because the magnetic pole that is near the geographic North Pole is actually a south magnetic pole! Confusion arises because the geographic term “North Pole” has come to be used (incorrectly) for the magnetic pole that is near the North Pole. Thus, “north magnetic pole” is actually a misnomer, it should be called the south magnetic pole.

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Magnetic Declination

Alongside the north arrow, there is usually an associated magnetic declination Links to an external site.. The symbol for this is used in conjunction with a compass for navigational purposes. The star represents the direction of the geographic north pole or true north. The GN represents grid north, which is associated with the map projection, typically the Mercator projection. The MN represents the direction of magnetic north, the direction a compass needle would point (Figure 2.7). The magnetic declination will allow the map reader to determine where the map is located relative to geographic north. The direction of magnetic north varies both with position on the earth's surface and over time, therefore magnetic declination values on old maps may no longer be accurate. Magnetic declination values can be obtained from NOAA’s National Centers for Environmental Information Links to an external site..

Map Scales

Map scale Links to an external site. is the relationship between distance on the map and distance on the ground. A ratio scale Links to an external site. usually is given as a fraction or a ratio, for example 1/24,000 or 1:24,000 (Figure 2.8). The first number (map distance) is always 1. The second number (ground distance) is different for each scale; the larger the second number is, the smaller the scale of the map. These "representative fraction" scales mean that 1 unit of measurement on the map, 1 inch or 1 centimeter, represents 24,000 of the same units on the ground. The scale 1:24,000, states 1 centimeter on the map would represent 24,000 centimeters on the ground. The scale used for most US topographic mapping is 1:24,000. USGS maps at this scale cover an area measuring 7.5 minutes of latitude and 7.5 minutes of longitude and are commonly called 7.5- minute quadrangle maps Links to an external site.. A 7.5-minute quadrangle map covers an area of 49 to 70 square miles (130 to 180 km²). Other common quadrangles may represent 15 minutes of latitude and longitude.