Page 259 - Laboratory Manual in Physical Geology

1:24,000.” Knowing that 1 inch on the map equals 24,000 of true north (star symbol), then subtract the degrees of
inches on the ground is not very convenient, because no one declination from your compass reading (imagine that you
measures big distances in thousands of inches! However, if you are rotating your compass counter-clockwise to compensate
divide the 24,000 inches by 12 to get 2000 ft, then the scale for declination). If the MN arrow is to the west of true

suddenly becomes useful: “1 in. on the map = 2000 ft on north, then add the degrees of declination to your compass
the ground.” An American football field is 100 yards (300 ft) reading (imagine that you rotated your compass clockwise).
2

long, so: “1 in. on the map = 6 football fields.” These adjustments will mean that your compass readings
3
On a map with a scale of 1:63,360, “1 inch equals are synchronized with the map (so long as you used the lat-
63,360 inches” is again not meaningful in daily use. But est declination values obtained from NOAA).
there are 63,360 inches in a mile. So, the verbal scale,
“1 inch equals 1 mile” is very meaningful. A standard How to Set a Compass for Declination
1:62,500 map (15-minute quadrangle map commonly Some compasses allow you to rotate their basal ring
used in parts of Alaska) is very close to this scale, so “one graduated in degrees to correct for the magnetic
inch equals approximately one mile” is often written on declination. If the MN arrow is 5° east (right) of true
such a map. Note that verbal scales are often approximate north, then you would rotate the graduated ring 5° east
because their sole purpose is to help the reader make gen- (clockwise, to subtract 5° from the reading). If the MN
eral sense of how the map relates to sizes of real objects on arrow is 5° west (left) of true north, then you would
the ground. rotate the graduated ring 5° west (counter-clockwise, to
add 5° to the reading).
Declination and Compass Bearings
How to Determine a Compass Bearing on
Directional information is summarized as a trident-shaped a Map
symbol like the one in the lower left corner of FIGURE 9.3
(Ritter Ridge Quadrangle). Because longitude lines form To determine a compass bearing on a topographic map,
the left and right boundaries of a topographic map, north follow the directions in FIGURE 9.5 . Then imagine that you
is always at the top of the quadrangle. This is called grid are buying a property for your dream home. The boundary
north (GN) and is usually very close to the same direction of the property is marked by four metal rods driven into the
as true north on the actual Earth. Unfortunately, magnetic ground, one at each corner of the property. The location
compasses are not attracted to grid north or true north (the of these rods is shown on the map in FIGURE 9.5 (left side)

geographic North Pole). Instead, they are attracted to the as points A, B, C, and D. The property deed notes the dis-
magnetic north pole (MN), currently located northwest of tances between the points and bearings between the points.
Hudson Bay in Northern Canada, about 700 km (450 mi) This defines the shape of the property. Notice that the
from the true North Pole. northwest edge of your property lies between two metal rods
located at points A and B. You can measure the distance

between the points using a tape measure. How can you mea-
What Is Declination?
sure the bearing?
The trident-shaped symbol on the bottom margin of topo- First, draw a line (very lightly in pencil so that it can
graphic maps shows the declination (difference in degrees) be erased) through the two points, A and B. Make sure
between compass north (MN) and true north (usually a the line also intersects an edge of the map. In both parts
star symbol). Also shown is the declination between true of FIGURE 9.5 , a line was drawn through points A and B so

north ( star symbol) and grid north (GN). The magnetic that it also intersects the east edge of the map. Next, orient
pole migrates very slowly, so the declination is exact only a protractor so that its 0° and 180° marks are on the edge
for the year listed on the map. You can obtain the most of the map, with the 0° end toward geographic north. Place
recent magnetic data for your location from the NOAA the origin of the protractor at the point where your line
National Geophysical Data Center ( http://www.ngdc. A–B intersects the edge of the map. You can now read a
noaa.gov/geomag-web/#declination ). bearing of 43° east of north. We express this as a quadrant
bearing of “North 43° East” (written N43°E) or as an azi-
What Is a Compass Bearing? muth bearing of 43°. If you were to determine the opposite
A bearing is the compass direction along a line from one bearing, from B to A, then the bearing would be pointing
point to another. If expressed in degrees east or west of southwest and would be read as “South 43° West,” or as
true north or south, it is called a quadrant bearing . Or it an azimuth of 223°. Remember that a compass points to
may be expressed in degrees between 0 and 360, called an Earth’s magnetic north pole (MN) rather than true north
azimuth bearing , where north is 0° (or 360°), east is 90°, or grid north (GN). When comparing the bearing read
south is 180°, and west is 270°. Linear geologic features directly from the map to a bearing read from a compass,
(faults, fractures, dikes), lines of sight and travel, and linear you must adjust your compass reading to match true north
property boundaries are all defined on the basis of their or grid north (GN) of the map, as described above.
bearings. But because a compass points to Earth’s magnetic You also can use a compass to read bearings, as

north (MN) pole rather than the true North Pole, one must shown in FIGURE 9.5 (right). Ignore the compass needle

correct for this difference. If the MN arrow is to the east and use the compass as if it were a circular protractor.
Topographic Maps and Orthoimages ■ 231