08 studying the earths surface

7/27/2019 08 Studying the Earths Surface

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CHAPTER 8

STUDYING EARTHS LANDFORMS

PURPOSE

To understand the different ways in which geologists study the surface of the Earth. To be able to select the most appropriate image for studying an aspect of Earths surface To practice interpreting landform models

MATERIALS NEEDED

A clear plastic ruler marked in millimeters and/or tenths of an inch and a circular protractor. A globe and maps provided by your instructor

8.1 INTRODUCTION

It is easier to study Earths surface today than at any time in history. To help understand surface

features, we can now make detailed surface models from satellite elevation surveys and download

images of any point on the planet at the click of a mouse using Google Earth and NASAs WorldWind.

Geologists were quick to understand the scientific value of satellite imaging technology and adapt new

methods as quickly as they are developed. You will use images in this manual that even researchers

didnt have at their disposal a decade ago. The study of the Earths surface is almost as dynamic as the

surface itself!

And you dont have to be in a college laboratory to use this technology because much of it can

be downloadedfree of chargefrom the Internet. Google Earthand NASAs World Wind show satellite

images of the entire globe and can be used to get three-dimensional views. Microdem can build

realistic digital elevation models of the landscape, draw topographic profiles, measure straight-line

distances and the length of meandering streams, and estimate slope steepness.Other resources are

relatively inexpensive: National Geographics TOPO! provides continuous topographic map coverage of

each state for about $100 and also draws profiles and measures distances.

8.2 IMAGES USED TO STUDY EARTHS SURFACE

2009 Allan Ludman and Stephen Marsh

W.W. Norton & Company


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The best way to study landforms would be to fly over them to get a birds-eye view and then walk

or drive over them to understand them from a human perspective. Thats not practical for a college

course, so we will have to bring the landforms to you instead. To do so, we will use traditional tools like

maps (Figure 8.1a) and aerial photographs (8.1b), a new generation of Landsat and other satellite images

(8.1c), digital elevation models (DEMs; 8.1d), and Google Earth and World Wind images that were

science fiction when we began planning this book.

Figure 8.1 shows a part of eastern Maine using four of these methods. A topographic map

(Figure 8.1a) uses contour lines to show landforms (see Chapter 9). Topographic maps used to be drawn

by surveyors who measured distances, directions, and elevations in the field. They are now made by

computers from aerial photographs and radar data.Aerial photographs(Figure 8.1b), including United

States Geological Survey (USGS) Orthophotoquads, are photographic images taken from a plane and

pieced together to form a mosaic of an area. Landsat images(Figure 8.1c) are made by a satellite that

takes digital images of Earths surface using visible light and other wavelengths of the electromagnetic

spectrum. Scientists adjust the wavelengths to color the image artificially, to emphasize specific

features. For example, some infrared wavelengths help reveal the amount and type of vegetation.Digital

elevation models(DEMs; Figure 8.1d) are computer generated three-dimensional views of landforms

made from radar satellite elevation data spaced at 10- or 30-meter intervals on the Earths surface. A

new generation based on 1-m data is now being released and provide a more accurate model of the

surface than anything available to the public 5 years ago.

Each type of image is particularly helpful for different types of study and each has its drawbacks,

so that no one image is the right one. The latitude and longitude of the corners of a topographic map,

DEM, or Landsat image are normally listed and there would be a scale for measuring distance on the

map and DEM. North is assumed to be at the top of all images unless otherwise indicated.

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Figure 8.1: Representations of an area in eastern Maine in different kinds of images

a. Topographic map b. Aerial photograph

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The images in Figure 8.1 show the same area but look different because each emphasizes

different aspects of the surface. Which is best? For learning the names of mountains or lakes, the

topographic map (Fig. 8.1a) is the obvious choice; for studying the network of roads in the area, the

DEM would be useless. For analyzing landforms and their evolution, geologists can choose the image

that works best for their particular project.

EXERCISE 8.1: WHICH IMAGE WORKS BEST?

a. Examine the images in Figure 8.1 and rank them (1-4) in Table 8.1 by how well they show the mapelements indicated (1 would be the most effective, 4 the least effective. Ties are allowed)

b. Which of the images enables you to recognize the topography most easily? Why?

c. Landsat 7 image (artificial color) d. Digital elevation model (DEM)


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c. Which is least helpful in trying to visualize the hills, valleys, and lakes? Why?

d. Erosional agents often produce a topographic grain, an alignment of elongate hills, ridges, andvalleys. Which images show the topographic grain in this area most clearly? Once youve seen it onthose images, can you recognize it on the others?

e. Which images show highways most clearly?f. Which images show unpaved lumber roads most clearly?g. Which image do you think is the oldest? The most recent? Explain your reasoning.

h. Which image(s) would you want to have if you were planning a wilderness hike? Why?

8.2.1 Map projections

These images are flat, two-dimensional pictures but Earth is a nearly spherical three-dimensional

body. The process by which a three-dimensional sphere is converted to a two-dimensional map is called

making a projection. There are many different projections and each produces maps that look different

and are used for different purposes (Figure 8.2). Some, like the Mercator projection, preserve

Table 8.1: Evaluating landscape images

Topographic map Aerial photograph Landsat image DEM

Location

Direction

Elevation

Changes in slope

Distance

Names of features

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accurate directions between points; others preserve the true areas of features, and some the true distances

8.3 MAP ELEMENTS

All accurate depictions of Earths surface must contain certain basic elements: location, a way to

show precisely where the area is; a way to measure the distance between features; and an accurate

portrayal of directions between features. It is also important to know elevationsof hilltops and other

features, and the steepness of slopes.

8.3.1 Map Element 1: Location

Road maps and atlases use a simple grid system to locate cities and towns, e.g. Chicago is in

grid square A8. This is not very precise because many other places may be in the same square, but is

good enough for most driving. More sophisticated grids are used to locate features on Earth precisely.

Maps published by the USGS use three grid systems: latitude/longitude, the Universal Transverse

Mercator (UTM) grid, and, in most states, the Public Land Survey System. The UTM grid is least

familiar to Americans but is used extensively in the rest of the world.

8.3.1a Latitude andlongitude

The latitude/longitude grid is based on location north or south of the equator and east or west of

an arbitrarily chosen north-south line (Figure 8.3). Aparallel of latitudeconnects all points that are the

same angular distance north or south of the Equator. The maximum value for latitude is 90N or 90S

between points. All projections must to some extent distort 3-D reality to fit on a 2-D piece of paper.

Figure 8.2 Four common map projections and their different views of the world

a) Orthographic b) Mercator c)Polyconic d)Sinusoidal

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(the North and South Poles, respectively). A meridian of longitudeconnects all points that are the same

angular distance east or west of thePrime Meridiana line that passes through the Royal Observatory in

Greenwich, England. The maximum value for longitude is 180E or 180W, the International Date Line.

Remember: You must indicate whether a point is north or south of the Equator, east or west of the

Prime Meridian.44 Latitude could be in the northern or southern hemisphere.

Latitude and longitude readings are typically reported in degrees (), minutes (), and seconds ()

where there are 60 in a degree and 60 in a minute, e.g. 403744N, 734509W. For reference, one

degree of latitude is equivalent to approximately 69 miles (111 km), one minute of latitude is about 1.1

mile (185 km), and one second of latitude about 100 feet (31 m). The same kind of comparison can only

be made for longitudeat the equatorbecause the meridians merge at the poles and the distance between

degrees of longitude decreases gradually toward the poles (Figure 8.3b). hand-held Global Positioning

System (GPS) receivers and those used in cars and planes can locate points to within a second.

EXERCISE 8.2: LOCATING CITIES USING LATITUDE AND LONGITUDE

a.With the aid of a globe or map, determine the latitude and longitude of your geology laboratory asaccurately as you can. How could you locate the laboratory more accurately?

b.If you have access to a GPS receiver, locate the corners of your laboratory building. Draw a mapshowing the location, orientation, and distances between the corners.

c. Locate the following U.S. and Canadian cities as accurately as possibleNome, Alaska______________________________ Seattle, Washington_____________________

Chicago, Illinois_____________________ Los Angeles, California__________________

St. Louis, Missouri_________________________ Houston, Texas __________________

New York, New York_______________________ Miami, Florida _________________________

Saint Johns, Newfoundland __________________ Ottawa, Ontario_________________________

Calgary, Alberta __________________________ Victoria, British Columbia __________________

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30 30

30 N. Latitude

30 S. Latitude

Equator

90 S. Latitude

South Pole

60 N. Latitude

60 S. Latitude

90 N. Latitude

North Pole

b.Latitude is measured in degrees northor south of the Equator

60

Prime Meridian

0 E or W Longitude

60 W. Longitude

75 W. Longitude

90 W. Longitude

90 E. Longitude

a.Longitude is measured in degrees east or west ofthe Prime Meridian (Greenwich, England)

0 N or S Latitude

0 E or W

45 N. Lat., 15 E Long.

30 S Lat. , 75 W Long.

c) Locating points using the completed grid

Figure 8.3 The latitude/longitude grid

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d. Which of these cities above do you think is closest in latitude to each of the following world cities?Predict first, without looking at a map or globe, then check. Were you surprised by any?

City Predicted North American Latitude and Longitude Actual best match

Oslo, Norway

Baghdad, Iraq

London, England

Paris, France

Rome, Italy

Beijing, China

Tokyo, Japan

Quito, Ecuador


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Cairo, Egypt

Capetown,South Africa

8.3.1b Public Land Survey System

The Public Land Survey System was created in 1785 to provide accurate maps as America

expanded from its 13 original states. Most of the rest of the country is covered by this system, except for

Alaska, Hawaii, and Texas, and the southwestern states surveyed by Spanish colonists before they

joined the Union. Points can be located rapidly to within an eighth of a mile in this system (Figure 8.4).

The grid is based on accurately surveyed north-south (Principal Meridian) and east-west (Base

Line) lines for each survey region. Lines parallel to these at six mile intervals create grid squares 6 miles

on a side forming east-west rows calledtownshipsand north-south columns calledranges (Figure 8.4a).

Townships are numbered north or south of the Base Line and Ranges east and west of the Principal

Meridian (Figure 8.4a). Each 6-mile square is divided into 36 sections, each one mile on a side,

numbered as shown in Figure 8.4. Each section is divided into quarter sections mile on a side and

each of these is further quartered, resulting in squares mile on a side. The location of the star in the

red box in Figure 8.4 is described in the series of blow-ups:

T2S, R3Elocates it somewhere within an area of 36 square miles (inside a 6 mi x 6 mi square)

Section 12,T2S, R3Elocates it somewhere within an area of 1 square mile

SE of Section 12, T2S, R3Elocates it somewhere within an area of square mile

SW of the SE of Section 12, T2S, R3E locates it within an area of 1/16 square mile


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A

SE of Section 12, T2S, R3E

SW of the SE ofSection 12, T2S, R3E

mile

NW NE

SE SW

Base Line PrincipalMeridian

R6W

R5W

R4W

R3W

R2W

R1W

R1E

R2E

R3E

R4E

R5E

R6E

T5N

T4N

T3N

T2N

T1N

T1S

T2S

T3S

T4S

T5S

6 miles

6miles

T2S, R3E

6 5 4 3 2 1

7 8 10 11 129

3631 32 33 34 351mile

1 mile

18 17 16 15 14 13

19 20 21 22 23 24

30 29 28 27 26 25

NW NE

SE SW

Section 12, T2S, R3E

mile

B

Figure 8.4 Locating with the Public Land Survey Grid

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EXERCISE 8.3: LOCATING POINTS WITH THE PUBLIC LAND SURVEY GRID

Determine the location of points A and B in Figure 8.4, additional points your instructor indicates ontopographic maps.

A_________ B ____________

Determine the location of points indicated by your instructor on topographic maps

8.3.1c Universal Transverse Mercator (UTM) grid

The UTM (Universal Transverse Mercator) grid divides the Earth into 1200 segments, each

containing 6 of longitude, and 8 of latitude (Figure 8.5). North-south segments are assigned letters (C-

X); east-west segments are called UTM zonesand are numbered 1-60 eastward from the International


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Date Line (180W) one 2 from 174 to

168 W Longitude, etc. The 48 conterm

to 67 W

shapes of Greenland and Antarctica in Figure 8.5,

To locate a point, begin with the grid box in which the feature is located. For example, the red

box in Figure 8.5 is grid S22. UTM grid readings tell in metershow far north of the Equator (northings)and east of the central meridian for each zone (eastings) a point lies. The central meridian for each UTM

zone (the line of longitude that runs through the center of the zone; Figure 8.6) is arbitrarily assigned an

easting of 500,000 m so that no point would have a negative easting. Points east of a central meridian

will thus have eastings greater than 500,000 m, those west of the central meridian less than 500,000 m.

Figure 8.6 UTM zones for the 48 conterminous United States

The red line is the central meridian (105W) for UTM Zone 13;the blue line is the central meridian for Zone 18

. Thus, UTM Zone 1 extends from 180 to 174 W Longitude, Z

Longitude. Because of the polar distortion in

inous United States lie within UTM zones 10-19, roughly 125

the Mercator projection evident in the sizes and

the UTM grid covers only latitudes 80N to 80S.

Figure 8.9 The worldwide UTM grid


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ma

Grid labels across the top and bottom of a map are eastings(indicated by E), those along the side of the

map northings (indicated by N). There are two kinds of labels, one a shorthand version, the other

complete. In Figure 8.11b,560

000E means 560,000 m east of the central meridian for the UTM zone.

Remember that each grid box is 1,000 m square. The marker immediately west of 560000E must be

559,000 m east of the central meridian but only every tenth value is written fully. Abbreviations like 559

are for the intervening grid markers. Northings are similar. The marker

42

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000

N means 4,281,000 mnorth of the Equator (4,281 km). And 4282, the marker, 1,000m north, is 4,282,000 m.

b) UTM gridlines used to locate a road intersection

Figure 8.7 shows how the UTM grid appears on USGS topographic ma

the border of every map define a grid containing boxes 1,000 m (1 km) on a side

ps, the grid lines are drawn (Figure 8.7b).

Figure 8.7 Using UTM grid marks to locate features

a)UTM tick marks (blue) along the sides and latitude/longitude (red) at the corner of a topogr

ps. Blue tick marks along

(Figure 8.7a). In newer

aphic map.

3915N

12030W 716 717

4348

UTM grid lines


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T

Figure 8.8 Using a UTM grid tool

o determine the location of the red star in Figure 8.8, measure proportionally the distance east

of 559 and north of 4282: 559450 m E, 4282182 m N. Rather than doing a lot of arithmetic, you can

make a UTM tool for each major map scale (Figure 8.8) and determine location easily to within 10 m.

EXERCISE 8.4: LOCATING POINTS WITH THE UTM GRID

a) Use the appropriate UTM grid tool from your tool kit to determine the location of the red star in

Figure 8.7b and 8.8. _______________________

b)Give the UTM coordinates of the top of Grey Mountain in Figure 8.10c: ____________________

8.3.2 Map Element 2: Direction

Geologists use theazimuth methodto indicate direction, based on the dial of a compass (Figure

8.9). The red-tipped compass needle in Figure 8.9a is pointing northeast -- somewhere between north

and east but how much closer to north than to east? On an azimuth compass (Figure 8.9b) 0 or 360 is

due north, 090=east, 180 = south, 270 = west. The direction 045 is exactly halfway between north

and east. The direction of the needle in Figure 8.9 can be read as 032.

UTM

grid tool

Tool chosen

for correct

map scale

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place its cen

EXERCISE 8.5: GIVING DIRECTIONS

Using the circular (azimuth) protractor in your tool kit, give the directions in Fig 8.9 from:

a)from A to C _____ C to A _____ B to C _____ C to B _____ .

b) from Point A to Point B in Figure 8.4 _________

8.3.3 Map Element 3: Distance and scale

A map of the entire world or your campus can fit onto an 8 x 11 sheet of paper if we scale

the Earth down so it fits. A map scale indicates how much an area has been scaled down so we can

relate inches or centimeters on the map to real distances on the ground. Figure 8.10 shows three maps of

the same general area, made at different scales. The more we scale down an area, the more detail we

lose; the closer the map is to the real size, the more detail we can see.

The three map segments in Figure 8.10 are the same size on the page but the area each covers is

different because of their different scales. The map in Figure 8.10a has been scaled down more than

Use the circular protractor in your tool kit to determine the direction between any two points.

Draw a line between the points, align the protractors registration lines in a N-S or E-W position and

ter point on one of the two points. (Figure 8.9c). The direction from that point to the other

is where the line intersects the azimuth scale; in this case, the direction from A to B is 235.

Figure 8.9 Using the azimuth method to describe direction between two points

N

E

S

W

0

090270

180

045315

225 135

0

090270

180

045315

225 135

A

B C

a)A simple compassb)Compass with azimuth markings

c) Using a circular protractor to determine thedirection between points


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four tim

es as much as Figure 8.10c (1:100,000 vs 1:24,000) and therefore covers much more of the land

surface on the same sized piece of paper.

Figure 8.10 An area in eastern Maine mapped at three common map scales

a)Scale = 1:100,000 b)Scale = 1:62,500 Note the latitude and longitude valuesat the southwest corner of this map segment, and the UTMgrid ticks along the bottom and western edge

c. Scale = 1:24,000


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8.3.3a Different ways to describe map scale

Map scale may be expressed verbally, proportionally, or graphically. A verbal scale, used on

many roadmaps, uses words like one inch equals approximately 6.7 miles to describe the scaling of

map and real distances. A driver can estimate distances between cities but not very accurately.

The most accurate way to describe scale is aproportional scale, one that tells exactly how much

the ground has been scaled down to make the map. Proportional scales like 1:100,000 (one to one

hundred thousand), mean that distances on the map have been scaled down to one 100,000 th of the

ground distance. The larger the number, the more the ground has been scaled down. A proportional scale

refers to all units. Thus, one centimeter on the map equals 100,000 centimeters (one kilometer) on the

ground, one inch on the map equals 100,000 inches (1.58 miles) on the ground, etc.

The metric system is ideally suited for scales like 1:100,000,000 or 1:100,000 because it is based

on multiples of 10: 1 cm=10 millimeters, 1 m=100 centimeters, and 1 km=1,000 meters. In the United

States, we measure ground distance in miles but map distance in inches. Unfortunately, relationships

among inches feet, and miles are not as simple as in the metric system.There are 63,360 inches in a mile

(12 inches/foot x 5,280 feet/mile), so the proportional scale 1:63,360 means that one inch on a map

represents exactly one mile on the ground. Old topographic maps use a scale of 1:62,500. For most

purposes, we can interpret this scale to be approximately 1 = 1 mile, even though an inch on such a

map would be about 70 feet short of a mile. Other common map scales are 1:24,000 (1 = __________

feet), 1:100,000 (see above), 1:250,000 (1= _______ miles), and 1:1,000,000 (1 = ________miles).

Map scale can also be shown graphically, using a bar scale (Figure 8.11) to express the same

relation as the verbal scale. Depending on how carefully you measure, a bar scale can be more accurate

than a verbal scale, but not as accurate as a proportional scale.

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Figure 8.11 Scale bars used with three common proportional scales

8.4 VERTICAL EXAGGERATIONA MATTER OF PERSPECTIVE

DEMs show the land surface in three dimensions and must therefore use an appropriate vertical

scale in order to show how much taller one feature is than another. It would seem logical to use the same

scale for vertical and horizontal distances, but we dont usually do so because mountains wouldnt look

much like mountains and hills would barely be visible. Landforms are typically much wider than they

are high, standing only a few hundred or thousand feet above or below their surroundings. At a scale of

1:62,500, one inch represents about a mile. If we used the same scale to make a three-dimensional

model, a hilltop 400 feet above its surroundings would be less than one tenth of an inch high. A

mountain rising a mile above its base would be only one inch high.

We therefore exaggerate the vertical scale compared to the horizontal to show features from a

human perspective. For a three-dimensional model of a 1:62,500 map, a vertical scale of 1:10,000 would

exaggerate apparent elevations by a little more than 6 times (62,500/10,000= 6.25). A mountain rising 1

mile above its surroundings would stand up 6.25 in the model; a 400 hill would be about half an inch

tall more realistic than the 0.1 if the 1:62,500 horizontal scale had been used vertically.


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The degree to which the vertical scale has been exaggerated is, logically enough, called the

Figure 8.12 DEMs of part of the area in Figure 8.1 showing the effects of vertical exaggeration (VE)(V.E.)

vertical exaggeration. Figure 8.12 shows the effects of vertical exaggeration on a DEM. With no

vertical exaggeration, the prominent hill in the center of Figure 8.12a is barely noticeable. One of the

authors of this manual has climbed that hill several times and guarantees that climbing it is far more

difficult than Figure 8.12a would suggest. On the other hand, Figure 8.12d exaggerates too much; the

hill did not seem that steep, even with a pack loaded with rocks.

Is there such a thing as too much vertical exaggeration? The basic rule of thumb: Dont make a

mountain out of a molehill. Vertical exaggerations of 2-5 X generally preserve the basic proportions of

landforms while presenting features clearly. We will return to the concept of vertical exaggeration when

we discuss drawing topographic profiles from topographic maps.

No Vertical Exaggeration V.E. = 5X VE = 10X VE = 20X

08 studying the earths surface

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