OBJECTIVE: To understand how the use of contours is employed in the study of topographic mapping.
MAIN CONCEPT: Topographic maps are incredibly important to the geologist. Further, knowing how to read a topographic map is imperative for anyone navigating on foot and off paved roads. Topography, or the shape of the land, is represented on topographic maps through the use of contours. A contour line is an imaginary line that connects lines of equal or constant elevation and thus express topography. This exercise is designed to understand contour lines, contouring, profile mapping and their relationship with actual landforms.
Contour lines display a region’s relief. Relief refers to the difference between the highest and lowest points of a region. It can be calculated by subtracting an area’s lowest from its highest elevation.
Important rules regarding contour lines
- Every point on a contour line is of the exact same elevation. This means that if you were walking along a contour line, you would not be going up or down hill, but staying at the same elevation.
- A contour lines are closed and therefore rejoins itself to form a loop. Often when viewing a topographic map, you cannot see the competed loops as they continue beyond the boundaries into adjacent maps.
- Contour lines never split.
- Contour lines do not cross each other because that would suggest there are two different elevations at the same place.
- Steepness of an area is given by the spacing of contour lines:
a. Closely spaced contour lines represent a steep slope
b. Widely spaced lines represent a gentle slope
- Hills are represented by concentric circles (closed circles)
- Depressions are represented by closed contours with hachures on the downhill side of the contour lines.
- Contour lines form a “V” pattern, always pointing upstream, as they cross streams or stream beds. Gentle Slope Contours always point
Steep Slope upstream in a “V” pattern
The relief of a region refers to the difference between the highest and lowest elevations. It can be qualitatively described as high, medium or low or it can be calculated and expressed quantitatively by subtracting the lowest elevation from the highest elevation.
A related term is gradient which expresses the steepness of a slope. This is determined by dividing the relief between two points by the distance between them. This is usually expressed in feet per mile or meters per kilometer.
All topographic maps have a contour interval. The contour interval (CI) of a topographic map tells the difference in elevation represented by adjacent contour lines. This is always consistent within a map. In regions of low relief the contour interval maybe only 5 or 10 feet. In mountainous areas, however, they may be as high as 100 or 200 feet.
On most topographic maps contour lines appear brown. Also, every fifth contour line is bolded. These are referred to as index contours because they typically labeled and serve as a starting point when reading elevations. See figure below.
Contour Interval – 20 ft
Note – Index Contours
NAME ___________________ DATE _
Contours and Topography
Exercise 1: To study a simple relationship between contouring and land forms. Following are six topographic landforms (1 – 6) depicted by contour lines and six landform (A – F) profiles. Match the topographic landform on the left with the corresponding profile on the right using the appropriate letter.
Exercise 2: To construct a topographic map using spot elevations and following the rules of contouring
Relief and Gradient
Constructing a Topographic Map
- Elevations of certain points above sea level are first plotted on a blank map. These are “spot elevations”. The greater the number of spot elevations the more accurate the topographic map. In the field some spot elevations are set with a permanent round marker, usually made of bronze, referred to as a benchmark. On a topographic map these locations are indicated with the symbol (BM).
- An appropriate contour interval (CI) must be selected that will accurately reflect the topography. Contour intervals are typically divisible divided by 5 or 10.
- Contour lines are then drawn by observing the rules of contour lines and interpolation.
- One rule of thumb is to assume a steady slope between two points of unequal elevation. For example if there is a 17 and 26 foot elevation adjacent to each other (and nothing between them), we have to assume a steady uphill walk from 17 to the 26. Obviously at some point we will cross a place where the elevation is 20. This, we assume will be slightly closer to the 17 than the 26.
- Another good rule to keep in mind is that when you are drawing a contour line between two points, one point should be of higher value than the line your are drawing and the other should be a lower value. See figure below.
Note that the value of contour lines can be repeated (e.g. There can be more than one 20 foot contour lines.)
Procedure: Using a pencil and the rules of contouring draw all contours using the spot elevations given in each of the two following figures. LABEL all contours with the appropriate elevations.
The following figure shows several spot elevations. Draw the 50-foot contour.
. 38 . 41 .43 . 44
.32 . 55 . 54
. 52 .63 . 61 . 48
. 39 . 50 . 47
Draw and label the contour lines in the figure below.
Remember to observe the rule of “Vs” when contours cross streams.
Exercise 3: To draw a topographic profile from a topographic map.
Materials: blank piece of paper to use as a straight-edge; pencil
Procedure: According to instructions given below, draw a topographic profile along line A – B from the map and grid on the following page.
Drawing a Topographic Profile
- locate line A – B on map. Place a blank piece of paper along the line and mark the starting and ending points of the line (label them A and B).
- Starting at one end move along the edge of the paper, making a mark on the paper every time a contour line touches the edge of the paper. Make sure you label each mark with the right elevation so that you can transfer that point to the correct elevation on your profile. See figure below. Also, you may also want to mark where rivers or streams occur.
- For every mark you made on the paper place a vertical point on the graph directly above thatpoint corresponding to the correct elevation. See figure below.
- Connect the dots on the graph paper. You have a completed topographic profile.
Topographic Profile A – B NOTE each horizontal row = 5 feet
Topographic profile C – D
TOPOGRAPHIC MAPS – LOCATION AND DISTANCE
Maps are simply representations of a land surface that has been shrunk to scale. They have been around as long as humans have had a reason to navigate and a means to create them. Today, there are many types of maps that suit numerous purposes and represent land at many different scales. In this first of three exercises dealing with maps, you will learn how the Earth’s grid system works, how maps accurately represent land surfaces, and how to use a map to judge distance and pinpoint locations.
USGS topographic map (Paxton Springs, NM) ∙ globe ∙ calculator ∙ ruler
In order to specify an exact location on Earth, a grid system is typically used. However, as you undoubtedly know, our planet is not flat, but (roughly) spherical. In order to accurately express distance and location on a spherically-shaped object, the grid system must be able to account for the curvature of the Earth
There are several commonly used grid or coordinate systems in use today (Public Land Survey [PLS], Universal Transverse Mercator [UTM], Global Positioning System [GPS]). One of the most common (and oldest) grid systems in use, however, is the latitude and longitude system, which will be used in this lab for pinpointing locations on the globe as well as on a topographic map of just a few square miles in area.
Latitude and Longitude
If you’ve ever looked closely at a globe, you have certainly noticed that there are a number of lines that run east-west and a number of lines that run north-south. These represent major lines of latitude and longitude, respectively. The latitude and longitude system uses angular measurements (degrees) to describe a position on the surface of the Earth.
Lines of latitude, also known as parallels, run east-west across the globe (and maps), and as you might imagine, are parallel to each other – they do not cross, meet, or change in distance apart from one another. The best known line of latitude is the Equator or 0°. The Equator makes a right (90°) angle with Earth’s axis of rotation (the imaginary line that runs through the poles about which the earth rotates). All lines of latitude represent the angle formed by the point on the surface, the center point of the Earth, and the closest point on the equator (Figs. 1, 2). Therefore, the numerical value for degrees of latitude increases moving away from the Equator in either direction, to a maximum of 90° at the poles.
Lines of latitude measure distances north or south of the Equator. Because the Equator splits the Earth into two hemispheres (half spheres), we must always designate a direction – North or South (N or S), when indicating latitude. Everything north of the Equator is the northern hemisphere, while everything south is the southern hemisphere.
Lines of longitude are fundamentally different than lines of latitude. They are meridians (or half circles), which run north-south from pole to pole, across the globe (and maps). They are NOT parallel, but converge to a point at both poles. The best known line of longitude is the Prime Meridian, or 0°, which runs north-south through the Royal Observatory in Greenwich, England. Note, however, that since it is a meridian, it does not continue on the opposite side of the Earth. The opposing line of longitude (180°) closely corresponds to the International Dateline (which runs mostly through the Pacific Ocean and delineates one calendar day from the next).
All lines of longitude represent the angle formed by the point on the surface, the center point of the Earth, and the closest point on the Prime Meridian (See Figs 1, 2). Therefore, the numerical value for degrees of longitude increases moving away from the Prime Meridian in either direction, to a maximum of 180°.
Lines of longitude measure distances east or west of the Prime Meridian. Because the Prime Meridian splits the Earth into two hemispheres (half spheres), we must always designate a direction – East or West (E or W), when indicating longitude. Everything east of the Prime Meridian is the eastern hemisphere, while everything west is the western hemisphere.
Figure 1 Comparison of latitude (parallels) and longitude (meridians)
Figure 2: Earth’s grid created by parallels and meridians
The length of one degree of latitude on Earth (as measured along a meridian) is about 111 kilometers (or 69 miles). The width of a degree of longitude, however, varies depending on where the measurement is taking place. A degree of longitude at the Equator is 111 km, but 0 km at the North and South Poles. The distance between adjacent lines of longitude gets shorter and shorter with proximity to the poles. Note that this is because the lines of longitude converge at the poles, as shown in Figure 8 above.
USGS Topographic Quadrangles
A topographic map is a two-dimensional representation of a three-dimensional landscape. This lab is primarily concerned with the east-west and north-south dimensions. The third (“up-down”) dimension – elevation, is the topic of the next exercise (Contours).
The United States Geological Survey (USGS) has been publishing topographic maps, called quadrangles, since the
1800s with a standard format that makes them very useful and consistent. A quadrangle is a rectangle-shaped section of
Earth’s surface that is bound by lines of latitude to the north and south and lines of longitude to the east and west. Most of these maps, however, represent a very small area of Earth’s surface, relative to the globe. Therefore, degrees are divided up into much smaller increments so that the grid system can be used for such small areas (say, only a few square miles or less). Each degree is divided into 60 equal parts, called minutes, signified by a single tick mark (’), and each minute is divided up into 60 equal parts, called seconds signified by a two tick marks (”).
1 sphere or circle = 360˚
1˚ (degree) = 60’ (minutes)
1’ (minute) = 60” (seconds)
There are many “parts” to a map, all with a specific purpose for the user. The following section describes some the most important parts to a USGS quadrangle, including those which are relevant to this lab exercise.
The scale of a map conveys to the user the amount land area represented and in how much detail. More specifically, it indicates the relationship between distance on the map itself and the distance on the actual land surface it represents. This relationship is usually expressed in two main ways:
A Representative Fraction (RF) or fractional scale gives a ratio of the distance on a map to the actual distance on the ground. For example, the commonly used RF scale of 1:24,000 (or 1/24,000) tells us that 1 of any unit on the map is equal to 24,000 of those same units (inches, centimeters, micrometers, etc.) on the ground. Therefore, 1 inch on a 1:24,000 map is equal to 24,000 inches, or 2,000 feet, or about .38 miles on the ground. A map with a RF scale of 1:24,000 and is considered a large-scale map. Large scale maps depict a small area and thus are able to show great detail and hence are generally preferred by geologists. On the other hand small-scale maps depict a large area but shows little detail (e.g., 1:250,000).
A Graphic Scale consists of a bar that is subdivided into segments that correspond to miles, kilometers, or feet. This is a very easy and convenient scale to use when measuring distance between two points on a map because it can be transferred to a piece of paper and moved around to different locations on the map.
The location of the scale on USGS quadrangles is at the bottom center.
North on a map almost exclusively refers to true geographic north, or true north, which is the North Pole (or 90˚ N).
However, the needle of a compass is not attracted to true north, but is instead attracted to magnetic north.
Unfortunately, magnetic north wanders significantly over time due to the movement of the iron-rich outer core deep in the Earth. The location of magnetic north it is closely monitored so that maps can be revised and updated accordingly. When navigating with a compass, it is important to adjust its settings so that it will point to true north (instead of magnetic north) so that it will be compatible with regional maps.
The USGS has an extensive list of symbols found in their maps. A few are shown on the page in the D2L content section for this lab. Colored symbols are posted in the lab as well as found on the USGS website (usgs.gov)
Other important elements of a map include:
- Title of the map is shown at the top and is typically named after a town or important location shown in the map
- Size the map (in angular measurements) is shown at the top (typically 7 ½’, 15’ or 30’)
- Names of adjacent quadrangles are shown either along the margins of the map or demonstrated with the square pattern at the bottom.
- Year of publication is shown in the lower right.
- Location within the state is shown by a black rectangle in the outline of the state at the bottom.
Figure 3: From U.S. Geological Survey
Topographic Maps – Location and Distance
Reading Latitude and Longitude
Use the globe to answer the following questions:
- How many degrees separate each line of latitude and longitude shown on the globe?
a. Latitude _ b. Longitude __
- Name two countries that each of the following lines run through:
a. Equator ___________________
b. 30˚ N ___________________
c. 23 ½ ˚S ___________________
d. 60˚ S ___________________!
e. Prime Meridian ___________________
f. 100˚ W ___________________
- Give the latitude and longitude of the following locations (in degrees only):
a. Barrow, Alaska _________________________
b. Bangkok, Thailand _________________________
c. Sydney, Australia _________________________
d. Santa Maria, Brazil _________________________
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