TOPOGRAPHIC MAPS

LEARNING OBJECTIVES

• Identify topographic map features
• Read map scales and determine distances
• Determine gradients and slope
• Recognize topographic patterns and elevation changes
• Be able to draw contour intervals
• Determine latitude and longitude coordinates as well as Universal Transverse Mercator (UTM) measurements

MATERIALS

• Pencil
• Calculator
• Ruler
• String
• A large round Earth globe
• Topographic quadrangle maps – Valley Forge, PA 7.5 minute Quadrangle, 1973, 1992 or 2010

INTRODUCTION

There are many types of maps: thematic maps, dynamic maps, geologic maps, topographic maps, weather maps and many more.  The choice depends on what information you seek.  Thematic maps are commonly used to visualize and identify a theme such as world life expectancies or population distribution (figure 1). Dynamic maps are very familiar to most people who use their mobile phone for directions or to watch an animation or video; they are changeable and interactive (figure 2). Map types can be combined or overlain to show a variety of land uses or changes over time (figure 3).  Geologic maps are very colorful and add rock types such as granite and limestone as well as rock ages to a map surface (figure 4).  

Figure 1. A map of world life expectancies is an example of a thematic map. Image CC BY_SA 3.0 https://2012books.lardbucket.org/books/geographic-information-system-basics/s06-01-maps-and-map-types.html

Figure 2.  If you use your mobile phone to find directions this is an example of a dynamic map.  Image: https://2012books.lardbucket.org/books/geographic-information-system-basics/s06-01-maps-and-map-types.html

Figure 3. A map overlay process that allows multiple layers of information to be combined. Image: https://2012books.lardbucket.org/books/geographic-information-system-basics/s06-01-maps-and-map-types.html

Figure 4. Geologic Map of Pennsylvania. This type of map combines rock type with rock ages using different colors. Free copies of geologic maps can be downloaded from www.dcnr.pa.gov. Image: https://www.dcnr.pa.gov/Education/GeologyEducation/Pages/default.aspx

This lab will focus on topographic maps which show the shape of the land by including elevation information (figure 5).  A topographic map is an extremely useful type of map by adding a third dimension (vertical elevation) to an otherwise two-dimensional flat map.  This third dimension is represented by contour lines which are imaginary lines drawn to represent elevation above sea level or mean sea level.  From these contour lines we can gather details about the shape of the Earth’s surface, such as steep cliffs, valleys and the direction that streams flow.  Topographic maps are used to aid in the visualization of the shape of the land and can be used to identify areas prone to geologic hazards such as landslides and flooding.  These maps are used by hikers, campers, engineers, land developers and anyone who needs to know about the topography of a region.  The addition of contour lines representing elevation are what make a topographic map different from a more familiar planimetric map, such as a highway map. Topographic maps are produced by the United States Geological Survey (USGS).  Free topographic maps are available by the U.S. Geological Survey online at www.usgs.gov.

Let’s Look at the Valley Forge, PA Map

Figure 5. A small segment of a topographic map, from Stowe Vermont, showing brown contour lines (intervals) to represent elevation above mean sea level as well as black building locations and blue stream paths. Image CC Public Domain USGS. https://store.usgs.gov/map-locator  https://www.amazon.com/YellowMaps-Stowe-topo-map-Historical/dp/B07L2TM2JC

Map Symbols

All USGS maps are oriented North at the top of the map.   The right and left sides are East and West respectively.  Looking at the map provided by your instructor, notice the many colors, scales and symbols.  Topographic maps contain cultural features as well as land use, political boundaries, waterways and road type symbols.  Use the legend on the map and the map symbol chart at the end of this lab to find and discuss some of the varied and interesting features.

What is the title of this map?

Describe the features represented by the many different colors.

What is the difference between the purple and black geometric shapes?

What is the difference between the pink shaded areas and the white and green shaded areas?

Where is this map located?  Notice the symbol of the state (bottom) and the black square representing the map location within that state.

Why do you think this location information is important?

If you are interested in continuing to follow a feature off the edge of this map what map would you need to acquire?

Notice the Schuylkill River, which maps are needed to follow it up or down stream?

In what year was this map published?

Use the legend at the bottom on the map: What type of road is Darby road?

What government organization creates these maps?

If you have access to more than one year of publication (Valley Forge, 1973, 1992, or 2010), describe some of the differences seen on the maps over time.

Scale of Maps

Maps are a reduced size of the real world.  The scale and bar symbol at the bottom -center of the map gives you important actual distance measurements. There is no standard scale for all topographic maps.  You will need to look at each map for this specific information. When you look at the scale bar, notice where the zero marker is on the scale.  It is not to the extreme left. The subdivision to the left of the zero marker allows you to make a more precise measurement (figure 6).

Figure 6. Three common map scales typically found at the bottom center of a topographic map.  Note that the bar scales zero marker is in the interior of the bar scale. The part of the bar to the left of the zero further subdivided the unit for more accurate distance determination. Miles, Kilometers and Feet are typically shown. Also note that the scale selected for a topographic map can vary.  The figure shows examples that range from 1:20,000 to 1:62,500. Image: Map author USGS. License Public Domain.

Let’s Practice Measurements

List the Ratio scale on the Valley Forge, PA map. Explain its meaning.

How many miles are in 1 centimeter on this map?

How many feet are in 3 centimeters?

How many centimeters are in 1.5 kilometers?

Measure the distance from Valley Forge Junior High school to Conestoga High School, in kilometers. Show your work.

The ratio-fractional scale is 1:24,000.  Notice that this scale has no units attached to it.  This means that one of any unit of measurement is equal to 24,000 of that same unit.  For example, 1 centimeter is equal to 24,000 cm in the real world.  Or 1 foot is equal to 24,000 feet.  

The Pennsylvania Turnpike runs east to west across the map.  Using the ratio scale, how many miles are represented on this map. Note, there are 12 inches in a foot and 5,280 feet per mile.

One advantage to using a fractional or ratio scale is that any unit of measure can be used, and conversions are easy to make when needed. For example, for a 1:12,000 map 1 inch on the map is equal to 12,000 inches on the Earth’s surface, and since there are 12 inches in 1 foot, we can also say that 1 inch on the map is equal to 1000 feet on the Earth’s surface (convert the 12,000 inches to feet by multiplying by the conversion factor 1ft/12in). Simply verbalizing this scale by saying “on this map, 1 inch represents 1000 feet” is another type of map scale, which for obvious reasons is called a verbal scale. Writing the phrase “1 inch equals 1000 feet” is a way of adding a verbal scale to your map.

What is the verbal scale of this map?

Table 1. Some useful conversion factors

Contour Lines

Contour lines allow for a 2-dimensional flat map to represent the 3-dimensional world with elevation.  This added dimension represents the elevation of the land above sea level or mean sea level. Each individual contour line is drawn in brown color and represents a line of equal elevation.  If you trace along a single contour line you are staying at the same elevation, therefore you are not walking up or down a hill.  If you move to a different line you are moving up or down elevation.  These lines of equal elevation are called Contour Lines. Contour intervals or the elevation change between two lines is not a standard unit.  Contour Interval is printed at the bottom of your map.  Since topographic maps are created by the USGS the elevation change is in feet not metric units.  Contour lines are not labeled with each line, but you will notice that each fifth line is a darker brown and is labeled with an elevation.  These darker brown lines are called Index Contours and serve as a starting point when reading elevations.  You can quickly determine which way is up or down hill by looking at a few numbered index contours. If a point lies on a contour line you can simply read its elevation.  If it lies between two lines you can calculate up or down and make an estimate (figure 7).  Elevations at specific points or features are marked with an “x” and the letters “BM” and are known as Benchmarks.  These surveyed points are exact elevation and are commonly used to mark the elevations of mountains, hilltops, and airport runways.  

Figure 7.  A portion of a USGS topographic map of Bat Cave quadrangle, Arizona. Notice the brown contour interval lines and the darker labeled Index Contour lines with elevation above sea level in feet. Image original from USGS Public Domain, portion removed by Harris openpress.usask.ca/geolmanual

Rules for Contour Lines

Contour lines are used to represent elevation and details about the shape of the surface.  The spacing and distribution of the lines can tell you about the slope of an area.  The closer together the lines, the steeper the slope or change in elevation over a distance (figure 8).  Contour lines can never cross each other, as this would represent being at two separate elevations at the same physical point.  Ridges and valleys can be represented by repeating upward and downward contour lines.  Circular contour lines represent a peak, while a depression can also be represented by circular contour lines with specific hachure marks on the contour lines pointing inwards to the depression (figure 9).    

Figure 8. Closely spaced contour lines represent a steep slope versus farther spaced lines representing a gentle slope. This tells you that you are changing elevation very quickly up or down over a short distance.  Image courtesy of REI.com at .rei.com/learn/expert-advice/topo-maps-how-to-use.html

Figure 9.  Contour line representation of ridges and valleys showing repeating contour lines (top). The hachure marks represent depression.  Image: oer.galileo.org.usg.edu Karen Tefend (2017) CC BY-SA 3.0 

Relief and Gradient

The range of elevation represented by the contour lines on a topographic map is called the relief and is easily determined by finding the highest (usually at the top of a mountain) Recall that contour lines create circles at the top of elevation points, and lowest points (usually near the base of a stream) on the map.  The overall relief of this map is the difference between the highest and lowest elevations represented on the map.  Gradient is the measure of the steepness of a slope. You can determine the slope of a location by determining the elevation difference between two points (local relief) or change in elevation and dividing by the distance between them.  This slope is also known as rise over run.  Recall that elevation change is measured in feet.  So, you only need to choose a unit for the distance between two points. In figure 10 below, you see that a circle of contour lines and the x represent a high point on a mountain at 859 feet above mean sea level.  The red line is the slope and distance that you need to measure using the scale bars.  If the contour interval is 20 feet, the elevation of the contour line at equation creek is 680 feet. The change in elevation is 859-680 = 179 feet. To determine the slope or gradient divide 179 feet by the distance shown on the red line (approximately 2.5 miles). This yields a slope/gradient of 71.6 feet per mile.

Figure 10. An example of a calculation of slope/gradient from the benchmark peak at 859 feet and the contour line of 680 feet at Equation creek. Divide the difference in elevation by the distance to determine slope/gradient of the hill. Image: https://serc.carleton.edu/mathyouneed/slope/slopes.html

Let’s Practice Elevation: Valley Forge, PA

Find a few places on the topographic map where the lines are close together.

Find a few places on the topographic map where the lines are far apart.

Determine the elevation of a high and low point.

Measure the change in elevation and then the distance. Determine the slope/gradient.

On your Valley Forge, PA map find the elevation of the New Eagle school building.  

If you are walking from the New Eagle school building to the Valley Forge Country club are you traveling uphill or downhill? How do you know?

What is the gradient of this trip?  Calculate the distance and elevation change.

Looking at Mount Misery, measure the slope of the hill between the lookout tower at the top and the stream below.

What does the Benchmark indicate about the elevation of the peak of the Lookout Tower?

What is the highest elevation represented on this map?

What is the lowest point?

Locate a location with circular contour lines, determine its elevation.

Locate a location with hachure marks.

Let’s Practice Drawing Topographic Contour Lines

Elevation data are collected and plotted at discrete locations.  The data do not necessarily form an organized grid. Start by determining the assumed elevation between any two points.  For example, between the 203 and 255 data points all of the elevations higher than 203 and lower than 255 exist.  If you were to draw a contour line with the elevation of 225, it would be placed between these two points.  An elevation of 225 would not exist between the points 255 and 235.  So do not draw the elevation line representing 225 between them.  Using the block diagram below draw contour intervals at 25-foot intervals.  The 300 feet elevation lines are already drawn. Lines will disappear off the grid edge (figure 11).

Figure 11. Practice creating elevation contour lines every 10 feet. Image: curtesy of instruct.uwo.ca/geog/2240/drawlab4

Direction of water flow and contour intervals

Rivers and streams are represented in blue on Topographic maps.  There are generally two methods for determining the direction in which a waterway is flowing.  Brown contour lines can be seen crossing over streams and creek waterways.  The direction that the lines cross the waterways can be used to help determine the direction that the water is flowing.  The rule of “V’s” states that there is a deflection of the contour lines on a map as they cross the valley produced by a stream. Contour lines form a “v” shape as they cross the water, and that the pointed end of this “v” is pointing in the upstream direction and streams always flow downstream (figure 12).  Note the contour lines crossing a stream may appear as a ‘U’ shape across wider rivers. A change in elevation is the second method for confirming the direction of water flow.  Find the elevation of the nearest brown contour line closest to the water at two separate locations.  The water will flow toward the lower elevation.

Figure 12. Rule of V’s showing the direction of a blue colored stream flowing to the right of the image. Also notice the peak at b and a depression at a. Image CC BY SA Commons WVC.edu 3.0 international R. Dawes. https://commons.wvc.edu/rdawes/g101ocl/labs/TopoMapsLab.html

On your map determine the direction using the rule of V’s and the change in elevation method of the Schuylkill river.

In which direction does the Little Valley Creek flow?

Map Orientation – Global – Latitude and Longitude

Topographic maps created in the United States are oriented with north at the top of the map.  Therefore, if you locate a position on the map, and move toward the top of the map you are moving in a northerly direction.  The Earth has two sets of imaginary lines: Latitude and Longitude.  Latitude lines circle the globe parallel to the equator which is Latitude 0 degrees.  Latitude lines are 0 - 90 0 north of the equator in the northern hemisphere, and 0 - 90 0 south of the equator in the southern hemisphere.  Longitude lines connect at the North and South pole and are therefore not parallel to each other.  They converge at the poles.  Longitude 0 degrees is called the Prime Meridian and passes through Greenwich, England (figure 13).  You are either West, as in North America of England or you are east, as in Asia of the Prime Meridian.  Longitude lines run 0 - 180 degrees east or west of the Prime Meridian and meet in the Pacific Ocean at the International Date line.  This is where the worlds day begins. The top and bottom of the maps are parallel to Latitude lines on a globe.   Maps are oriented with their sides parallel to lines of Longitude (figure 14).

Figure 13. Graphical representation of Latitude and Longitude lines. Image by unknown author - web.archive.org/web. Public Domain. Commons. Wikimedia

Figure 14. Globe showing Latitude and Longitude lines on a circular globe. The Equator represents 0O latitude and represents the division between the Northern and Southern Hemispheres.  The Prime Meridian is the 00 Longitude line separating East and West. Image CC BY-SA 3.0 K. Tefend. Oer.galileo.usg.edu/geo-textbooks/1

Topographic Quadrangles

The grid system of Latitude and Longitude allows a position on the Earth to be precisely located.  A single degree latitude or longitude represents a large distance.  Therefore, degrees have been further subdivided into minutes (‘) (there are 60 minutes in a degree, just like hours of time).  Minutes are further subdivided into seconds (“).  Therefore, coordinates are stated in degrees, minutes’ and seconds”.  There are 60’ minutes in 1 degree and 60” seconds in each minute (figure 15).  

Note that each map has two latitude lines, one defining the top and bottom of the map.  They are both North latitude if the map is located in the northern hemisphere.  There are also two longitude lines, one on the right and one on the left edge of the map.  If your map is in the United States, they are both west of England and therefore both have a west longitude designation.  Most maps represent either 15 minutes or 7.5 minutes (7’ 30”) of distance.  This is much less than 1 complete degree.  Note latitude is listed first.  Coordinates are printed in full at all four corners of a USGS topographic map.

Figure 15. A small portion of a topographic map showing 400 07’30” North latitude and 750 30’ West longitude on the Valley Forge, PA 7.5 minute quadrangle. Image USGS public Domain. Map section image by author.

Magnetic Declination

Maps are drawn with north at the top.  North on a topographic map is known as True North, usually represented by a star symbol.  At most places on Earth a compass needle does not point toward the north pole but toward the magnetic north pole – ‘MN’ (figure 16).  The Magnetic North Pole is currently in the Canadian Arctic.  The Earth’s magnetic pole migrates slowly over geologic time.  The angular distance between true north and magnetic north is the magnetic declination.  Notice the symbol at the base of your topographic map.  This represents the difference between true north and magnetic north at the time your map was printed.  Geographic North (GN) represents the map's grid north; this is used for the Universal Transverse Mercator (UTM) grid system.  Note that it may have changed if your map is old.  If you are navigating with a compass you must adjust the declination of the compass, as a compass is aligned to magnetic not geographic - true north.  

Figure 16.  (Left) Magnetic North (Nm) is different from geographic North (Ng).  Your compass points toward Magnetic North, while most navigation is toward geographic North. The difference between the two is called declination and is location specific.  (Right) The declination symbol from the Valley Forge, PA 7.5 minute Topographic quadrangle located at the bottom of your map. Image (left) Wiki Commons license https://en.wikipedia.org/wiki/Magnetic_declination and USGS Public Domain, map section image by author.

Mercator Projection Maps

One major disadvantage of latitude and longitude is that it is difficult to relate degrees, minutes and seconds to actual distances on the ground.  For example, 10 degrees of longitude near the poles is a much smaller distance than 10 degrees of longitude near the equator.  The traditional Mercator projection of a map uses latitude and longitude symbols and since the longitude lines converge at the poles the upper portions of the northern and southern hemispheres are distorted in size relative to land closer to the equator (figure 17). The traditional Mercator also distorts the visual area as it gets closer to the Northern and Southern Poles, expanding their appearance. Many projections eliminate the land north and south or 70 degrees to minimize this effect. In order to represent a circular globe on a flat map cut outs along oceans are used (figure 18).

Figure 17. Mercator projection distorts the areas of land represented.  It expands land as it approached the Northern and Southern Poles.  https://www.britannica.com/science/Mercator-projection#/media/1/375638/231099

Figure 18. Mercator projection map with oceanic cut-outs to reduce distortion near the poles. Image By Strebe CC BY-SA 3.0, commons.wikimedia. https://www.iflscience.com/environment/how-maps-can-lead-you-wrong-idea/. Strebe, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons,

Universal Transverse Mercator System - UTM

The Universal Transverse Mercator System (UTM) was set up in order to produce a handy grid of 1000 meter squares on most types of maps.  This makes it easy to determine accurate grid coordinates from paper maps and to determine distances between points on the ground.  It is important to note that this grid system is based on the international system of measurements which recognizes metric units. Most global positioning systems (GPS) units allow for UTM coordinates as well as latitude and longitude.  The UTM system divides the 360 degrees of longitude into 60 north-south zones each 6 degrees wide.  The zones are numbered from west to east beginning at the International date line in the Pacific Ocean. The symbols on the right and left edges of the North American maps represent north grid lines as they are north of the equator.  They are similar to latitude lines.  The symbols that appear on the top and bottom edge in North American maps are marked East (E), as they are east of the Prime Meridian in England. For simplicity in map making the full symbol is reduced to the kilometer and meter designation. Each grid represents 1000 meters distance (figure 19).

Figure 19. Lower right corner of the Valley Forge, PA 7.5-minute quadrangle showing both Latitude and Longitude along with North UTM measurement. Image USGS public Domain. Image section from author.

Let’s Practice Latitude and Longitude and UTM on the Valley Forge, PA Map

On the block drawn below that represents the topographic map, label the two Latitude lines and the two Longitude lines.  Add the UTM coordinates.

Both Latitude coordinates:

Both Longitude coordinates:

Subtract the amount of time represented by Latitude and Longitude.  Look at the top right corner of the map to confirm that this is a 7.5 minute Series map. Representing 7.5 minutes on Latitude and 7.5 minutes Longitude.

Note that each 7.5 minutes of latitude and longitude is subdivided into 2.5 minute segments.  Find the notations.

Determine the Latitude and Longitude of the St. David’s Church.

Using the UTM grid, Find the feature located at 4434000mN and 467000mE.

Figure 20. Symbols for topographic maps. Image USGS Public Domain. Usgs.gov. https://pubs.er.usgs.gov/publication/70039164