2.3: Map Scale and Projection - Geosciences

Map scale is defined as: The ratio of distance on a map to distance on the ground Map scale is generally expressed as a ratio, such as 1:100,000. This means that one unit on the map is equal to 100,000 units on the ground, or the map representation of a widget is 1/100,000th of the actual size of the widget.

One way that I find useful to visualize map scale is to think about a map of the world. The length of the equator at different scales is a good way to think about the actual size of a map at that scale. The table below lists the distance on the map (as if you were to lay a ruler along the map and measure the equator) for each map scale. As you can see, a 1:400,000,000 scale map would probably fit across two pages of your textbook, while a 1:10,000,000 scale map would take up a wall of your classroom. At 1:1,000 scale, a map of the world would stretch across the county!

The Length of the Earth's Equator at Different Map Scales
Map Scale
Distance of the Equator on the Map (m)
Large Scale Maps vs. Small Scale Maps

“Large Scale”

Small features are large

  • A map of this room
  • A map of this campus
  • A map of this city

“Small Scale”

Large features are small

  • A map of this country
  • A map of this world

Resolution is defined as: The smallest feature that is represented on the map

  • A city such as Los Altos Hills probably would not be represented on a map of the USA (at a scale 1:10,000,000). The resolution of this map is therefore too coarse to represent our city.
  • A city such as Los Altos Hills probably would be represented on a map of the Bay Area (scale 1:500,000). The resolution of this map is therefore fine enough to represent our city.

Map projection is the way in which we represent the spherical earth on a flat map (see below). You may want to do some searches to find different types of map projections. The most important thing to remember about map projections, however, is that there will always be some distortion. Some map projections preserve relative areas, some projections preserve shape (such as the shape of coast lines), some try to do both and end up doing neither. But the result is that there will always be some error in a projected map.

One example of error in map projection can be seen in maps of the United States. The graphic below shows three different projections of the United States overlain. Note that the further you get from the center of the projection (the hot pink dot in the middle of the figure below) the bigger the distortion gets. Look in particular at Florida, Washington and Maine.

Another example of distortion caused by map projection is the represented area of Australia and Greenland. The landmass of Greenland is 1/3 that of Australia. That is to say, Australia is three times as big as Greenland. The map on the left is a Mollweide Equal Area projection which preserves the relative area of landforms. The projection on the right is a Gall projection (very similar to a Mercator projection), and it preserves the shape of landforms but not the area.

All maps will have an indicator of the scale of the map. A map that doesn’t conform to a specific scale will be indicated by the words “not to scale” (or NTS). This notations is most commonly found on graphic style maps such as the “we are here” or “how to get here” style maps used on invitations. Since GIS relies on a minimum threshold of accuracy and precision, all GIS based maps will have a scale.

There are three ways to show the scale of a map: graphic (or bar), verbal, and representative fraction. Graphic scales, also know as bar scales, as indicated by the name, show the scale graphically.

Bar scale showing graphically the ratio of map units to ground units. The top scale shows the comparable ground measurement in kilometers (km) and the bottom bar scale shows the comparable length in miles.

A verbal scale is text based, with the scale shown as a number and type of unit measurement equal to a specified unit measurement on the ground. The left side of the verbal is the unit of measurement on the map and the right side of the ratio is the unit measurement on the ground. For example the verbal scale, 1″ = 100′ means that one inch measured the the map represented 100 feet on the ground. This type of scale is sometimes confused with Representative Fraction (RF) scales.

RF scales is also a text based scale but no units are shown. The scale is a simple ratio of map to ground measurement with a colon between the two measurements . For example, a RF scale of 1 : 1,200 means that every one unit on the map is equal to 1,200 units on the ground. There is no notation of the actual unit type used on a RF scale. Therefore a RF scale of 1:1,200 is the same scale as a verbal scale of 1″ = 100′.

2.3: Map Scale and Projection - Geosciences

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MOLA Products and IAU Coordinates

The final version (Version L) of the MOLA PEDR data set was generated using the IAU 2000 coordinate system. The final version of the MOLA topographic maps (MEGDRs) was generated from the Version L PEDRs, and therefore also uses the IAU 2000 coordinate system. Older versions of MOLA PEDRs and topographic maps were created in the IAU 1991 system. For more information, see the PEDR Data Set Information under Data Set Overview.

Author information


Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland

Sonia I. Seneviratne, Annette L. Hirsch, Edouard L. Davin, Martin Hirschi & Micah Wilhelm

ARC Centre of Excellence for Climate System Science, and Climate Change Research Centre, University of New South Wales, Sydney, Australia

Steven J. Phipps & Markus G. Donat

Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia

ARC Centre of Excellence for Climate Extremes, and Climate Change Research Centre, University of New South Wales, Sydney, Australia

CSIRO Oceans and Atmosphere, Tasmania, Australia

Pacific Northwest National Laboratory, Richland, WA, USA

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S.I.S. designed the study together with S.J.P. and A.J.P. S.J.P. performed the climate model experiments with inputs from S.I.S. A.L.H. conducted complementary simulations. S.J.P., M.G.D., S.I.S. and M.H. performed the analyses. S.I.S., E.D. and M.W. compiled Table 1. S.I.S and A.J.P. wrote the first version of the manuscript. All authors commented on the manuscript.

Corresponding author

Section Assessment

I Different types of projections are used for different purposes.

I Geologic maps help Earth scientists study patterns in subsurface geologic formations.

I Maps often contain a map legend that allows the user to determine what the symbols on the map signify.

I The map scale allows the user to determine the ratio between distances on a map and actual distances on the surface of Earth.

1. iman4TTHB Explain why distortion occurs at different places on different types of projections.

2. Describe how a conic projection is made. Why is this type of projection best suited for mapping small areas?

3. Determine On a Mercator projection, where does most of the distortion occur? Why?

4. Compare and contrast Mercator and gnomonic projections. What are these projections commonly used for?

5. Predict how a geologic map could help a city planner decide where to build a city park.

6. Determine the gradient of a slope that starts at an elevation of 55 m and ends 20 km away at an elevation of 15 m.

40 Chapter 2 • Mapping Our World

40 Chapter 2 • Mapping Our World


I Compare and contrast different types of remote sensing. I Discuss how satellites and sonar are used to map Earth's surface and its oceans. I Describe the Global Positioning System and how it works.

Review Vocabulary satellite: natural or human-made object that orbits Earth, the Moon, or other celestial body

New Vocabulary remote sensing Landsat satellite TOPEX/Poseidon satellite sonar

Global Positioning System Geographic Information System

Figure 2.12 Notice the differences between the two Landsat photos of New Orleans.

Interpret Which image was taken after Hurricane Katrina in 2005? Explain.

2.3: Map Scale and Projection - Geosciences

Dot Products and Projections

The Dot Product (Inner Product)

There is a natural way of adding vectors and multiplying vectors by scalars. Is there also a way to multiply two vectors and get a useful result? It turns out there are two one type produces a scalar (the dot product) while the other produces a vector (the cross product). We will discuss the dot product here.

The dot product of two vectors a =<a_1,a_2,a_3> and b =<b_1,b_2,b_3> is given by

An equivalent definition of the dot product is

where theta is the angle between the two vectors (see the figure below) and | c | denotes the magnitude of the vector c . This second definition is useful for finding the angle theta between the two vectors.


The dot product of a =<1,3,-2> and b =<-2,4,-1> is

which implies theta=45.6 degrees.

An important use of the dot product is to test whether or not two vectors are orthogonal. Two vectors are orthogonal if the angle between them is 90 degrees. Thus, using (**) we see that the dot product of two orthogonal vectors is zero. Conversely, the only way the dot product can be zero is if the angle between the two vectors is 90 degrees (or trivially if one or both of the vectors is the zero vector). Thus, two non-zero vectors have dot product zero if and only if they are orthogonal.


<1,-1,3> and <3,3,0> are orthogonal since the dot product is 1(3)+(-1)(3)+3(0)=0.

One important use of dot products is in projections. The scalar projection of b onto a is the length of the segment AB shown in the figure below. The vector projection of b onto a is the vector with this length that begins at the point A points in the same direction (or opposite direction if the scalar projection is negative) as a .

Thus, mathematically, the scalar projection of b onto a is | b |cos(theta) (where theta is the angle between a and b ) which from (*) is given by

This quantity is also called the component of b in the a direction (hence the notation comp). And, the vector projection is merely the unit vector a /| a | times the scalar projection of b onto a :

Thus, the scalar projection of b onto a is the magnitude of the vector projection of b onto a .

Suppose you wish to find the work W done in moving a particle from one point to another. From physics we know W=Fd where F is the magnitude of the force moving the particle and d is the distance between the two points. However, this relation is only valid when the force acts in the direction the particle moves. Suppose this is not the case. Let the force vector be F =<2,3,4> and the displacement vector be d =<1,2,3>. In this case, the work is the product of the distance moved (the magnitude of the displacement vector) and the magnitude of the component of the force that acts in the direction of displacement (the scalar projection of F onto d ):

Thus, the work done by the force to displace the particle from say the origin to the point (1,2,3) is

Note that this is the easiest way to compute the dot product since the angle between the vectors F and d is unknown.

Formatting Guidelines

Following are some guidelines for formatting a Map chart's Series Options. To display the Series Options for your map chart you can right-click on the outer portion of the map and select Format Chart Area in the right-click menu, or double-click on the outer portion of the map. You should see the Format Object Task Pane on the right-hand side of the Excel window. If the Series Options aren't already displayed, then click the Series Option expander button on the right side and select the Series "value" option that corresponds with your data.

Next, select the Series Options button to display the Series Options and Color choices:

Note: The Map projection, Map area and Map labels options are the same for either type of map chart, but the Series Color options are only available for value charts.

Excel will automatically select the Map projection option that it thinks is best, but you can choose from any of the available selections (not all options will be available depending on the scope of your chart).

Excel will automatically select the Map area option that it thinks is best, but you can choose from any of the available selections (not all options will be available depending on the scope of your chart).

You have the option to not display country or region names, or display where possible based on the Best fit only, or Show all options. Label display depends greatly on the actual size of your chart - the larger the chart, the more labels can be displayed.

Series Colors are only available for Map charts that display values. You can choose from the Sequential (2-color) option, which is the default color setting, or the Diverging (3-color) option. You can experiment with changing the Minimum, Midpoint and Maximum values, as well as the colors assigned to each.

Color formatting a category map chart

Although there are no Series Colors options for a map chart based on a category, you can adjust the individual category colors. Select the data point of interest in the chart legend or on the chart itself, and in the Ribbon > Chart Tools > Format, change the Shape Fill, or change it from the Format Object Task Pane > Format Data Point > Fill dialog, and select from the Color Pallette:

2.3: Map Scale and Projection - Geosciences

Maps are essential tools in geology. Maps are as important in geology as written texts are in the study of literature. By studying maps, a geologist can see the shape and geology of the earth's surface and deduce the geological structures that lie hidden beneath the surface. Geologists are trained in map reading and map making. Many geologists have experience mapping some part of the earth's surface.

It takes some training to read maps skillfully. You are not expected to become a geological expert in reading maps. However, you will be expected to develop your map reading skills as you use maps to help you learn geology.

What are topographic maps and why are they important?

A topographic map is one type of map used by geologists. Topographic maps show the three-dimensional shape of the land and features on the surface of the earth. Topographic maps are also used by hikers, planners who make decisions on zoning and construction permits, government agencies involved in land use planning and hazard assessments, and civil engineers. The topographic maps drawn and published by the U. S. Geological Survey portray the grids that are used on deeds to identify the location of real estate, so homeowners and property owners sometimes find it useful to refer to topographic maps of their area.

Most topographic maps make use of contour lines to depict elevations above sea level. The contour lines reveal the shape of the land in the vertical direction, allowing the 3-dimensional shape of the land to be portrayed on a 2-dimensional sheet of paper or computer screen. When you know how to read contour lines, you can look at them on a topographic map and visualize the mountains, plains, ridges, or valleys that they portrays.

Topographic maps are important in geology because they portray the surface of the earth in detail. This view of the surface shows patterns that provide information about the geology beneath the surface.

The landforms of the earth result from surface processes such as erosion or sedimentation combined with internal geological processes such as magma rising to create a volcano or a ridge of bedrock pushed up along a fault. By studying the shape of the earth's surface through topographic maps, geologists can understand the nature of surface processes in a given area, including zones subjected to landsliding, places undergoing erosion and places where sediment is accumulating. They can also find clues to the underlying geologic structure and geologic history of the area.

In addition to a topographic map, a complete understanding of the underlying geologic structure and history of an area requires completion of a geologic map and cross-sections. A topographic map provides the frame of reference upon which most geologic maps are constructed.

Reading a Topographic Map

Reading a topographic map requires familiarity with how it portrays the three-dimensional shape of the land, so that in looking at a topographic map you can visualize the shape of the land. To read a topographic map, you need to understand the rules of contour lines.

Rules for Contour Lines

  1. A contour line connects all the points on a map area that are at a specific elevation. For example, every point on a 600-foot contour line represents a point on earth that is 600 feet above sea level. You can visualize a contour line as the shoreline that would exist if the ocean were to cover the land to that elevation.
  2. The contour interval is the vertical distance, also known as the elevation difference, between adjacent contour lines. On a map with a 40-foot contour interval, the vertical distance between two contour lines that are next to each other, is 40 feet, regardless of the horizontal distance between the two lines on the map.
  3. Contour lines do not intersect each other, because a point on the surface of the earth cannot be at two different elevations. (However, in the rare case of a vertical cliff showing up on a topographic map, contour lines along the cliff may appear to join together into a single line.)
  4. Circles that are closed contours generally signify hills.
  5. Depressions that have no outlet are signified by closed contours with short lines that stick out of them and point toward the center. (The short lines sticking out of the contour lines are called hachures, hatch marks, or tick marks.)
  6. Contour lines on standard US Geological Survey topographic maps are brown -- except on the surfaces of glaciers, where the contour lines are blue.
  7. The elevation of a point on the map that is not on a contour line must be estimated as greather than the elevation of the nearest contour line below it, and less than elevation of the nearest contour line above it. For example, a point lying midway between the contours 5440 ft and 5480 ft would be at approximately 5460 ft elevation.
  8. Contour lines curve upstream when they cross a valley. This leads to the "Rule of Vs": Where they cross streams, contour lines make Vs that point upstream.
  9. Where contours are close together, the topography is steep where contour lines are far apart, the slope is gentle or flat.
  10. The relief on a landscape is the elevation difference between two given points. The maximum relief on a topographic map is the elevation difference between the highest and lowest points on the map.

Map Quadrangles, Latitudes, and Longitudes

Standard United States Geological Survey topographic maps cover a quadrangle. A map quadrangle spans a fraction of a degree of longitude east-to-west and the same fraction of a degree of latitude north-to-south. Because lines of longitude degrees (also called meridians) in the Northern Hemisphere come closer and closer together the nearer they get to the North Pole, whereas lines of latitude degrees remain the same distance apart as they circle the earth, quadrangle maps span less distance east-to-west than they do north-to-south.

Latitude is how far north or south of the equator a point is on earth, measured in degrees, from 0° at the equator to 90° at the poles. When specifying a latitude, always state whether it is in the Northern Hemisphere (N) or Southern Hemisphere (S).

Longitude is how far east or west, up to a maximum of 180°, a point on earth is from the Prime Meridian. The Prime Meridian, 0° longitude, is a north-south line that runs through Greenwhich, England. When specifying a longitude, state whether it is in the Western Hemisphere (W) or Eastern Hemisphere (E).

Meridians, lines of longitude, run from the South Pole to the North Pole, converging (coming together) at the poles. Because the meridians converge at the poles, a degree of longitude gets smaller and smaller near each pole. In contrast, a degree of latitude remains approximately 69 miles across, no matter how near or far from the poles or equator it is.

Degrees of latitude and longitude are divided into arc minutes and arc seconds. In this context, they are usually just called minutes and seconds, but it must be kept in mind that these minutes and seconds are units of angles, not units of time. These units, which divide angles into smaller parts, work as follows:

  1. There are 60 arc minutes in 1 degree.
  2. The symbol for minutes is a single apostrophe: '.
  3. In symbols, 60' = 1° means there are 60 minutes in 1 degree.
  4. There are 60 arc seconds in 1 arc minute.
  5. To convert arc minutes into a decimal fraction of a degree, multiply the number of arc minutes by 1°/60'. For example, to convert 15' into a decimal fraction of a degree, 15' x 1°/60' = 0.25°. In simpler terms, just divide the number of arc minutes by 60 to convert to decimal degrees.
  6. The symbol for arc seconds is a double apostrophe or quotation mark: ".
  7. In symbols, 60" = 1' means there are 60 seconds in 1 minute.

Two common quadrangle sizes are 7.5 minutes (1/8 of a degree), and 15 minutes (1/4 of a degree).

Name, Size, and Latitude-Longitude of a Topographic Map Quadrangle

Click on the map to open a larger version in a new window

  1. The name of the quadrangle (Juniper).
  2. The state(s) in which the quadrangle is located (Oregon and Washington).
  3. The size of the quadrangle (7.5 minutes).
  4. The name and fractional scale of the quadrangle map that is located adjacent to the northeast of the corner corner (Wallula, 1:125,000).
  5. The longitude of the eastern boundary of the map (119° 00').
  6. The latitude of the northern boundary of the map (46° 00').

Map Scale, Contour Interval, and Magnetic Declination

Click on the map to open a larger version in a new window

  1. The map scale. The map scale is listed in terms of the fractional scale as 1:24,000. This means that 1 inch on the map corresponds to 24,000 inches in the real world represented by the map, or 1 cm equals 24,000 cm in other words, distances on the map have been shrunk by a factor of 24,000 from their real-world size. Beneath the fractional scale, the map scale is also depicted a different way, in bar scales using three different units. One of the bar scales is in miles, one is in units of thousands of feet, and one is in kilometers.
  2. The contour interval, the difference in elevation between adjacent contour lines on the map, is listed below the map scale as 20 feet.
  3. There is also a reminder that elevations shown on the map are elevations above mean (average) sea level on earth.
  4. (You might note that this map does something unusual for a topographic map. It shows depths in the Columbia River in feet below the surface of the river when the river is backed up in its reservoir behind a dam to a normal pool surface elevation of 340 feet above sea level.)
  5. To the left of the bar scales, the magnetic declination is shown as an arrow diverging from a line oriented toward true north. True north is the direction toward the geographic North Pole. The geographic North Pole is where the northern end of earth's axis of rotation is located. The magnetic North Pole is in northeastern Canada. In 1962, the magnetic North Pole, as measured from the Juniper quadrangle, was located 20.5° east of true north. If you took a magnetic compass to the Juniper quadrangle in 1962, its north arrow would point 20.5° east of true north, so you would have had to set your magnetic compass to compensate for the declination. The magnetic North Pole wanders a few miles every year and the magnetic declination of 20.5° east of true north was determined in the year 1962 it may be slightly different now.

Constructing a Topographic Profile

One of the important tools you can use to extract the vertical information from a topographic map, and see more clearly the shape of the earth's surface that it represents is a topographic profile.

Construction of a topographic profile allows you to visualize the vertical component of a landscape. A topographic profile is similar to the view you have of a landscape while standing on earth, looking at hills and valleys from the side rather than from above.

Given a topographic map such as the one below, here's how to construct a topographic profile.

Click on the map to open a larger version in a new window

  1. Determine the line of profile, the line across that part of the map that you want to see in profile or cross-section view. Depending on which part of the map you want to see in profile, you can draw your line of profile in any direction you choose, across any part of the map you choose. For the map used in this example, we choose to draw the profile from A to A' as shown in the diagram below, to see the entire length of the hill in profile.
  2. Draw a grid that will contain the profile. The width of the grid should be the same as the length of the line of profile. To draw the profile, the grid must be crossed by evenly-spaced horizontal lines that represent the contour elevations. The grid must extend high enough to span the elevation range of the contour lines spanned by the line of profile. You can see that the grid, shown below, includes the range of elevations that the line of profile crosses on the map. In addition, the grid must have an extra horizontal line at the bottom and top to accomodate the parts of the profile that go above the highest contour elevation and below the lowest contour elevation. That is why the grid in the example below goes below 400 feet and above 500 feet in elevation.

Click on the map to open a larger version in a new window

  1. Transfer the contour elevations from the topographic map to the profile grid. The point where each contour line crosses the line of profile on the topographic map determines the horizontal coordinate of each corresponding point on the grid of the topographic profile. The elevation of each contour line corresponds to the vertical coordinate of each corresponding point on the profile grid, as shown on the diagram below.

Click on the map to open a larger version in a new window

  1. Now that you have marked the elevation points on the profile grid, draw a smooth line connecting the data points as shown below. Note that the ends of this profile go below the 400 foot contour elevation but they do not extend to the 380 foot elevation because on the map the line of profile did not reach the 380 foot contour line. Also note that the top of the profile reaches a peak above 520 feet but less than 540 feet because the line of profile does not cross the 540 foot contour line.

Click on the map to open a larger version in a new window

  1. The completed topographic profile and the map it was drawn from are shown below. Topographic profiles are usually constructed without drawing any lines on the map. Instead, the edge of a piece of paper is laid along the line of profile and the contour line data is transferred to the edge of the piece of paper. From the edge of the piece of paper, the data are transfered to the profile grid, which is on a separate piece of paper.

Click on the map to open a larger version in a new window

Notice on the topographic profile constructed above that the peak of the hill is above 520 ft, but below 540 ft. Similarly, the ends of the profile are below 400 ft but above 380. This is consistent with the elevations of those parts of the line of profile on the map.

Note that the vertical scale on the profile is very different from the horizontal scale on the map. In this example, the map covers 0.25 miles horizontally in less distance than the profile covers 100 feet vertically. As a result, the topographic profile is greatly exaggerated vertically. In an actual view of the hill, looking at it from the side, it would not look nearly as steep as it does in the topographic profile that we have constructed.

If the vertical scale on a topopgraphic profile is different from the map scale, as it is in this case, then the profile will exhibit a vertical exaggeration. The vertical exaggeration of a topogrpahic profile can be calculated. It is the fractional scale of the topographic profile's vertical axis, divided by the fractional scale of the map. For example, if the vertical scale on the profile is 1:200 and the map scale is 1:24,000, the vertical exaggeration is (1/200)/(1/24000). To divide by a fraction, you can invert and multiply, so this becomes (1/200)x(24,000/1) = 24,000/200 = 120. A topographic profile with a VE of 120 would be a very exaggerated topographic profile. It would be as if a rubber model of the landscape has been pulled in the vertical direction, until it is 120 times taller than it really is.

If the vertical scale of a topographic profile is different from the map scale, the vertical exaggeration should be listed next to the profile, such as VE=10 or VE 10x if the vertical exaggeration is 10.

Compare the profile to the topographic map. You will see that the hill is steeper on the west (left) side than on the east (right) side. This is consistent with the contour lines being spaced more closely on the west side of the hill and farther apart on the east side of the hill. This accords with the rules of contour lines, which state that slopes are steeper where contour lines are more closely spaced, and slopes are less steep where contour lines are more widely spaced.

If you drew a profile from north to south across the peak of the hill, do you think the profile would be symmetric or asymmetric?

Checklist for a Complete Topographic Profile
A properly drawn topographic profile will have the following attributes:

  1. The topographic profile is drawn on a rectilinear graph with evenly spaced grid lines. (Vertical grid lines are not required.)
  2. Elevation lines are labeled along the left-hand vertical axis.
  3. The profile is a smooth curve where its gradient changes, rather than straight-line segments connecting the dots and only bending at the dots.
  4. If the vertical scale on the profile is different from the map scale, the resulting amount of vertical exaggeration is listed.
  5. The ends and any high points or low points of the topographic profile should be above or below elevation lines, not on them, except in cases where an end, high point, or low point of a line of profile happens to fall right on a contour line.

What are geologic maps and why are they important?

A geologic map shows mappable rock units, mappable sediment units that cover up the rocks, and geologic structures such as faults and folds. A mappable unit of rock or sediment is one that a geologist can consistently recognize, trace across a landscape, and describe so that other people are able to recognize it and verify its presence and identity. Mappable units are shown as different colors or patterns on a base map of the geographic area.

Geologic maps are important for two reasons. First, as geologists make geologic maps and related explanations and cross-sections, they develop a theoretical understanding of the geology and geologic history of a given area.

Second, geologic maps are essential tools for practical applications such as zoning, civil engineering, and hazard assessment. Geologic maps are also vital in finding and developing geological resources, such as gravel to help build the road you drive on, oil to power the car you travel in, or aluminum to build the more fuel-efficient engine in your next vehicle. Another resource that is developed on the basis of geologic maps is groundwater, which many cities, farms, and factories rely on for the water they use.

Essential Components of Geologic Maps

  • an accompanying explanation of the rock or sediment units
  • geologic cross-sections of the map area.

The legend or key to a geologic map is usually printed on the same page as the map and follows a customary format. The symbol for each rock or sediment unit is shown in a box next to its name and brief description. These symbols are stacked in age sequence from oldest at the bottom to youngest at the top. The geologic era, or period, or epoch--the geologic age--is listed for each rock unit in the key. By stacking the units in age sequence from youngest at the top to oldest at the bottom, and identifying which interval of geologic time each unit belongs to, the map reader can quickly see the age of each rock or sediment unit. The map key also contains a listing and explanation of the symbols shown on the map, such as the symbols for different types of faults and folds. See the Table of Geologic Map Symbols for pictures and an overview of the map symbols, including strikes and dips, faults, folds, and an overview.

The explanations of rock units are often given in a separate pamphlet that accompanies the map. The explanations include descriptions with enough detail for any geologist to be able to recognize the units and learn how their ages were determined.

If included, cross-sections are usually printed on the same page as the geologic map. They are important accompaniments to geologic maps, especially if the map focuses on the geology of the bedrock underneath the soil and loose sediments.

Geologic Cross-Sections

A geologic cross-section is a sideways view of a slice of the earth. It shows how the different types of rock are layered or otherwise configured, and it portrays geologic structures beneath the earth's surface, such as faults and folds. Geologic cross-sections are constructed on the basis of the geology mapped at the surface combined with an understanding of rocks in terms of physical behavior and three-dimensional structures.

Open Source Web Links

To learn more about topographic maps visit the US Topo page:

The USGS web site shows the standard US topographic map symbols.

Geoscientists at Idaho State University and an affiliated map skills group have put together the following lessons on topographic maps. These lessons are more advanced than Geology 101, but the Web site is a good resource that you can search for key information and explanations that you need about topographic maps: ld_Exercise/topo maps/index.htm

For explanations of types of geologic maps published in the United States visit: lists/booklets/usgsmaps/usgsmaps.php#geologic maps

Unless otherwise specified, this work by Washington State Colleges is licensed under a Creative Commons Attribution 3.0 United States License.

Watch the video: How to draw a cross section. No activities, just how to draw one (October 2021).