Topographic maps as a way to visualize the surface of the Earth
Topographic maps show the three-dimensional shape of the landscape by representing
equal elevation with lines on a two-dimensional map; they are in essence a type of
contour map (also used in meteorology
and
oceanography). Although these can be
mathematically derived, most geologists
create
them by measuring the elevation (and
position) in
the field (or on an aerial photo), plotting
elevation
on a map and connecting lines of equal
elevation
(much like connect-the-dots except that
all the
same numbers are connected, rather
than in
sequence).
Why should I become familiar with all aspects of topographic maps?
Topographic maps will be familiar to those of you who are hikers or outdoor enthusiasts;
they are used to understand the landscape over which you will hike (or rock climb or ski,
etc). They are also commonly used by field geologists for a variety of applications. The
ability to read and interpret topographic maps is considered a basic skill for all
geoscientists and geoscience students. Topographic maps are used to understand the
shape of the land, whether a slope will fail, how glaciers are changing, and geologic
history, among many other things. Geoscientists make and use them to construct
geologic maps, to find the best building sites, to estimate where flooding will take place,
and to determine the best sites for archeological or paleontological digs. Parts of this
module teach you about how to read topographic maps so that you can complete other
applications that these types of contour maps are used for.
Students of the geosciences
who want to
become proficient in reading
topographic
maps must also learn about
scale - the
relation of the size of the map to the size of the area in real life. Why should you care
what the scale of the map is? The scale helps you understand how far you'll have to
hike to get to that lake, or the distance between two points on a road. Parts of this
module will take you through the types of scales, how to determine scale, and the
standard types of scales for topographic maps.
One of the most practical exercises found in geoscience
textbooks involves calculating the slope of a hillside or
other part of the ground surface. Why would you
want to calculate the slope of a hillside? The slope
can tell us whether this is a good site to build, whether
a road will become covered with debris, or how difficult
it will be to hike to that peak. It also influences volcanic
hazards, limits permissible land use (such as farming
and development), and much more. Parts of this
handout will walk you through how to calculate the
slope of a hillside or groundwater table surface or anything for which you know the
distance and difference in elevation.
Drawing a topographic profile is related to slope and understanding the shape of the
land. Why would you want to draw a profile of the landscape? Many geoscientists
like to visualize the shape of the land as if they had sliced through it. This helps them to
see hazards, draw conclusions about the strike and dip of the geology beneath the land,
among other applications. Parts of this module will walk you through the steps to
making a topographic profile, something geoscientists do often.
How do I calculate slope/gradient?
"Rise over run" in the geosciences
Many of us know that the slope of a line is calculated by "rise over run". However, the
application of slope calculation can seem a little more complicated. In the geosciences,
you may be asked to calculate the slope of a hill or to determine rate by calculating the
slope of a line on a graph. This page is designed to help you learn these skills so that
you can use them in your geoscience courses.
Why should I calculate slope or gradient?
In the geosciences slope can play an important role in a number of problems. The slope
of a hill can help to determine the amount of erosion likely during a rainstorm. The
gradient of the water table can help us to understand whether (and how much)
contamination might affect a local well or water source.
How do I calculate slope (or gradient)
Gradient in the case of hill slope and water table is just like calculating the slope of a line
on a graph - "rise" over "run". But how do you do that using a contour (or topographic)
map?
1. First get comfortable with the features of the topographic map of interest. Make
sure you know a few things:



What is the contour interval (sometimes abbreviated CI)?
What is the scale of the map?
What is the feature for which you want to know the slope?
2. First, you need to know "rise" for the feature. "Rise" is the difference in elevation
from the top to bottom (see the image above). So determine the elevation of the
top of the hill (or slope, or water table)
3. Next you need to know "run" for the feature. "Run" is the horizontal distance from
the highest elevation to the lowest. So, get out your ruler and measure that
distance. If you know the scale, you can calculate the distance. Most of the time
distance on maps is given in km or mi.
4. Now comes the rise over run part. There are two ways that you may be asked to
make calculations relating to slope. Make sure you know what the question is
asking you and follow the steps associated with the appropriate process:
a. If you are asked to calculate slope (as in a line or a hillside), a simple
division is all that is needed. Just make sure that you keep track of units!
b. You may also be asked to calculate percent (or %) slope. This calculation
takes a couple of steps. And it mostly has to do with paying attention to
units. The units on both rise and run have to be the same.
Calculating slope
Avalanche hazards
The following questions relate to the map below.
You have recently purchased land around Pioneer Ridge (on the map above). You plan to
build a ski resort and need to decide where to build your lodge. You have chosen two
sites that have the aspects you want for your lodge (a view of the river and proximity to
some nice steep (double diamond) slopes. You know that avalanches are most likely to
happen on slopes that are 60-100%, so you need to know which of the sites will be less
likely to be buried in an avalanche. Use the information on the map to calculate % slope
and then decide which site is more realistic for your lodge.
Water Table
You are working in an area with an important aquifer that has been contaminated by a
buried tank. You need to know the slope of the water table so that you can calculate
how quickly the contaminants will get to the nearby wells. You measure the depth to
water in two wells: Well A has water at 649 m elevation. Well B has water at 937 m
elevation. The two wells are 0.7 km apart. What is the slope of the water table (in
m/m)?