Lesson 6:
Topo Maps and Watershed Boundaries



In this lesson, we will answer the following questions:



Reading Assignment

There is no reading assignment for this lesson.




Reading Topo Maps

What Is a Topo Map?

A map is a representation of all or part of the earth.  A topographic map, also known as a topo map, is a map which shows changes in elevation by using contour lines.  Contour lines are imaginary lines that join points of equal elevation on the surface of the land above or below a reference surface such as average sea level.  You can think of a contour line as a trail for a lazy hiker who never wants to climb up or down.  Instead, he just walks around the side of a hill at the same elevation.  Contour lines make it possible to show the height of mountains, depth of the ocean bottom, and steepness of slopes on a topo map.


The map above is an example of a topo map and the brown lines are contour lines.  Notice that contour lines never cross, but instead make concentric rings around hilltops. 

The thicker brown lines are known as index contours.  Index contours are used to help determine the specific elevation at a certain location on the map.  The elevations of some index contours are actually marked on the map while you can determine the elevations of other contours from context.  For example, find a circular index contour near the upper left hand side of the map above - you can see that it is labelled "2400."  The label means that the contour is at an elevation of 2400 feet above sea level. 

The contour interval is the difference in elevation between two adjacent contour lines on a topo map.  You can determine the contour interval by counting the number of contours between labelled index contours and then by using the formula shown below. 

Contour interval formula


For example, on the map above you can see that there are 15 contour lines between the 2400 index contour and the 1800 index contour.  (There are actually 14 contour lines between the two index contours, but you always count one of the index contours as well as all of the contours in between.)  So the contour interval is calculated as follows:


As you can see, there is a 40 feet change in elevation between each contour line on the map above.  The contour interval is selected to best show the general shape of the terrain.  A map of a relatively flat area may have a contour interval of 10 feet or less while maps in mountainous areas may have contour intervals of 100 feet or more.

Reading a Map

Topo map

Interpreting the colored lines, areas, and other symbols is the first step in reading a map.  The first features usually noticed on a topographic map are the area features such as vegetation (green), water (blue), information added during update (purple), and densely built-up areas (gray or red).  On the map above, notice that there is a large tract of forest, shown as green, on the left hand side of the map.  The right side of the map is primarily unforested---probably pasture and lawns.  

Features are shown as points, lines, or areas, depending on their size and extent.  For example, individual houses may be shown as small black squares.  You can see dozens of houses scattered along the upper half of the map above.  For larger buildings, the actual shapes are mapped---notice the larger, variously shaped buildings on the upper right side of the map.  In densely built-up areas such as cities, most individual buildings are omitted and an area tint is shown.  On some maps, post offices, churches, city halls and other landmark buildings are shown within the tinted area.  You can tell that may of the buildings shown on the map above were built between when the map was first drawn and when it was updated since they are shown in purple. 

Many features are shown by lines that may be straight, curved, solid, dashed, dotted, or a combination.  The colors of the lines usually indicate similar kinds or classes of information: brown is used for topographic contours; blue for lakes, streams, irrigation ditches, etc.; red for land grids and important roads; black for other roads and trails, railroads, boundaries, etc.; and purple for features that have been updated using aerial photography, but not field verified.  The red line in the upper half of the map in this section is a highway while there are several smaller black roads elsewhere on the map. 

Various point symbols are used to depict features such as buildings, campgrounds, springs, water tanks, mines, survey control points, and wells.  Names of places and features also are shown in a color corresponding to the type of feature.  Many features are identified by labels, such as "Substation" or "Golf Course."  Notice that the town of Jonesville is labelled on the map in this section as is the Jonesville Campground.  The highway, 58, is labelled in red since the highway is red. 

The scale is usually found at the bottom of a map.  A typical scale is 1:24,000 meaning that every inch on the map is equivalent to 24,000 inches on the ground.  An arrow at the bottom of the map points to magnetic north.  The map shown in this section does not include a scale or an arrow. 


Reading Topography

Illustration of ground configuration shown by contours
Illustration of ground configuration shown by contours

The picture on the left above is a three dimensional representation of the land shown in the topo map to the right.  In this section, you will learn how to read a topo map so that you can visualize how the land shown on the map looks in three dimensions. 


The first step in reading a topo map is to determine which way is uphill.  You can do this by finding labelled index contours since the contour with the higher elevation is always uphill of the contour with the lower elevation.  For example, on the map below, you can see that the 2400 index contour is near the top of the map while the 1800 index contour is nearer the bottom of the map, so you can tell that the land goes downhill as you read it from the top to the bottom.

Hilltops are easy to find on topographic maps because they are shown as closed circles.  A peak is labelled on the map above.  Valleys, in contrast, are usually shown as Vs or Us with the point of the V being the upstream end.  I have labelled one valley on the map above, but there are several other small valleys you should be able to pick out.  When looking for valleys, be sure to distinguish them from ridges in which the point of the U or V is at a lower elevation than the legs of the V. 

Contours that are very close together represent steep slopes. Widely spaced contours, or an absence of contours, means that the ground slope is relatively level.  On the map above, the contour lines are relatively close together, showing that the hillside is relatively steep. 



Plotting Your Boundaries on the Topo Map

My property is located on the topo map section shown below:

Topo map of my property

In order to plot my property boundaries onto the topo map, I have to go back to the deed to find features which might show up on the topo map.  The deed for my property gave me the following information:

Boundary drawing

Now you will want to find these features on the topo map. You need to locate at least one corner of the property boundary on the topo map so that you can draw on the rest of the boundary.  I was able to locate the divide of two hollows, the creek, and the foot of the hill, and place the boundaries on my topo map as shown below:

Boundaries on topo map.

Using the information on the topo map, you can add even more features to your drawing.  In this case, I added the location of the creek and the road.  You may be able to add buildings, sinkholes, cliffs, and other features.  You should also add utility lines such as electricity, sewer, water, telephone, cable tv, and fire protection.

Scale drawing
The final drawing, including physical features.




What is a Watershed? 


A watershed is an area where all rainfall collects into a common location.  The common location could be a stream, a pond, a river, etc.  

Watershed boundary




How do we find the boundaries of the watershed?  Edges of a watershed are usually found in the highest areas around.  There, water falling as rain on one side of the mountain or hill runs down into one watershed while water falling on the other side of the mountain or hill runs down into another watershed.   


Defining Watershed Boundaries on a Topographical Map

Finding watershed boundaries for a creek on a topo map is as simple as finding the highest points around the creek and connecting the dots. 

 A creek's watershed boundary

Starting at the creek, go uphill in every direction (except directly downstream) until you reach the highest points.  Sometimes the highest point above the creek may be a peak, shown as a closed circle.  Other times, the highest point is a ridge, shown as an elongated U- shape.  In any case, on the other side of these highest points, a new watershed begins.  Connect the highest points around the creek with a line and you have drawn in the watershed boundary. 

Your Watershed



The first step in determining your construction site's watershed boundaries is to mark the location of all of the wet and dry creeks around your construction site.  For example, I've marked the drainage patterns onto the topo map on the right as blue lines.  (The topo map on the left is the original, unmarked topo map.)  The proposed construction site is shown as a red rectangle. 

Watershed boundaries

Next, you need to determine the watershed boundaries between the creeks as explained in the last section.  I have marked all of the watershed boundaries onto the map above as red lines. 

Where the water flows

Once your watershed boundaries are marked onto the map, it will become clear which creek or creeks the water from your construction site will flow into.  I have marked with an X the creek and the location in the creek which water from the construction site will flow into.

The construction site's watershed boundaries

The final step is to mark the lower boundaries of the watershed.  The X marks the lower boundary of the construction site's watershed, so I have marked the construction site's watershed boundaries in orange. 



Topo maps show elevation changes of the land using contour lines.  Other features such as streams and roads are also shown by the map. 

A watershed is an area of land from which all of the water collects into a central location.  In order to find watershed boundaries, go to the highest point in all directions from the body of water into which the watershed's water flows. 

New Formulas Used
To calculate the contour interval:

Contour interval formula



U.S. Geological Survey provided most of the information on reading topographic maps. 


Using the property you worked on in the last lesson, draw the property boundaries onto the following topo map.  The red dot marks the location of the beginning of the engineering description of the property's boundaries. 

Topo for assignment

You plan to build a subdivision in the area marked in red on the map below.  Draw the watershed boundary of this area onto your topo map. 

Subdivision area



Answer the questions in Quiz 1 .  When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and directly submit it into the database for a grade. .