This lesson explains the main features of topographic maps, including contour lines, index contours and contour intervals. It will also cover two rules that all contours follow and discuss geologic maps and the information they contain.
What Are Topographic Maps?
Imagine the best banana split ever. Golden banana slices on the bottom, two or three nice big scoops of frosty ice cream, warm caramel & chocolate, maybe even some bright sprinkles…Oh and don't forget the best part, the whipped cream. If you start out with a wide base and keep spiraling upward, you get that mountain peak at the top where the cherry lives. And if you were to look down at that sundae from above, (paying special attention to the whipped cream) you would see lots of lines. These lines would circle all the way around the mountain you just created and would be very similar to something called a topographic map.
A topographic map is a type of map that shows elevation. Topographic maps are used by people like hikers and others who need to know what the elevation of an area is. So what do these lines mean? Why are there so many, and why are they so crooked? This lesson will be looking at just that: We will cover what a topographic map is, what those lines are and how they work. Specifically, we will delve into the rules that those lines follow and how to make sense of the numbers associated with them. Later on in the lesson, we'll learn how this information connects to a true geologic map and how to read all those colors.
Topographic Map Basics
We begin our focus on the basic part of a topographic map, all those lines. These are known as contour lines. Contour lines connect points of equal elevation. All that means is that where there's a line, everything on that line is the same height or altitude.
Think of it this way, if you had a tub of water with a big rock in it and you started filling the tub with water but paused every minute to draw a line where the water had risen to - those lines would be like contour lines. If you did this for the entire rock, then drained all the water out, the rock would have lines all over it. If you looked at this from above, just like the whipped cream from before, you would see all those lines connecting and forming circles. A topographic map shows those lines. On most maps like this, these lines are shown with a light brown color.
A topographic map
If you were to look at a topographic map, you would realize that there's not just one line, but many. Each line is a different elevation, but some of these lines have numbers on them. These are called index contours. Index contours display the exact elevation of one specific line. Why not just put numbers on every line? Space. There just isn't enough room on the map to keep putting numbers everywhere, and it would get awfully cluttered if we did. We'll touch on this idea again later on.
To put this into perspective, imagine you are somewhere with a big staircase. Maybe you're at the Pyramids of Giza or one of those big concrete sports arenas or any other place where there's lots of steps. Imagine you are at the very bottom of those steps. If you start hiking up the stairs, the tenth step up would be painted light brown. Not just in one spot, the entire step, all the way across. It would represent 10 feet high. This would be one of your contour lines.
If you went up another 10 steps, there would be another step painted brown there as well. If you keep going, eventually you will probably lose count of how many steps you took. Don't believe me? Go run stairs somewhere and try to remember what floor you are on, try a tall hotel stairwell. After a while, you'll get tired and start to forget. Well lucky for you, after your 50 stairs, you see a painted step that has a big 5-0 printed on it. Do you remember what this line is called? If you said index contour line, you are correct! If not, that's OK. Why don't you have a seat, catch your breath and take a look around. This step with a big 50 painted on it means that you're now 50 feet above sea level. Each index contour might only be labeled in two or three other places - it keeps the clutter down.
One different thing about topographic maps: They have rules. Specifically, the contour lines have rules - and every topographic map follows them:
1. Lines cannot cross.
2. Lines form circles around hills or depressions.
Let's unpack those rules a bit more. First, the contour lines cannot cross, but why? Think about what those lines do first. What are they there for? I know you remembered that they show elevation! If two lines crossed, it would actually mean that one location had two different elevations. While that would be pretty cool, being at two different elevations at the same time would cause some serious issues with physicists because basically you would need to be in two different places at one time, and that can't happen.
While we're focused on lines here, look at the space between any two lines. This is called the contour interval. Contour interval is the distance between any two side-by-side lines. For example, a contour interval of 10 means you would change elevation 10 feet up or down by crossing over those lines on the map. It depends which way you're traveling. Every space between every line on that map would be 10 feet of elevation change, no matter how close or spread out those lines are. This information will either be printed on the map somewhere or you can figure it out by looking at the index contours.
Now, lines can (and often do) get really close to each other. The closer those lines, the steeper the hill would be. A sheer cliff would have lines all piled up to show that drastic elevation change, and a gentle slope would have contour lines much more spread out. See how these lines are all bunched up here? This is a topographic map that would represent this type of cliff landform.
The other rule we need to look at states that contour lines close around hills and depressions. This is probably best explained by going back to our step idea. If you were to run all the way around the base of one of those pyramids, it would make a big square. If you were to then go up 10 feet to that first painted level of step and run around that that, it would make another square. This would be a little bit smaller, but the same exact shape.
If you kept doing that, eventually you would you would probably be arrested, but you would end up with a bunch of concentric squares. Eventually, it would look like a square bull's eye with the center being the smallest square. The same is true for hills and mountains. As you go up in elevation, that circle will get smaller and smaller. Just like lines in the whipped cream, the lines here would completely surround the pyramid.
Up until now we've only talked about contour lines going up in elevation. But what would happen if there was a section of the map that had a circle with little hairs coming off of it? Here's a sample of what I mean:
Looks strange, right? What are those things? Those little lines are called hachures. Hachures are how topographic maps show depressions, or holes. Hachures look a bit like teeth in an open mouth.
If you had a topographic map of a footprint in the sand, it would have hachures like those shown below. When you see those lines you know that you are going to be dipping down into a bowl shape. Some volcano maps have these, and places with a dip in an otherwise flat landscape have them, too. Ehh, that's actually a bit terrifying. Let's move on!
Many parts of a geologic map include things that we have already talked about. A geologic map contains topographic lines and ages and types of rocks. Basically it shows geologic features. This is similar to the topographic map that we've already been looking at, but it has much more information layered right on top of it. See why we needed to be so conservative with our contour lines before? Speaking of lines, if you see lines or markings that are black, those are typically geologic markings which contrast the brown contour lines showing elevation.
The most obvious feature here is color, lots of color. Each color represents either a different type of rock or a different age of rock. For example, below you can see some pink rocks with circles on them from Yosemite National Park.
This pink color represents granite rocks. It's not the real color of the rock, just a way to show difference. It's similar to the way that we color countries on a map - no country is actually purple!
While coloring can vary from map to map, the United States usually uses reds and oranges for igneous rocks, which were once molten. For rocks changed by heat and pressure, called metamorphic rocks, we tend to color those with shades of brown or olive green. Usually, solid colors show the rock is marine formed, and the ones with patterns on them show that the rock is terrestrial or formed on land. These features are usually explained on the map in the key or the legend.
Fully understanding maps like these will take time and practice. Here are the main points we discussed in this lesson: A topographic map shows elevation. It does that by using lines called contours. Each contour line connects points of equal elevation. Some of those lines have numbers on them and are called index contours. These lines show the exact elevation on one specific line. There's also a space between these lines called a contour interval, which is a set number for each map and shows elevation change from one line to the next.
There were also two rules we looked at. Contour lines cannot cross, and they form circles around hills and depressions. Contour lines that show depressions are called hachures.
The key to understanding geologic maps begins with first knowing how a topographic map works. Only after that can we look at a geologic map, which displays both topography and the types and ages of rocks. We also saw how different rocks are shown with different colors. And don't worry if you can't remember what all those colors represent, this will normally be displayed by a key within the map.
Thanks for watching!
Once you are done with this lesson, you should be able to:
- Explain the purpose of a topographic map
- Describe contour lines and recall the two rules that contour lines must follow
- State what hachure lines indicate
- Recall the meaning of the numbers marked on and between contour lines and what they are called
- Summarize what contour intervals are
- Interpret a geologic map and explain the color selections
- Discuss how a topographic and geologic map are connected