Hello and welcome to this article of the online course Wilderness Navigation Masters, where I will talk about what are these lines that we found on a topographic map and how to use them to get an idea of the relief.
The first thing to know, these lines are named contour lines or elevation lines.
What does a contour line show?
If you wonder what does a contour line show, the contour lines (or topography lines) are the brown lines that you usually see on every topographic map, which allows you to determine the elevation on every part of the map.
How map makers do that?
They do that by taking a mountain for example and start linking all the points that have the same elevation within this mountain.
So if you hike following exactly a contour line, you will stay at the same height above sea level.
Types of contour lines :
There are three types of contour lines :
- Index lines: it’s the thick brown lines that are labeled with a number that represents the elevation above the sea.
- Intermediate lines: they are the brown thin 5 lines between index lines, they don’t have an elevation value to keep the map readable.
- Supplementary lines: These dashed brown lines are what map makers add between intermediate lines, in areas where there isn’t too much elevation difference. All that, to help us understand well the topographic of the area.
Depression contour lines :
Before I go to what represents the space between contour lines, if you have dashes in one side of a contour line, those dashes mean that there is a depression in that side. .. like a hole in the ground or it’s a low area like the sides of what we see sometimes on a highway.
Yes, in a topographic map, contour lines are often brown, but that does not mean that we will not found blue contour lines sometimes.
Glacier or permanent snowfield contour lines :
Because when a mapmaker wants to represent a glacier or permanent snowfield, they use the blue color instead of the brown.
Contour interval :
The space between two contour lines is what we refer to as the contour interval, it’s the value of the vertical elevation change.
It’s one of the essential information of a topographic map, and we found it usually underneath the map scale.
How to read topographic maps contour lines
Topo-map reading #1
Now that you know the importance of the contour lines, and how many types and colors you will found on a topographic map, I want you to learn how to read these contour lines to distinguish where is the summit ?, where is the Ridgeline ? where is the valley? the saddle? etc
The first thing I notice when I first see this map excerpt is that we have some hydrography features here, and when I read the toponymy, I know that, here, it’s a creek named Fish creek, and a lake named Lake Estes.
After that, I can see that there are some mountain tops here.
And between these two, I have a ridgeline.
Why mountains and not depressions?
Because if it’s the case, they will use the little dashes in the inside of the contour line circles.
Topo-map reading #2
This is another topographic map excerpt.
I see here that there 6 peaks, one, two, three, four, five, and sex.
Another map feature that I notice is this intermittent river, like here, for example, that goes down into this little pond here.
Why it goes down this way and not the opposite because just from these two index contour lines, this area is high in altitude (with an approximate elevation between 8400 and 8600 feet).
For this place, using this index contour line, I know that it’s an area with only 8000 feet in elevation …. and because water always goes down the slope, this is the direction of these intermittent rivers.
In this topographic map excerpt, we have some vegetation that is represented by this green color, a river here, and some campgrounds.
And from these two elevation values, I know that the direction of the slope is this way … but with this river, I know that the slope goes down from this place just to the river, then it starts going up to this flat area.
It’s not a flat area, but with no much elevation, because the contour lines are away from each other, unlike this place.
Is this a valley or a ridge?
Now I want to talk to you about a problem that happens to the majority of the newbies, when they try to differentiate between a valley and a ridge, especially in maps without shading like this.
This line for example, is this the uphill and this is a ridgeline that goes down, or this is the uphill, and it’s a valley path that goes downhill from this place to this?
To know how to answer this question, you need to know where is the area with the highest elevation.
For that, you may use the toponymy, or the elevation values in the index lines, because, having just 2 index lines can give you the direction of the slope.
After you know where is the highest point, the Vs or the Us (depending on the relief), point to a direction. If this direction is toward lower altitudes, this is a ridgeline, if it’s toward high altitudes, it’s a valley.
I know that this may sound complex to you the first time you hear it. So, let’s see an example :
Is this the mountain top and this is a Ridgeline that goes down this way, or this is the mountain top …. and this is a river path that goes down this way.
To answer this question … not only, but the simple key to answering this is to know where is the high altitude.
If I slip this elevation value following the index line, it will be here.
From these two elevation values, I know that the high altitude place is this, and the low altitude place is this.
So because this little Us point toward the low altitude, this is a ridgeline …. and because this little Vs point toward the high altitude, this is a valley that goes down following this path.
Another way to differentiate between a valley and a Ridgeline without using contour lines is to know that it’s impossible for water to be up in a ridgeline.
In this example, it’s not a Ridgeline that has water in the top line because it’s impossible, so it’s a valley.
From that, this is the high altitude place, and all the rain get accumulated and go down here.
If you still don’t know how to differentiate between a valley and Ridgeline on a topographic map, no problem, because you will have another chance in this next question.
Quizzes & questions
That’s all for this article, don’t forget that you can interact with us in the comments section below sharing for example, what is the questions that you get correct and what’s not.
Thank you for watching and see you in the next video.
Last modified: June 7, 2021