What Contour Lines Mean and How to Interpret Them
Have you seen maps covered with swirling, curly lines and wondered what that’s all about? Contour maps are two-dimensional maps or surveys or site plan drawings that are imbued with elevation information through the expression of those lines curving and swirling in spaced proximity to each other. They are not only informative, but also quite beautiful, both literally and conceptually. The literal beauty stems from the shapes that can seem to pop off the page and the color combinations that some maps are created with. The conceptual beauty comes from the contour line itself.
A contour is a line, very often curved, that is shown on a topography plan connecting points of equal height as related to a datum. The datum may be mean sea level, but more likely in the present day the datum is a geodetic reference.
Each contour line represents an interval of vertical elevation, or height, and the spacing of the lines represent the steepness or gentleness (or flatness) of sloping land.
Any drawing or map is an abstraction of the three dimensional world in which we live, and some are more abstract than others. The abstraction of a drawing arises from the fact that while every worldly thing manifests itself in (at least ) three dimensions, a drawing can only host two, and so something must be done with that third one. A floor plan handles the problem be representing two dimensions, the x and the y, on a static, level plane in space, usually at a height of three feet above the floor. There is a drawing convention that permits things above the plane of the plan to be represented by a dashed line. Anything shown below the plane of the “cut” is shown as a solid line. Sometimes elements that are beneath the surface of the floor are also shown dashed, but for the most part there is not a substantial presence of vertical information provided in a floor plan.
Like a Floor Plan, But Different
If you grasp the concept of a floor plan as representing a level horizontal section cut though a building at a static height above a datum (ie the floor surface), then you will grasp the concept of a contour map representing a series of successive, level, horizontal cuts at regular intervals through the ground. A contour line emerges at the edge perimeter of each cut, and once all the edge perimeters of all the cuts are perfectly overlaid, one onto the next, into a system of contours, then you have the basis of a contour plan or map. Of course, this is not actually the exact work and process that goes into developing a contour map, but it’s an easy way to think about it on the way to grasping the concept.
A Thought Experiment for Comprehending Contours
Another way to think about it involves a kind of thought experiment. It goes like this (you can do this, or just imagine doing it) :
Place an object in a deep container with one inch increments marked on the side wall of the container. The object should have an obvious bottom, top, and sides. A sphere would work, but it would work less well than a cone, for instance. The best would be an object with a wavy, undulating surface, like a human face or the like.
Assign a zero value to the bottom of the container. Each one inch increment that you marked on the side of the tray represents a successive rise in elevation: 1”, 2”, 3” and so on.
Now pour a liquid into the container up to the first inch mark. Where the liquid meets the surface of the object, a line is formed that demarcates wet (below the line, in the liquid) and dry (above the liquid). This line denotes an increment of elevation along the shaped surface of the object. This is contour elevation 1”.
Now continue pouring liquid into the container until you reach the next inch mark. The surface of the water is now at a 2” vertical height, and the line that is formed by the water on the surface of the object denotes the 2” contour. You can keep pouring liquid into the container to the 3”, 4”, 5” elevation and so on, pausing at each incremental interval to observe the line that forms.
Again, you may be served well enough just to imagine this exercise, as opposed to actually doing it. But while it so far demonstrates the relationship between the shaped surface of an object and the continuously flat and level plane of liquid rising in vertical elevation, you will need to engage your imagination one step further to grasp how this relates to a contour drawing.
So now imagine that you had positioned a camera directly over the object and the container, set perfectly still and fixed in its position, and each time you reached a contour increment expressed by liquid in the container, you snapped a photo that captured clearly the contour line created by the meeting of the liquid and the object surface. If you then overlaid each photo in perfect registration with all the others to create a single composite image, you would begin to see the two dimensional relationship that the contour lines have together. Moreover, had you done all this in half inch increments, you would have twice as many photos, and therefore twice as many contour lines, and they would be spaced tighter together, by a factor of a half. This is an abstract representation of the object’s topography. This is not unlike a contour map of the ground, if the ground were the object.
Learn the lay of the land from a quick study
By keeping the vertical interval the same from one cut to the next, a graph-like operation in your mind can help you to understand the relationship of the vertical value to the horizontal value. For instance, if the vertical interval from one cut (ie the contour line) to the next is two feet, and there is eight horizontal feet from one contour line to the next contour line, then you know that ground slope is two feet of rise over eight feet of run, or 25%.
With this in mind, you can then appreciate that if the horizontal distance from one contour line to the next was only two feet, then the slope would be two feet of rise over only two feet of run, or 100%. slope. Therefore, tightly space contour lines represent steeper slopes than widely spaced contour lines, which represent a gentle slope. No contours in an area of a contour map means there is no slope, or it is too gentle to register. If you reduce the vertical interval ever smaller, then the contours of a super gentle slope will eventually emerge.