How To Read a Contour Map

One of the most useful skills to have in agriculture, is the ability to read and utilize contour maps. A contour map is simply a map that uses lines to represent different levels of elevation. Topographic maps that show hills, valleys, and mountains is a type of contour map. Today I will be discussing some ways to interpret these lines that we often find useful in sustainable agriculture. In particular contour maps help us identify geographical feature, and pathways that help us understand how water interacts with the landscape.

Obtaining the Contour Map

Getting a contour map of your area, may seem challenging because there are several ways to go about doing it. However, one of the easiest and straightforward approaches is to use the online contour map generator. Unfortunately, it's not free, but then again it does most of the heavy lifting for you. In fact this is the best tool I've found to do smaller land areas than what typical topographic maps can offer. There are a few other online contour map generators out there, but are either more expensive or abandoned, so use at your own discretion. For larger areas you can use this global online topographic map, at no cost, or if the area in question is based in the US, you can try this site. Finally, when it comes to generating a detailed contour map, especially anything you generate online, it's best to verify it with your own land survey. In permaculture, this is usually accomplished using an A-Frame or Bunyip Water Level. Many resources are available to help you use these tools, but for a quick overview see this link.

Understanding Topographic Patterns

After you have obtained a contour map, let's take a look at different ways contours can reveal topographical features that will be of use to you on your farm or project. To reiterate, each line you see represents an elevation, usually in meters, and is measured from sea level. The proximity of these lines will tell you the shape of a slope. Let's look at the following figures.

Determining Slopes

Figure 1a shows how the closer the contour lines the steeper the slope. Conversely, distant contours create a gentler slope. The next example, Figure 1b shows how progressively closer lines with increasing elevation create a concave slope, and how progressively distant lines with increasing elevation creates a convex slope.

Fig 1a. Steep Slope vs Gentle Slope

Fig 1b. Concave Slope vs. Convex Slope

Identifying Peaks

Peaks are easily spotted as the highest point on the contour map often times as small circles. The following Figure 2a, shows how these peaks may be flat like a plateau or pointed like a mountain. However, sometimes dips in the landscape may appear as peaks as in Figure 2b. Pay close attention to elevations to avoid this mistake.

Fig 2a. Flat-top vs. Pointed

Fig 2b. Dip vs. Hill

Ridgelines and Spurs

A very important feature to identify on a contour map is the ridgeline. Start by finding the highest peak, then drawing a line connecting the closest, next highest peak as in Figure 3a. You may notice V-shaped lines pointing down in elevation from the peaks. This feature is a spur and breaks up the flow of water (Figure 3b).

Fig 3a. Ridgeline

Fig 3b. Spur

Water Pathways

Similar to the spur, a water pathway can be found by tracing the V-shaped contour lines pointed up in elevation (Figure 4a). Often times thes V shapes are more pronounced than the lines associated with a spur. In Figure 4b you can see how spurs also act to divert water along separate paths (bottom), or the immediate transition from spur to water pathway is an indication of water pathway convergence (top).

Fig 4a. Water Pathways

Fig 4b. Water Convergence and Division

And that wraps up this post for today. Try it out for yourself. Perhaps start with just the contour map you get online and try to find the ridgelines, and then the water pathways using what you've learned here. Then go out into the world and see how it aligns. Often, you will need to fine tune your map in order to correct for the real world. However, once it's done, your understanding of how water flows through your land will help you with planning your next garden layout, building a house, or understanding your flood risks.

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