Drawing a contour cross section helps visualize the topography along a specific line on a map.
Creating a contour cross section, also known as a topographic profile, involves a few key steps to translate the information from a 2D contour map into a 2D vertical representation of the terrain. Based on the process, here's how you do it:
1. Define and Draw Your Profile Line
First, decide where you want to slice through the terrain. On your contour map, draw a straight line across the contours where you want your cross-section to be. This line is called the profile line or section line.
2. Mark Points and Note Elevations
Along the profile line you've drawn:
- Mark every point where your line intersects a contour line.
- Note the elevation of each contour line at the point of intersection.
- Also, mark points where the line crosses important features like hilltops, valleys, or stream beds, estimating their elevations if they don't fall exactly on a contour.
3. Measure Distances
For each marked point along your profile line on the map, note how far each point is from the next on your map. You'll also need to know the total distance of the line. These horizontal distances will be used to space your points correctly on your cross-section graph.
4. Draw the Cross-Section
Now you'll transfer this information to a graph or plotting surface:
- Set up your graph: Draw a horizontal line representing the length of your profile line. This is often referred to as the base horizon or the horizontal axis. Above this line, draw a vertical axis representing elevation. Choose a suitable vertical scale; often, the vertical scale is exaggerated to highlight topographic changes.
- Transfer points: For each point you marked on your map line, measure its distance from the start of the line and mark that position on the horizontal axis of your graph.
- Plot elevations: Above each marked position on the horizontal axis, plot a point corresponding to the elevation you noted for that location on the map.
- Connect the dots: Once all points are plotted, connect them with a smooth line. This line is your completed contour cross section, illustrating the terrain's profile along your chosen line.
Practical Tip
Using a piece of paper placed along your profile line on the map can help in transferring points accurately. Mark the beginning and end of the line, and then mark each contour intersection and feature point on the paper, noting the elevation next to each mark. You can then use this marked paper strip as the horizontal axis guide for your graph.
Here’s a simple representation of how you might organize the data points before plotting:
| Point # | Distance from Start (Map Units) | Elevation (Elevation Units) |
|---|---|---|
| 1 | 0 | Starting Elevation |
| 2 | [Distance to Point 2] | [Elevation of Contour 2] |
| 3 | [Distance to Point 3] | [Elevation of Contour 3] |
| ... | ... | ... |
| Last | Total Line Length | Ending Elevation |
By following these steps, you transform a 2D topographic representation into a vertical profile that clearly shows the rise and fall of the landscape.
