To draw contour lines for a valley, the fundamental rule is that contours should always point upstream in valleys.
Understanding Contour Lines on Topographic Maps
Contour lines are lines drawn on a topographic map that connect points of equal elevation above a specific reference point, usually sea level. They are used to represent the three-dimensional shape of the land on a two-dimensional surface.
The Key Rule for Valleys
According to the provided reference, a critical rule for interpreting or drawing contour lines is:
1. Contours should always point upstream in valleys.
This means that when a contour line crosses a valley or a stream, it forms a 'V' shape, and the tip of this 'V' points towards the area of higher elevation, which is typically upstream.
Why Do Contours Point Upstream in Valleys?
Understanding why contours point upstream helps in accurately drawing them:
- Elevation Change: Valleys are areas of low elevation between higher ground (ridges or hills). Water flows downhill, so the elevation decreases as you move downstream in a valley.
- Equal Elevation: Contour lines connect points of equal elevation.
- Forming the 'V': As a contour line representing a specific elevation reaches a valley, it must bend upstream to stay at that same elevation level. The sides of the 'V' point downstream, and the tip of the 'V' points upstream (towards higher elevation).
Contour Line Behavior: Valleys vs. Ridges
Contour lines behave differently for valleys and ridges, reflecting their opposing topographic shapes:
| Feature | Contour Shape | Direction of 'V' Tip | Implication |
|---|---|---|---|
| Valley | Forms a 'V' shape | Points upstream (towards higher elevation) | Indicates water flow direction is opposite to the 'V' tip |
| Ridge | Forms a 'U' or rounded shape | Points downridge (towards lower elevation) | Indicates higher ground |
Reference Rule for Ridges:
2. Contours should always point downridge along ridges.
This contrast highlights how the 'V' shape helps identify valleys and the direction of elevation change.
General Rules for Drawing and Interpreting Contours
Beyond the specific rules for valleys and ridges, several other fundamental principles govern how contour lines are drawn and interpreted. These rules ensure the accuracy and consistency of topographic maps:
- Adjacent Contours:
3. Adjacent contours should always be sequential or equivalent.
This means that contour lines next to each other on a map will either represent consecutive elevation values (e.g., 100m, 110m, 120m) or, in the case of a single line looping back, represent the same elevation. The interval between contour lines (the contour interval) is constant on a map. - No Splitting:
4. Contours should never split into two.
A single contour line represents a single, continuous elevation. It cannot branch. - No Crossing or Looping (Generally):
5. Contours should never cross or loop.
Except in rare instances like a vertical cliff or an overhang (which are often represented specially), contour lines cannot cross because a single point on the ground cannot have two different elevations. A contour line also forms a closed loop eventually, although the loop may extend beyond the map boundaries. - No Spiraling:
6. Contours should never spiral.
Contour lines represent relatively smooth elevation changes, not a vortex or spiral. - No Stopping Mid-Map:
7. Contours should never stop in the middle of a map.
Unless they run into a boundary of the map, contour lines must complete their loop, which often means extending off the edge of the mapped area.
Adhering to these rules, particularly the rule about pointing upstream, is essential for accurately drawing or understanding the representation of valleys on a topographic map.
Summary
Drawing contour lines for a valley primarily involves ensuring that the contour lines form a 'V' shape that points upstream, towards higher elevation. This is a direct application of the rule that "contours should always point upstream in valleys," which, along with other standard contour rules, allows for the accurate depiction of terrain.
