Two contour lines of same elevation:

Question

Prepp Team · Accepted Answer

Understanding Contour Lines and Their Properties

Contour lines are fundamental in representing the topography of the Earth's surface on a two-dimensional map. A contour line is an imaginary line on the ground connecting points of equal elevation above or below a reference datum, such as mean sea level.

These lines provide valuable information about the shape and slope of the land. By observing the pattern of contour lines, one can understand features like hills, valleys, slopes, and ridges.

Key Properties of Contour Lines

Understanding the behavior of contour lines is crucial in surveying and mapping. Several standard rules govern how contour lines behave:

Contour lines close upon themselves, either within or outside the limits of the map.
A contour line represents a single, specific elevation.
The closer the contour lines are to each other, the steeper the slope. The farther apart they are, the gentler the slope.
Contour lines generally do not cross each other, except in the rare case of an overhanging cliff or a vertical cliff face where multiple elevations exist at the same horizontal location. In such cases, multiple contour lines would merge at a point but not continue as a single line representing different elevations or unite and continue as one line representing the same elevation.
Contour lines usually run perpendicular to the direction of the steepest slope.
Contour lines bend uphill when crossing a valley or stream and bend downhill when crossing a ridge.

Why Two Contour Lines of Same Elevation Cannot Unite and Continue

The question asks about two contour lines of the same elevation uniting and continuing as one line. Let's consider the implication of this:

If two distinct contour lines, both representing, say, an elevation of 100 meters, were to unite and continue as a single line, it would mean that the single line now represents all the points that were previously on the two separate lines. However, each point on a single contour line must represent only one specific elevation. If the two lines represent the same elevation (100m), their merging into one continuous line doesn't inherently violate the 'single elevation per line' rule *at that specific elevation*. But the fundamental principle is that a contour line traces a single path of constant elevation. Two separate paths of the same elevation exist because there is land *between* them at a different elevation (either higher, forming a depression/peak, or lower, forming a ridge/saddle). For these two separate contour lines at the same elevation to unite and continue as one, the land between them would have to cease to exist or abruptly change in a way that is physically impossible in typical terrain representation.

The rule that contour lines cannot unite and continue as one line (except in the theoretical case of a truly vertical feature where they merge at a point) stems from the fact that a single line must consistently represent one elevation. If two separate lines at the same elevation were to unite and continue, it would imply an anomaly in the terrain that is not represented by standard contouring, or it would require the single line to represent areas previously covered by two separate lines, which goes against the principle of how contours map unique paths of constant elevation.

Consider two separate hills, both reaching a peak above 100m. The 100m contour line would circle each hill separately. These two 100m lines cannot merge and continue as one line unless the topography changes dramatically (e.g., the land between the hills rises to exactly 100m over a wide area, effectively merging the 100m elevation region, in which case the original separate lines wouldn't exist as drawn). Standard contouring rules state this merging and continuing does not occur for lines of the same elevation.

Analyzing the Options for Contour Line Behavior

Let's examine the given options based on the properties of contour lines:

Option	Analysis	Validity based on Contour Rules
1. Cannot unite and continue as one line.	This option states that two contour lines of the same elevation cannot merge and then proceed as a single continuous line. This aligns with the standard rule that each contour line represents a distinct path of a single elevation, and two such distinct paths at the same elevation do not merge and continue.	Valid
2. Cannot unite and but continue as one line.	This option is grammatically incorrect and unclear. It seems to imply they cannot unite but somehow continue as one line, which doesn't make sense in the context of contouring.	Invalid
3. Can unite and but not continue as one line.	This option suggests they can unite but not continue as one line. While contour lines can touch or overlap at a point (like a vertical cliff face), they wouldn't typically "unite" in the sense of merging their paths for a distance. Even where they overlap at a point, they do not continue as one line thereafter if they represent different slopes/features on either side. For lines of the same elevation, uniting at a point would mean the land is vertical at that elevation, which is possible, but they still wouldn't typically "unite and continue". The primary rule is against uniting and continuing.	Partially relevant to vertical features touching, but the phrase "unite and continue as one line" makes it mostly invalid for standard cases. The stronger rule is "cannot unite and continue".
4. Can unite and continue as one line.	This option directly contradicts the fundamental rule of contour lines which states that two lines of the same elevation cannot unite and continue as a single line under normal circumstances and standard representation.	Invalid

Based on the fundamental rules of surveying and mapping regarding contour lines, two contour lines representing the same elevation cannot merge and continue as a single continuous line.

Revision Table: Understanding Contour Line Properties

Property	Description	Behavior
Elevation	Each line represents a single, specific elevation.	Constant value along the line.
Crossing	Generally do not cross.	Cross only in rare cases of overhanging or vertical features.
Merging/Uniting	Cannot unite and continue as one line (especially lines of the same elevation).	Maintain separate paths except potentially touching at a point for vertical features.
Spacing	Indicates slope steepness.	Close spacing = steep slope; Wide spacing = gentle slope.
Closure	Must close upon themselves.	Forms closed loops on or off the map.

Additional Information on Contour Maps

Contour maps are essential tools used in various fields like civil engineering, geology, environmental science, and urban planning. They provide a visual representation of the terrain, which is crucial for tasks such as planning construction, analyzing drainage patterns, or understanding geological structures.

The difference in elevation between two consecutive contour lines is called the contour interval. This interval is constant for a given map and is chosen based on the scale of the map and the nature of the terrain. A smaller contour interval is used for detailed maps or flatter terrain, while a larger interval is used for maps covering larger areas or very hilly terrain.

Index contours are usually shown as bolder lines, typically every fifth contour line, and have their elevation value labeled. This helps in easily reading the elevation values on the map.

Understanding the rules governing contour lines, such as their inability to unite and continue as one line when representing the same elevation, is key to accurately interpreting topographic maps.