Contour lines are one of the most important features used in topographic maps to represent the shape and elevation of the land surface. These lines are imaginary and connect points of equal elevation above a reference point, which is usually the mean sea level or a specific datum surface. The line joining the points having the same elevation above the datum surface is called a contour line.

Explanation:

1. Contour lines:
- Contour lines are lines drawn on a topographic map to represent the elevation of the land surface.
- They are used to depict the shape and relief of the terrain, including hills, valleys, and other landforms.
- Each contour line represents a constant elevation above a reference point or datum surface.

2. Datum surface:
- The datum surface is a reference point or a horizontal plane from which elevations are measured.
- It is typically the mean sea level, which serves as a common reference for different locations.
- Other reference points can also be used, such as local benchmarks or specific elevation points.

3. Points of equal elevation:
- The contour lines connect points of equal elevation above the datum surface.
- This means that all the points along a particular contour line have the same height or elevation above the reference point.
- For example, if a contour line is labeled as 100 meters, it means that all the points along that line are 100 meters above the datum surface.

4. Contour line characteristics:
- Contour lines are typically represented as closed loops, but they can also be open or irregular depending on the shape of the land surface.
- They never intersect or cross each other, as that would imply multiple elevations at the same point.
- Contour lines that are close together indicate steep slopes, while lines that are far apart represent gentle slopes.

5. Importance of contour lines:
- Contour lines are crucial for understanding the topography of an area and for various engineering and environmental applications.
- They help in visualizing the shape and elevation of the land surface, which is essential for activities like construction, land use planning, and flood mapping.
- By interpreting contour lines, one can identify valleys, ridges, cliffs, and other landforms on a topographic map.

Therefore, the line joining the points having the same elevation above the datum surface is called a contour line. It plays a vital role in representing the elevation and shape of the land surface on a topographic map.