Understanding the term "contour":
A contour refers to a line that joins points of equal elevation or depth on a topographic map. It represents a specific elevation above or below a reference point, such as sea level. Contours are used to depict the shape, relief, and slope of the land. They provide valuable information for understanding the topography of an area, including the presence of hills, valleys, and other features.

How contours are numbered:
Contours are numbered in a systematic manner to provide information about their elevation. The process of numbering contours involves the following steps:

1. Interval determination:
The first step in numbering contours is to determine the contour interval, which is the difference in elevation between two adjacent contours. This interval is usually mentioned on the map and remains constant throughout the map area. The contour interval is chosen based on the scale of the map and the relief of the area being represented.

2. Index contour selection:
Index contours are selected at regular intervals, usually every fifth contour line, and are labeled with their elevation. These lines are usually thicker and darker than other contours, making them easily distinguishable. The index contours serve as reference points for determining the elevation of other contours.

3. Counting contours:
Starting from the lowest elevation, each contour line is counted and assigned a number based on its relationship to the nearest index contour. The index contour is given the elevation mentioned on the map. Contours uphill from the index contour are numbered by adding the contour interval to the elevation of the index contour. On the other hand, contours downhill from the index contour are numbered by subtracting the contour interval from the elevation of the index contour.

4. Interpolation:
In some cases, contours may be missing or not explicitly labeled on the map. In such situations, interpolation is used to estimate the elevation of these missing contours. This involves examining the surrounding contours and making an educated guess about the elevation based on the contour interval.

5. Labeling:
Once all the contours have been numbered, the map is labeled with the elevation of the index contours. This provides a clear representation of the elevation patterns in the area.

Conclusion:
Contours are numbered in a systematic manner by determining the contour interval, selecting index contours, counting contours based on their relationship to the index contours, interpolating missing contours if necessary, and labeling the map with the elevations. This numbering system helps in understanding the elevation profile and topography of an area, enabling users to interpret the landscape accurately.