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Topography refers to the variations in elevation on the Earth's surface, and it includes mountains, hills, and plains. There are many ways to represent the terrain on a map, but in surveying work, contour lines are commonly used. Representing the terrain with contour lines allows not only to show the undulations of the surface but also its slope and the elevation of various points on the surface. This section discusses the method of representing landforms using contour lines. I. Concept of contour lines: Contour lines are continuous closed curves formed by connecting points on the ground that have the same elevation. There is a small hill located in a calm lake, and the water level at which the top of the hill is just submerged is 100 meters. Then the water level drops by 5m, exposing the mountain peak; at this point, there is a line of intersection between the water surface and the slope, and it is a closed curve. The elevation of all points on this curve is equal, and this is the contour line at an elevation of 95m. Subsequently, the water level dropped by another 5 m, and the slope and the water surface formed another intersection line, which is the contour line at an elevation of 90 m. By the same logic, for every 5m drop in water level, the water surface intersects the ground surface, leaving behind a contour line; thus, a set of contour lines with a height difference of 5m is obtained. Imagine projecting this set of contour lines in the field vertically onto the horizontal plane H, and then reducing them to scale on paper; this results in a contour map that represents the topographic features of that hill using contour lines. II. Height interval and horizontal distance between contour lines The difference in height between adjacent contour lines is called the height interval, which is often denoted by h. On the same topographic map, the contour interval is the same. The horizontal distance between adjacent contour lines is called the contour spacing, often denoted by d. Since the contour interval is the same on the same topographic map, the magnitude of the horizontal distance between contour lines d is directly related to the slope of the ground. The smaller the distance between contour lines, the steeper the slope of the ground ； The larger the horizontal distance, the smaller the slope ； The slope is the same, and the horizontal distance is equal. Therefore, the steepness or gentleness of the ground slope can be determined based on the density of the contour lines on the topographic map. It can also be seen that the smaller the contour interval, the more detailed the terrain is displayed ； The larger the contour interval, the more simplified the terrain appears. There are also certain special landforms, such as gullies and landslides; for their representation methods, see the topographic map symbols. III. Contour lines of typical landforms The shapes of landforms on the surface are diverse; upon careful analysis, it becomes clear that they are essentially a combination of several typical landforms. Understanding and becoming familiar with the characteristics of typical landforms represented by contour lines will help in reading, applying, and mapping topographic maps. Typical landforms include: 1. Hills and depressions (basins). The contour lines of hills and depressions are all sets of closed curves. The method to distinguish between hills and depressions on a topographic map is as follows: areas where the elevation values indicated by the inner contour lines are greater than those of the outer contour lines are hills, while those with lower elevation values are depressions. If there are no elevation annotations on the contour lines, slope indicators are used to represent them. A slope line is a short line perpendicular to the contour lines, used to indicate the direction in which the slope decreases. The slope line points from the inner circle to the outer circle, indicating that the area in the middle is higher while the areas around it are lower; it is a hill. The slope indicator points from the outer circle to the inner circle, indicating that the area around it is higher while the center is lower; hence it is a depression. 2. Ridges and valleys A ridge is an elevated area that extends in one direction. The highest ridge line of a mountain range is called the ridge line. Ridge contour lines appear as a set of curves that bulge toward lower areas. A valley is a depression that extends in one direction, located between two mountain ridges. The line that runs through the lowest point of a valley is called the valley line. The contour lines of a valley appear as a set of curves that bulge upward. Rainwater near a ridge inevitably flows in two directions, on either side of the ridge, along the ridge line as a boundary; therefore, a ridge is also known as a divide. In a valley, rainwater inevitably flows from the slopes on both sides toward the bottom of the valley, converging at the valley floor; therefore, the valley floor is also known as the watershed. 3. Saddle: The saddle is a saddle-shaped depression between two adjacent mountain peaks. The saddle is often the area where mountain roads pass, as well as the point where two ridges and two valleys meet. The characteristic of contour lines at saddles is that, within one large closed curve, there are two sets of smaller closed curves. 4. Cliffs and precipices: A cliff is a slope with a gradient of 70°. The steep cliffs mentioned above can be divided into rocky and soil-based types. A cliff is a steep slope with a protruding upper part and a recessed lower part; the contour lines of this landform intersect. Hidden contour lines when viewed from above are shown in dashed lines. IV. Classification of contour lines 1. First-order contours On the same map, the contour lines drawn at the specified contour interval are called first-order contours, or basic contour lines. It is a thin solid line with a width of 0.15 mm. 2. Count curves: For easier reading of the map, contour lines whose elevations are divisible by 5 times the basic contour interval are drawn in thicker lines; these are known as count curves. 3. Intermediate curves and auxiliary curves: When the main contour lines fail to show the characteristics of the terrain, contour lines drawn at half the basic contour interval are called intermediate curves, and they are represented on the map by long dashed lines. Sometimes, to show local landforms, contour lines drawn at intervals of one-quarter of the basic contour interval are used, and these are called auxiliary curves. It is generally represented by short dashed lines. V. Characteristics of contour lines 1. The elevation of all points on the same contour line is equal. 2. Contour lines are closed curves; if they do not close within this map sheet, they must close outside the map. 3. Except at cliffs or sheer drops, contour lines must not intersect or overlap on the map. 4. A small horizontal distance between contour lines indicates a steep slope, while a large horizontal distance indicates a gentle slope; if the horizontal distances are equal, the slope is also equal. 5. The contour lines are perpendicular to the ridge lines and valley lines. After the control survey is completed, the planar positions and elevations of features and topographic features can be determined using the control points, and these can then be reduced to a topographic map at the specified scale and with appropriate symbols. Methods for mapping topographic maps include the theodolite surveying method, the photogrammetric range finder surveying method, the combined use of a small planimeter and a theodolite for surveying, and the photogrammetry method.