What is a contour line? What are the characteristics and types of contour lines?

Contour refers to a closed curve connected by adjacent equal elevation points on topographic map. A closed curve connecting points with the same height on the ground. Vertical projection on the horizontal plane, and drawn in proportion on the map, you get the contour line. The contour line can also be regarded as the intersection line between the horizontal plane with different heights and the actual ground, so the contour line is a closed curve. The number marked on the contour line is the height of the contour line.

trait

1, the ground points on the same contour line have the same height. But points with the same altitude are not necessarily located on the same contour line. (e.g. contour line 40m on the right)

2. In the same picture, contour lines at different altitudes cannot intersect except cliffs.

3. The elevation difference between adjacent contour lines in the drawing section is generally the same, so the ground slope is inversely proportional to the horizontal distance between contour lines. The smaller the horizontal distance of contour lines, the denser the contour lines are arranged, indicating the greater the slope of the ground; The greater the horizontal distance of contour lines, the finer the contour line arrangement, which means the smaller the slope of the ground.

4. The contour line is a closed curve. If it cannot be closed in the same picture, it must be closed in an adjacent or other picture.

5. When passing through the ridge or valley, the contour line will change direction. Therefore, the ridge line or valley line should be perpendicular to the tangent line at the turning point of the contour line, that is, the contour line is orthogonal to the ridge line or valley line.

classify

According to the different functions of contour lines, contour lines can be divided into four types: first curve, design curve, intermediate curve and auxiliary curve.

1, the first curve, also known as the basic contour line. It is a contour line based on basic vertical interval surveying and mapping, which is generally described by a thin solid line (0. 15mm) and is the main contour line representing the landform state.

2, curve, also known as bold outline. In order to explain the elevation of contour lines, the drawn contour lines are thickened every four first curves from the elevation calculation surface. Generally, a thick solid line (0.3mm) is used and the marked elevation is disconnected at an appropriate position. The prefix faces uphill, and the curve is the basis of identifying contour elevation.

3, curve, also known as half distance contour. When the first curve cannot show some local topography, contour lines drawn at semi-vertical intervals should be used. Generally, slender dotted lines are used, which are used in local areas. Can not be closed, but symmetrical.

4. Auxiliary curve, also called auxiliary contour line, is a short dashed line drawn with a quarter vertical interval, which is used to represent a section of miniature landform that can not be displayed by indirect curve.