四、等高线地面上有高山、平原、盆地。局部也不是平坦如镜。用等高线表示地貌的原理,就是以平均海水面为标准水平面,把地面上相同高度的任何一个点,垂直投影到水平面上,按一定的比例缩小,把那些在地面上高程相同、并且相邻的点连成的 Fourth, there are mountains, plains and basins on the contour line. Part is not flat as a mirror. The principle of using contour lines to represent geomorphology is to use the average sea surface as the standard horizontal plane, project any point on the ground at the same height, and project it vertically to a horizontal plane, reduce it by a certain proportion, and put those elevations on the ground to be the same, and Neighboring points