当地下水抽出地面的井,而不是补充足够快,地下水位下沉。,区域的表面下沉。在当地,这种效应并不明显,因为高度指定的对象是相对于表面。高度的变化保持零的定义。但相对于周围土地的平均海拔不是连接在地下水方面,有一个变化。什么是改变土壤表面之间的距离和地球的重心。沉降是一个地球的形状的变化,所以它的重心变化本身,但变化是足够小的大小比表面变化忽略它。表面高程的变化如何地球表面上的一个固定的点来衡量吗?一种方法是使用表面这一事实的水量平衡到处是相同的高度,即使表面分离。水位在u型管两端是相同的。 One could use a long pipe that reaches from the measured location to a reference point. Connected bodies of groundwater are large, so that may require pipes of a length in the order of kilometers. Also, with a large distance between the end the air pressure may be different, so there is no equilibrium. The ground water level itself is a possible reference point, because the ground water can permeate the ground. But we know that it is not in equilibrium, because ground water is removed only in parts of the surface - the wells, and the permeation is slow. Ground elevation can be measured from satellites, as GPS does. I suspect that the accuracy is too low for many purposes because the relative change in the measured distance is very small. Also, the orbit of a satellite changes with the change of the center of gravity. Because the change is slow, elevation changes in distant regions influencing the satellite create noise in the measurement.