为了在一个类图上建模继承，从子类(要继承行为的类)拉出一条闭合的，单键头(或三角形)的实线指向超类。

To model inheritance on a class diagram, a solid line is drawn from the child class (the class inheriting the behavior) with a closed, unfilled arrowhead (or triangle) pointing to the super class.

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在GPS测量中，往往需要计算三角形、多边形闭合差，以便及时发现观测粗差、衡量GPS观测精度。

In GPS surveying, we usually need to compute closure difference of triangle or polygon, so as to search surveying gross errors and estimate GPS observing precision.

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