如果两个等高线相切，说明它们的切线相同，也就是说他们的法向量应该平行，我再在这里画个图。

If the two level curves are tangent to each other that means they have the same tangent line. That means that the normal vectors should be parallel. Let me maybe draw a picture here.

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本文根据圆的渐开线原理和两点间的距离公式，得出两种用放射式丈量法测画不分道跑起跑线的新画法。

This article utilizes the theory of involute of circle and the formula of the distance between two points to obtain two radiated measurement ways of defining the starting line.

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此画没有用透视法照样画出物象的空间和距离，是由于画家灵活运用了皴线和点。

This painting does not use the same perspective to draw images of space and distance, is due to the painter flexibility in the use of the Cun lines and points.

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