What is Sea Level, 什么是海平面？

Sea level seems like a pretty easy concept, right?

海拔听起来是一个很简单的概念，对吧?

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You just measure the average level of the oceans and thats that.

你只用测出海面平均高度就行了。

But what about parts of the earth where there arent oceans?

但地球上没有海洋的那些部分呢?

For example,when we say that Mt. Everest is 8850m above sea level,how do we know what sea level would be beneath Mt. Everest,since theres no sea for hundreds of kilometers?

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海例如，当我们说珠穆朗玛峰拔8850米时，我们怎么知道山底海平面的高度呢，因为几千米之内可都没有海水啊。

If the earth were flat then things would be easy - wed just draw a straight line through the average height of the oceans and be done with it. But the earth isnt flat.

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如果地球是平的，那么事情就容易了--我们只要在海洋的平均高度上画一条直线就可以了。但是地球并不是平的。

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If the earth were spherical,it would be easy, too, because we could just measure the average distance from the center of the earth to the surface of the ocean.

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如果地球是球形的，也会很容易，因为我们可以测量从地球中心到海洋表面的平均距离。

But the earth isnt spherical - its spinning,so bits closer to the equator (n.赤道) are thrown out by centrifugal effects, and the poles get squashed(n.壁球;摺皱不堪;拥挤嘈杂的人群;浓缩果汁;美国南瓜;vt.压制;vi.变扁)in a bit.

但地球不是球形的一它在自转，靠近赤道的部分因“离心效应”被甩了出去，而极点被压扁了。

In fact,the earth is so non-spherical that its 42km farther across at the equator than from pole to pole

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实际上，地球不是圆形的，所以赤道两端间距离比两极点距离长42公里。

That means if you thought earth were a sphere(n.范围;球体;vt.包围;放入球内;使...成球形)and defined(vt.定义;使明确;规定)sea level by standing on the sea ice at the north pole, then the surface of the ocean at the equator would be 21km above sea level.

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这意味着如果你认为地球是球体，并将海平面定义为站在北极海冰上的高度，那么赤道海平面的高度就成了海拔21千米。

This bulging(n.膨胀,肿起;vi.膨胀,凸出,鼓起) is also why the Chimborazo volcano in Ecuador,and not Mount Everest,is the peak thats actually farthest (adv. 最大程度地;最远地(far的最高级);adj.最远的;最久的(far的最高级))from the center of the earth.

这也是为什么尼儿多尔的钦博拉索火山，而不是珠程朗玛峰，是距离地球球心最远的山峰

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So how do we know what sea level is? Well, water is held on Earth by gravity,so we could model the earth as a flattened(vt.使......平坦;击败，摧毁:vi.变平;变单调) stretched spinning sphere(n.范围;球体;vt.包围;放入球内;使...成球形)and then calculate what height the oceans would settle to when pulled by gravity onto the surface of that ellipsoid(n.椭圆体).

那么我们怎么才能知道海拔高度呢?水因其重力而被保持在地球上，所以我们可以把地球建模成一个扁平的拉伸的旋转球体然后计算在重力作用下海水落到这个椭圆球表面的高度。

Except the interior(n.内部;本质;adj.内部的;国内的;本质的)of the earth doesnt have the same density(n.密度) everywhere,which means gravity is slightly stronger or weaker at different points around the globe, and the oceans tend puddle more nearer to the dense (adj.浓厚的;稠密的;愚钝的)spots.

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只不过地球内部各处的密度不同，这意味着地球上不同地点的重力有小幅波动，并且海洋在密集处聚集的更多。

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These arent small changes, either - the level of the sea can vary (vi.变化;违反;[生变异;vt.改变;使多样化;[音]变奏) by up to 1oom from a uniform ellipsoid depending on the density of the Earth beneath it. And on top of that

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literally,there are those pesky(adj.讨厌的;麻烦的;adv.极端) things called continents moving around on the Earths surface.

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这些并不是小变化--海水高度与椭球面最多相差可达100米，这取决于其下方的密度。而它的上面，是那些在地球表面到处晃悠的讨厌东西。

These dense lumps(n.块,块状;肿块;瘤;很多;笨人;vt.使成块状;忍耐;笨重地移动) of rock bump out from the ellipsoic and their mass gravitationally(adj.万有引力的;重力的)attracts oceans,while valleys in the ocean floor have less mass and the oceans flow awav.shallower (adi.浅的;肤浅的;n.浅滩).

这些稠密的岩石块从椭球体中碰撞出来，它们的质量在引力作用下吸引着海洋，而海床上的峡谷质量较小，上方的海水也因此流走。

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And this is the real conundrum. because the very presence of a mountain (continent on which it sits) changes the level of the sea: the gravitational attraction of land pulls more water nearby, raising the sea around it

这才是真正的难题，因为一座山(它所在的大陆)的存在改变了海平面：陆地上的万有引力吸引附近更多的水，使周围海面更高。

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So,to determine the height of a mountain above sea level, should we use the height the sea would be if the mountain werent there at all?

所以为了确定一座山的海拔高度，我们是否该用假定山不存在时的海面高度呢?

Or the height the sea would be if the mountain werent there but its gravity were?

或者仅保留山的重力时海平面高度?

The people who worry about such things, called geodetic scientists or geodesists, decided that we should indeed define sea level using the strength of gravity, so they went about creating an incredibly detailed model of the earths gravitational field.called,creatively,the Earth Gravitational Model.

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那些担心这些事情的人，被称为大地测量学家或测地学家，他们认为在测量海平面时应该考虑到重力因素，于是他们开始着手创建一个非常详细的地球重力场模型，这个模型很有创意，叫做地球引力模型。

Its incorporated(v.合并;组成公司;adi.合并的;公司组织的)into modern GPS

receivers so they wont tell you youre 1oom below sea level when youre in fact sitting on the beach in Sri Lanka which has weak gravity, and has allowed geodesists

themselves to correctly predict(vt.预报,预言;预知;vi.作出预言;作预料,作预报) the average level of the ocean to within a meter everywhere on Earth.

这个模型被植入了现代GPS接收器，所以当你坐在斯里兰卡的海滩上时它不会说 你当前在海拔-100米，那里重力略低，重力场图让测量者能够正确地使全球海平面的高度在上下一米之内浮动。

Which is why we also use it to define what sea level would be underneath mountains if they werent there. . .but their gravity were.

这也是为何我们用它来测量山底下海平面的高度视其身不在，而重力在。

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