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Wolfram Alpha为什么不显示低重力加速度的哈德逊湾吗?

2019 - 04 - 22 - t06:58:13z

我想是聪明和添加一个新的答案< a href = " //www.hoelymoley.com/q/15343/6031 " >哈得逊湾低重力吗?< / >通过使用Wolfram Alpha为不同位置报告重力加速度,但它看起来像我的聪明适得其反。

From this answer I found this view, a subsection of which is cropped and shown below.

I chose three points, the lowest, purple area near the West shore of the Hudson bay, a medium green-orange in lake Michigan near Chicago, and a high area just north of Iceland.

I was surprised to see Wolfram report such a high gravitational acceleration for the Hudson Bay area at 9.85 m/s^2! I was expecting something substantially lower than 9.81 m/s^2

Question(s):

Why is the gravitational acceleration for the west coast of the Hudson Bay so high from Wolfram Alpha?
Roughly what acceleration should I be getting there?
Could it be related to Wolfram Alpha's use of EMG2008 12th order for a model?
Why would it be reporting such large deviations in altitude for locations on water?

Here is the data:

 lat lon Total vert dev down west south elev Western Hudson Bay 'low' 60.9N 94.1W 9.85176 0.00302 9.85172 0.00967 0.02816 -22 Lake Mich. (~Chicago) 'med' 41.7N 87.3W 9.8188 0.00352 9.81874 0.01119 0.03274 +175 North of Iceland 'high' 66.6N 18.8W 9.86107 0.00259 9.86104 0.00845 0.02406 -445

Wolfram Alpha links and raw data screen captures:

答案由FSimardGIS Wolfram Alpha为什么不显示低重力加速度的哈德逊湾吗?

2019 - 06 - 27 - t00:50:01z

Wolfram Alpha的报告值9.78和9.88 m / s ^ 2之间根据不同的位置,这很奇怪,因为它应该呆在9.76 - 9.83范围遍布世界各地。它似乎也使用海底作为参考高程点落在水中。 使用< a href = " http://webapp.geod.nrcan.gc.ca/geod/tools-outils/gpsh.php?locale=en&" rel="noreferrer">Natural Resources Canada's tool, it reports a gravity of 981933.3 mGal (9.819333 m/s^2) for Hudson Bay at 60.9N, 94.1W at sea level, which seems more likely for that location.

Keep in mind that while the area near Hudson Bay has a negative gravity anomaly, this does not imply that it should have weaker gravity than Chicago (9.8026 m/s^2), which is much closer to the equator. It means that Hudson Bay's gravity is lower than what it should be at that latitude. Gravity is even weaker near the equator (around 9.78) because as you get closer to the equator, the centrifugal force is stronger and slightly counteracts the force of gravity, also the mass of the Earth is slightly further away from you, thus has less gravitational pull on you. These 2 effects alone create gravity variations up to 0.5% (around 0.05 m/s^2)

Gravity anomalies are relative to a theoretical normal gravity based on an idealized ellipsoidal rotating Earth, and these anomalies are small compared to the variation caused by cetrifugal forces and the oblateness of the Earth. Anomalies range between -50 and +50 mGal (+-0.0005 m/s^2). Normal gravity at the Equator for sea level is about 9.78 m/s^2, and near the poles, around 9.83 m/s^2. Altitude also has a significant effect, at 5000 meters, gravity is about 0.01 m/s^2 weaker than at sea level.

回答约翰Wolfram Alpha为什么不显示低重力加速度的哈德逊湾吗?

2019 - 06 - 28 - t13:37:17z

简短的回答,因为没有冰帽。 为什么哈得逊湾低重力,< a href = " https://en.wikipedia.org/wiki/Post-glacial_rebound " rel = " nofollow noreferrer " > < / >均衡反弹,曾经有一个冰盖,几英里的冰。,进一步推动大陆地壳在地幔,像一艘船在负载很重的。地幔的密度比大陆地壳。当冰盖融化的岩石开始反弹,但需要很长时间恢复平衡。地幔不是液体是不会很快反弹。冰的重量基本上取代部分地幔和大部分的地幔仍流离失所。直到它完全篮板少质量比的地方没有这样的位移。我们甚至可以测量< a href = " https://www.ontariobeneathourfeet.com/rising-land-isostatic-rebound " rel = " nofollow noreferrer " > < / >速度反弹,每年几毫米。 边注,也是为什么有一个海洋的大陆地壳,土地仍然是抑郁低于海平面。 It should take the area about ~1,000,000 year to rebound but the glaciers only meted about 11,000 years ago. So in about 989,000 years there will no longer be a gravitation depression there because the rock will have returned to equilibrium.

But why does the western shore not have it, because there was no glacier there.