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# Questions tagged [geodesy]

Geodesy is the science of accurately measuring and understanding three fundamental properties of the Earth: its geometric shape, its orientation in space, and its gravity field as well as the changes of these properties with time.

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### How to compute spherical harmonics coefficients using orthogonality?

I use the lunar gravity model GRGM900C to calculate the gravitational potential across a $360{^\circ}*360{^\circ}$ grid. This calculation follows the equation: V(\varphi,\lambda)=\sum_{\mathrm{n=0}}^...
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3 votes
1 answer
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### Is the surface of the Indian ocean really 106 meters below sea level?

CNN's July 24, 2023 There is a ‘gravity hole’ in the Indian Ocean, and scientists now think they know why links to the Geophysical Research Letter (Pal & Ghosh 2023) How the Indian Ocean Geoid Low ...
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4 votes
1 answer
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### What is the pitch and roll in a tiltmeter sensor?

I wonder if you could help me to understand what the pitch and roll mean for an tiltmeter sensor? I have tiltmeter data and I don´t understand at all how it works. If you can recommend literature on ...
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2 votes
1 answer
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### Is there a way to intuitively explain that the North of Sweden is closer to Australia than the South?

For reasons unknown, I started pondering what cities/towns in Sweden and Australia are the closest to each other. My hunch was that some town in the South such as Malmö or Karlskrona would be closest ...
• 392
2 votes
1 answer
127 views

### How to find out if three points on Earth are on the same great circle arc?

I am trying to figure out whether 3 particular points on Earth's surface are on the same arc (of a great circle) or not. Using Google Earth's (also ArcGIS Earth's) line tool, I see that the points are ...
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4 votes
2 answers
65 views

### What is the right term to describe the observation that your true bearing changes if you move in a non-cardinal direction on a globe?

I just came across a question on skeptics.SE that asked whether it is possible to sail a boat from the coast of Pakistan to the east coast of Russia in an uninterrupted straight line. A quick glance ...
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6 votes
2 answers
6k views

### Is it a coincidence that the circumference of the Earth in kilometers is almost $2^{12}$?

The circumference of the Earth has been measured to be $40,075 \,\pu{km}$, which is only $21 \,\pu{km}$ from $40,096 = 2^{12}$. For reference, $40,075 \approx 2^{11.992}$. This is probably a strange ...
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1 vote
0 answers
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### Does the magnitude of gravity stay the same at all points along the geoid [closed]

And how does this differ from the gravitational potential along the geoid (which is equal).
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1 vote
0 answers
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### Has the pear shape of the earth been measured?

I have heard Neil degrasse Tyson say the earth is pear shaped but I can’t find any source for this. We all know the earth has an equatorial bulge because it spins but Neil says “it’s slightly wider ...
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1 vote
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### How does one compute an ECEF vector? [closed]

I know how to compute an n-vector given the latitude and longitude of a location. I want to compute the ECEF (Earth-centered, Earth-fixed) vector of two positions so that I can subtract those vectors ...
26 votes
2 answers
6k views

### Is there a name for the great circle where latitude and longitude are equal?

Is there a name for the great circle where latitude and longitude are equal? I have attempted a google search but only the equator and the prime meridian are defined in the sources I can find. ( It is ...
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1 vote
2 answers
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### Centripetal Force Perfect Ball and Water

I made a question in WolrdBuilding as I was directed to. There a user said that with a perfect ball all water would pool to the equator because of centripetal force. What I do not understand is how ...
1 vote
1 answer
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### Possible cavern?

In eastern Ontario, Canada, there's a place where, when walking and stomping, you can hear a hollowness in the ground. The area is primarily igneous rock with many large boulders, and the spot in ...
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2 votes
1 answer
112 views

### If the accuracy of measurement of the earth's volume is not known, how can scientists assert that the earth is not expanding?

Volcanic eruptions, undersea earthquakes, fissures in the crust, meteoroid accumulation could all be seen as empirical evidence of expansion of the earth. Sink holes and other phenomena could be ...
3 votes
1 answer
383 views

### Great circle in UTM coordinates

I've got a question about calculation of great circles in Cartesian coordinates: Is it possible to compute points along a great circle in UTM coordinates, i.e. given two points P1(easting1, northing1)...
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84 votes
7 answers
59k views

### What place on Earth is closest to the Sun?

What is the location on Earth that is closest to the Sun? I've seen this question asked many times, and answered in varied and contradictory ways: The most common answer is “the summit of Chimborazo ...
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0 votes
1 answer
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### I need help with these geode like rocks [closed]

I found these rocks in my pasture and was excited because I thought I had found some geodes. The rocks were in a sandy bank and looked like eggs sticking out of the bank. I am located in Ness County ...
3 votes
1 answer
73 views

### Plate boundary lon/lat data

I'm working on a map in GMT and am having trouble finding lon/lat data for the minor plate boundaries (specifically the adriatic). Is there a database I can refer to?
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4 votes
0 answers
84 views

### What gravitational evidence do we have to counter the idea that oceans are formed when continents sink?

Is there some evidence for this based on gravity anomalies? I think I know how to tell if a mountain or structure is compensated based on bougeur and free air anomalies, but this really has me ...
9 votes
2 answers
1k views

### Terrain correction for gravity data using Nagy, 1973

I am working with an algorithm for terrain correction of gravity data. I am using Nagy, 1973. However, I could not get the results I need. So, this is the problem. I have this function where x,y,z ...
6 votes
1 answer
251 views

### Why do we measure eustatic sea-level from the center of earth?

Earth is rotating so it has a ellipsoidal shape. The measurement will therefore be different from equator and pole. So why do we measure eustatic sea-level from the center of earth?
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19 votes
2 answers
657 views

### To what accuracy and precision do we know the volume of the Earth?

Satellites such as GOCE and GRACE measure the geoid with unprecedented accuracy. Altimeters can determine local surface elevation with millimetre-precision. This makes me wonder: to what precision ...
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12 votes
1 answer
242 views

### Do normal modes of oscillation cause permanent deformation?

It is known that when a large earthquake occurs, say $M \ge 9.0$, The surface waves travel around Earth over and over, "ringing the surface like a bell". The GIF below is an example (Image Source): ...
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17 votes
3 answers
13k views

### Why is Earth not a sphere?

We've all learned at school that the Earth was a sphere. Actually, it is more nearly a slightly flattened sphere - an oblate ellipsoid of revolution, also called an oblate spheroid. This is an ellipse ...
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