Gravity Station - What Is It?

[+bicknell]

Can someone explain a gravity station in brief non-surveyer's terms? I understand a triangulation station (horizonal) and a benchmark (verticle). What's the purpose of a gravity station? I'm sure it has something to do with measuring gravity . What does gravity have to do with mapping and surveying?

[+Black Dog Trackers]

I did a search on gravity here and found this and this and this. They have links for further reading.

[+bicknell]

I only sort of understand. I mean, I understand measuring gravity all over, and I understand that it varies. What I don't understand is what impact this has on understanding the geoid and making maps and such. Sounds like something a geologist would be more interested than a surveyer.

[+Klemmer]

The US Department of Defense (and other similar organizations worlwide), as well as NASA, have spent many millions of dollars mapping the gravity variations worldwide. It is necessary to know this information for accurate orbital calculations. In fact, even the GPS constellation of satellites needs it! GPS would be useless if the control system didn't know the exact gravity data worldwide, since it effects the orbits. NGS data was fed to NASA, DOD and others. May still be, but it's not something that doesn't change much, as I understand it, barring major rapid earth movements (i.e. earthquakes). All part of mapping, just a different part.

[evenfall]

Bicknell,

 

You said "What I don't understand is what impact this has on understanding the geoid and making maps and such. Sounds like something a geologist would be more interested than a surveyer."

 

The impact is that Gravity distorts the shape of the earth. See The Earth, as you may know is not a perfect Sphere, it spins on it's axis and this centrifugal force distorts the Sphere into the shape of an Ellipsoid, and well, it doesn't stop there. This ellipsoid is further acted upon by the force of gravity, or distorted by gravity, so much so that we have found that the ellipsoid is not smooth, it is lumpy. This really goes beyond geology, and in fact it is part and parcel a determiner of geologic development. Further we have found that gravitational forces are not static, they change and is so doing the surface of the earth is constantly changing.

 

To add, The NGS, Decades before the DOD and NASA came along, was studying Gravity, and their work was used by DOD and NASA to aid the studies they have. A lot of what is studied and when goes along with the era, and the mission of the agency at the time. DOD first started studying Geodesy for Space purposes in the late 50's around 1959. The USAF had Geodetic Squadrons for about 35 years... Missiles you see, need to know where their landing strip is... Oh, and how to get there from well, you know.

 

How it matters to the Surveyor and the Map is that the map is supposed to be a replication or representation which defines the face of the Earth on a smaller scale, or what we should find and measure when we are at any given spot on a map. The Surveyor is the person who is responsible for taking the measurements. In the Chicken Egg theory, the Surveyor has a reference point, and in a way the map will try to represent it, but when they observe the point they may observe that the reference has changed. So not only does Gravity interest the Geologist, it interests the Surveyor and the Geodesist as well. It affects everything in what seems like a relative uniform way. When precisely measure it, it really isn't as uniform as we think.

 

The way we find that the gravitational references affect things the most are in the vertical. The vertical changes can of course affect horizontal locations as well, but the grid we measure with is fixed, it doesn't move. he earth moves. Latitude and Longitude are more stable than the surface of the earth. When we compare movements of the earth as compared to the static measure of Latitudes and longitudes as well as Orthometric, Ellipsoidal and Geoidal heights, We quickly see that the Earth is always on the move and that very much matters to all the different scientists and observers involved. We study it to keep track of all the points on the Earth which we have assigned importance to, and, there are a lot of them.

 

If I can help any further Please ask, and in the meantime, if you are interested in some of the science behind it, the NGS website has Many papers on Gravity and Geoidal work, and Google can hunt up more if you are interested. It really is more interesting and better reading than you might think.

 

Rob

Edited March 23, 2005 by evenfall

[+Klemmer]

Well said. Thanks, Rob. I sorta knew that in general terms, but you said it so well! You write for a living?

[+Renegade Knight]

... the grid we measure with is fixed, it doesn't move. he earth moves. Latitude and Longitude are more stable than the surface of the earth. ...

Just out of curiosity, how do you Fix the location of the reference system that doesn't change when everthing else does? I can see the value of such a frame of reference but it's just not clear how you can fix it so you know it's the same when everthing else isn't.

[evenfall]

Renegade,

 

You probably already understand this but it is worth a refresher...

 

In the case of Latitude you have : The angular distance north or south of the earth's equator, measured in degrees along a meridian, as on a map or globe.

 

In the case of Longitude you have: Angular distance on the earth's surface, measured east or west from the prime meridian at Greenwich, England, to the meridian passing through a position, expressed in degrees (or hours), minutes, and seconds.

 

So the equator fixes the frame of reference and the Sun is what determines the Equator. The Poles of the Earth fix the other frame of reference. The system of measuring all this is called a Polar Coordinate System, which is a grid that allows angular measurements on a curved surface and rotates with the Earth. Circular Trigonometry is used to figure it out. It is not dependent on the surface of the earth as a reference, but can and does allow for measurements which are both of Earth and moving on earth.

 

The equator and the poles are where they are and the earth moves rotationally through and around them. The surface of the earth moves in a manner relative to this grid largely due to Tectonic Plate movement. This would include Earthquake Fault lines such as the famous San Andreas, Gravitational changes and shifts, Volcanic Movement and explosions, (remember the current swelling under Mt. St. Helens and the explosive results of the same?) Geological movements such as sinking and slipping such as Glacial and Mud slides, and believe it or not sometimes dissimilar earthen materials simply reach a point where they slip due to weight and pressure. These ultra huge earth slips have been attributed to tidal waves just as tectonic plate movement has and that leads us to other forces which are not from geological forces but can affect them are Huge Tidal Waves, Weather, Tidal actions and river erosion, not to mention the Human Factor's man made earth moving efforts.

 

All these factors affect the Earth Surface and are constantly causing changes to that surface relative to the frame of reference.

 

Klemmer,

 

Writing for a living is something I don't currently do, but I have been wondering if I would enjoy it. I do enjoy the writing that I currently do, and have been musing about the future of it a bit. I do enjoy helping out here in the forums.

 

Thanks for asking,

 

Bicknell,

 

As you know, I am not the Briefest writer around. I fear I may have already put the Wordy Surveyor upon your question. I sure wish the answers were simple, but the simplest answers seem incomplete and only sprout more questions...

 

I hope we went where you had hoped this would go.

 

Rob

Edited March 23, 2005 by evenfall

[holograph]

I only sort of understand.  I mean, I understand measuring gravity all over, and I understand that it varies.  What I don't understand is what impact this has on understanding the geoid and making maps and such.  Sounds like something a geologist would be more interested than a surveyer.

Elevation is tied to the geoid, and the geoid is defined to be the imaginary surface at which the force of gravity would be identical across the world -- very close to mean sea level (but not identical to it). In the olden days, there were several designated tidal stations that were used to measure sea level, and that height was carried into the interior by leveling.

 

In modern times, there are extremely sensitive instruments that can measure gravity, and so the geoid is determined directly from gravity measurements to greater accuracy than leveling. Additional data on gravity is obtained from precise measurements of satellite positions.

 

Once the gravity measurements are obtained, they are tied into the NGS database of benchmarks via a dataset called "GPS on Benchmarks" and a mathematical model of the geoid is created so the the variation between the geoid and sea level is a small as possible.

 

As time goes on, more data is available and the model gets better and better. You see a reference to the model on the NGS datasheet when you see GEOID HEIGHT and the geoid model is GEOID03.

 

It's kind of interesting to see which benchmarks were used as part of the dataset. You can get a list of those benchmarks (and learn more than you ever wished to know about geoids) by going to this NOAA web site

[+bicknell]

I think you guys have answered my question now. That's the level of detail I needed.

 

That leaves only one question. Why gravity station discs? Why not just use existing triangulation stations or benchmarks if all you needed was a repeatable spot to perform the experiment?

[+BuckBrooke]

Existing stations may not be where you want to measure, and you may want to create a regular grid of data collection points that doesn't include any existing stations.

[evenfall]

Bicknell,

 

Testing at the most basic of levels is something where you define a criteria for observing something such as some specific property a certain way on a certain thing and then you observe it the same way many times to gather the desired information. One way the data could be looked at is how the observations compare over time.

 

The old CGS Gravity Station is actually a triangulated station with values for what ever order horizontal survey it had, and additionally it was chosen as a place where gravimeter observations would be made and tracked. So we have triangulated the where of the testing, meaning this is a location where we want to make repeated observations and then we measure the gravity here at whatever the prescribed frequency of testing is prescribed.

 

There is no way to collect meaningful data if a stringent practice is not followed. The procedure has a standard and methodology that has to be followed so the results remain consistent. If we come across this sort of station in the field we will immediately know that it holds additional importance and data that other stations may not have, just from seeing the additional information on the disc. There are also Magnetic stations for monitoring geomagnetism as well.

 

Rob

[Z15]

I did not see if this was mentioned but marks used for Gravity Observations cannot have anything magnetic in them. No steel etc. It will cause the Gravimeter to give inaccurate results.

 

I have never done any of these observations but my old agency did several projects. I did some recon looking for marks for them to use.

 

 

gra·vim·e·ter  Audio pronunciation of "gravimeter" ( P )  Pronunciation Key  (gr-vm-tr, grv-m-)

n.

 

  1. An instrument used to measure specific gravity.

  2. An instrument used to measure variations in a gravitational field.

Edited March 24, 2005 by Z15

[+T-boys]

Thanks Rob for your excellent explanation. It was written so even a mechanical engineer could understand. While I was reading it I had a small epiphany. I never knew (thought about, cared) why polar coordinates were called polar. As an engineer I’m very familiar with the polar coordinate system. To me it was no different than using the Cartesian. Just use what ever information you had to get from one point to another.

 

I learned something new today. Thanks again.

[Prof. Y. Lupardi]

Let me tell you something about in my country the Netherlands. It is very flat here and part is below mean sealevel. And yes, it is very muddy here most of the time in most places but we do not walk in wooden shoes to keep dry feet. We have rubber boots nowadays

So height has some importance to us: where want the surplus water goto? A question of millimeters.

Thanks to gravity measurement we now have RDNAPTRANS™ 2004 with a new geoide.

In the past the measurement equipment was transported by train late in the evening and the stuff was setup on a railwaystation platform. This was the gravity station. When trains stopped travelling and industry came to relative rest then measurements were taken of something falling in vacuum with precise (atomic?) clocks for timing.

Modern technical developments make it possible to transport the gravity station by road.

(for British readers: there is also mention of a Gravity Station in Doctor Who)

 

The result of all their work is this picture where you can see that water in the North-east of the country on a distance of 40.25 m above the WGS84 ellipsoid is in equilibrium with water in the south of the country on a distance of 46.25 above the WGS84 height. This over a distance of 200 km.

NLGEO2004 (in meters) ten opzichte van GRS80 (ETRS89/WGS84)

[evenfall]

It is notable to add that Professor Lupardi made reference that The Netherlands uses the WGS 84 Datum, The GRS 80 Ellipsoid Model, and the NLGEO2004 Geoid Model to help adjust their observations locally.

 

Many Countries use the WGS 84 Datum as a "best fit" for the shape of the earth as opposed to developing their own. It is however a "global" datum and doesn't always do the best job of describing a local area to the highest precision without some help.

 

It is also important to note a few things we probably don't think about. WGS 84 is a Datum which is used both vertically and Horizontally at the same time. Other Countries, such as the United States developed and use different Datum which they feel is a better fit for their specific area. In the U.S. instance this is NAD 83 for horizontal, and NAVD 88 for vertical. Both NGS Datum are three dimensional, just like WGS 84, however the NAD 83 is adjusted in only the Horizontal plane, and NAVD 88 in the Vertical Plane. Europe has it's own Datum which some countries there elect to use. It is called ETRF 89 (European Terrestrial Reference 89)

 

In the case of the Netherlands, a country where the Professor indicated they are very interested in elevations, the NLGEO2004 is to them what NAVD 88 is to us in the vertical reference, and they also utilize ETRF89 to accurize European Tectonic Plate Shift on WGS 84. All the slight differences are accounted for in this way.

 

I want to take something the Professor said apart just a little so we have a better look at it...

 

He said, "The result of all their work is this picture where you can see that water in the North-east of the country on a distance of 40.25 m above the WGS84 ellipsoid is in equilibrium with water in the south of the country on a distance of 46.25 above the WGS84 height. This over a distance of 200 km."

 

Look at the image The Professor provided of the NLGEO2004 geoid model. First let us keep in mind that this is a 2004 snapshot of gravitational measurement. If we do this again in 2005 or 6 we will note changes. It is pretty common to update Local Geoid Models pretty frequently. The sea level, due to the gravitational differences is 6 meters higher in the south of the country, when compared to the ellipsoidal reference (GRS80) than it is in the north of the country, just 200 Kilometers away. This is a 19.68 foot change in 124.27 miles distance. By equilibrium he meant that this is sea level, yet look at the differences in elevation. even though it only represents .003 percent change over the distance involved, it is still 19+ feet! This is why Mean sea level is a poor frame for reference because it is not only inconsistent in many ways, with a 19 foot difference in the Netherlands alone how would they know the elevations within their country if they were to assumed sea level is consistant. simply put, Gravity isn't stable and affects everything!

 

Rob

Edited March 25, 2005 by evenfall

[evenfall]

T-Boys,

 

If you like, try thinking of it like this...

 

If we are thinking in terms of using a Cartesian coordinate system, we are trying to locate places which reside upon a surface which is flat or otherwise thought of as a Plane and could have a shape like a rectangle would have a shape or a triangle would... We could even use a Circle as long as it's surface is flat, cubic, and we could still have a plane surface if it is even cylindrical.

 

If we are thinking in terms of using a polar coordinate system, then we are no longer trying to work in the Flat, we are instead working in the spherical. We are trying to locate specific places on the surface of a sphere or ellipsoid We could be trying to mark out the shape of a rectangle or triangle, even a circle, but the surface of those shapes would be draped over the face of the sphere and brings a different way of locating places to the table.

 

I suppose we could also think of one system as usable in two and three dimensional on a plane and the other is three dimensional on a sphere or ellipsoid.

 

In another thought, I suppose we could take an acre of ground and suppose it is planar in it's manner and try to describe locations within this acre with cartesian coordinates and we would likely have a high degree of accuracy. But if we grow the acre to 100, 1000, or 10,000 acres, the cartesian formulas will begin to fail in their methods of accurately expressing locations because they will not compensate for the spherical shape of the Earth

 

Initially, before we had GPS, which we use to radio triangulate to measure the earth from space, we did so Optically. We used Triangulation, and as a part of the methodology of course, was that the line of sight would be considered Planar by nature. We were able to make the triangles fit the sphere by noting how high above the ground level at each end of the survey location we were when we performed the survey. Naturally the higher you are, the further you can see, and when the mathematical formulae, (Trig) does not have the vertical component for the instrument height the numbers won't fit right. In other words, the height of the light on both ends of the survey was a component of the mathematical formula that helped us figure the spherical components of horizontal location. Without knowing them, the survey won't properly close.

 

Rob

Edited March 25, 2005 by evenfall