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Geodesy GME 108 > Gravity Effect, leveling, Earth's shape, size (Pres VI) > Flashcards

Gravity Effect, leveling, Earth's shape, size (Pres VI) Flashcards

1
Q

What is relative vertical positioning?

A

Determining the vertical position of one point with respect to another.

  • with determination of height difference
  • trigonometric leveling including deflection of the vertical
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2
Q

Why do we need deflection of the vertical?

A

To transform between the geoid & ellipsoid

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3
Q

What is the geodetic network?

A

It’s a set of points representing terrain, then monumented and determine position in some known coordinate system

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4
Q

What are geodetic height networks also known as?

A

Vertical network

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5
Q

How is Geodetic height network (1D) defined?

A

By H, which is the height above sea level and orthometric height,

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6
Q

Where is the orthometric height defined from?

A

Mean sea level

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7
Q

How is the horizontal network (2D) defined?

A

With known horizontal coordinates. Height is only weakly determined.(only needed to transform from surface to reference ellipsoid)

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8
Q

Where is raw GPS height measured from?

A

Ellipsoid

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9
Q

What is the 3D network?

A

x, y, and z combined in the Geodetic network

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10
Q

What are monument points called in Geodetic height networks?

A

Benchmarks

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11
Q

How are measurements made in Geodetic height networks?

A

Using trigonometric height difference or differential leveling

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12
Q

Which coordinates are very accurate and which are approximate in Geodetic height networks?

A

Heights are very accurate and horizontal coordinates are apprx

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13
Q

Height are usually measured from where in Geodetic height networks?

A

Mean sea level or more precisely the Geoid

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14
Q

What is the geoid called within a Geodetic height network?

A

Vertical geodetic datum

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15
Q

What is a datum?

A

A surface on which a coordinate has a constant value

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16
Q

What does H equal on the Geoid

A

0

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17
Q

How is mean sea level established?

A

Using tide gauges. Working inland from them

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18
Q

Where are leveling lines usually located due to the nature of the instruments?

A

Along roads or railways

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19
Q

How are height networks classified?

A

According to their order of accuracy…1st order, 2nd order, etc

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20
Q

What is the US horizontal datum?

A

NAD83

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21
Q

What is the US vertical datum?

A

NAVD88

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22
Q

What does GLOSS stand for?

A

The Global Sea Level Observing System

23
Q

What is the Global Core Network (GCN) of GLOSS and what is it for?

A

It has 290 sea level stations around the world for long term climate change and sea level monitoring

24
Q

What is a horizontal network?

A

Monumented points whose geodetic coordinates (x,y) are known with respect to some reference ellipsoid

25
Q

What is the ellipsoid called in the case of horizontal networks?

A

Horizontal geodetic datum

26
Q

Why do separate vertical and horizontal networks exist?

A

Because of the historical development of different instruments and techniques for determining heights and horizontal coordinates

27
Q

What are some disadvantages of 3D networks?

A
  • incomplete in themselves

- few common monuments

28
Q

What is another why to define a Datum?

A

Using a set of 8 constants

  • 2 for dimensions of ellipsoid (a,f)
  • 3 for location of origin of system (xo yo zo)
  • 3 for orientation (xyz)
29
Q

How to define the North American Datum?

A

Coordinate system and set of all points and lines whose coordinates, lengths, and directions have been determined by measurement or calculation

30
Q

Which ellipsoid did NAD27 use? And where is initial point

A

Clarke 1866

Meades Ranch, Kansas

31
Q

Why the update from NAD27 to NAD83?

A
  • The network expanded a lot since 1933
  • EDM was added/doppler satellite
  • Clarke 1866 was outdated
  • NAD27 was using locally best fitting ellipsoid, whereas NAD83 was using globally best fitting
32
Q

Which reference system does NAD83 use?

A

GRS80

33
Q

What is the National Adjustment of 2011?

A

Lat and longs adjusted for passive (in ground) monuments, but NOT orthometric height.

34
Q

What is the NGVD29?

A

A vertical control datum established for vertical control in the US. Mean sea level fixed by 26 tide gauges

35
Q

What is the NAVD88

A

The vertical control datum established in 1991 for Canada-Mexico-US.

36
Q

Where is the origin point of NAVD88?

A

Father Point, Quebec, Canada

37
Q

Why switch from NGVD29 to NAVD88?

A
  • Many km of leveling has been added.
  • 1000s benchmarks have been destroyed.
  • Crustal motion, glacial rebound, distortion of ground, and subsidence have moved monuments
38
Q

National Spatial Reference System of USA

A

800,000 control survey points in US
80 CORS pts (coast)
FBN (federal base network) - 100km apart monuments across US. Very precise
Cooperative based network - between FBN points 25-50 km apart

39
Q

What is the Geoid?

A

A physical surface at mean sea level

40
Q

Is Geoid same as mean sea level?

A

They don’t quite coincide in real life due to variations in tide changes from temp, salinity, winds, currents

41
Q

What is sea surface topography?

A

Departure of actual mean sea level from the geoid

42
Q

What is the most accurate way to define Geoid?

A

The best fitting equipotential surface to mean sea level

43
Q

Where is the height of sea surface topo from?

A

Geoid

44
Q

Range of geoidal height on global scale?

A

200m

45
Q

Datum at the tide gauges?

A

Orthometric

46
Q

Not a mathematical surface?

A

Geoid

47
Q

Physical surface?

A

Geoid

48
Q

Designed to appx the physical surface of earth?

A

Telluroid

49
Q

Quasi-Geoid

A
  • Quasi and geoid coincide in open sea

- Math surface - which is why is not equipotential

50
Q

What is height above ellipsoid called?

A

Quasi-geoid height or height anomaly

51
Q

What is more precise than Geoid?

A

Quasi-geoid

52
Q

Telluroid

A

Needed for physical geodesy
Not equipotential
Designed to appx earth’s surface

53
Q

Quasi-Geoid

A

Can be derived without assumptions about near surface mass distributions

Geodesy GME 108 (4 decks)

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