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

Question 1

What is relative vertical positioning?

Answer 1

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

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

Question 2

Why do we need deflection of the vertical?

Answer 2

To transform between the geoid & ellipsoid

Question 3

What is the geodetic network?

Answer 3

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

Question 4

What are geodetic height networks also known as?

Answer 4

Vertical network

Question 5

How is Geodetic height network (1D) defined?

Answer 5

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

Question 6

Where is the orthometric height defined from?

Answer 6

Mean sea level

Question 7

How is the horizontal network (2D) defined?

Answer 7

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

Question 8

Where is raw GPS height measured from?

Answer 8

Ellipsoid

Question 9

What is the 3D network?

Answer 9

x, y, and z combined in the Geodetic network

Question 10

What are monument points called in Geodetic height networks?

Answer 10

Benchmarks

Question 11

How are measurements made in Geodetic height networks?

Answer 11

Using trigonometric height difference or differential leveling

Question 12

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

Answer 12

Heights are very accurate and horizontal coordinates are apprx

Question 13

Height are usually measured from where in Geodetic height networks?

Answer 13

Mean sea level or more precisely the Geoid

Question 14

What is the geoid called within a Geodetic height network?

Answer 14

Vertical geodetic datum

Question 15

What is a datum?

Answer 15

A surface on which a coordinate has a constant value

Question 16

What does H equal on the Geoid

Answer 16

0

Question 17

How is mean sea level established?

Answer 17

Using tide gauges. Working inland from them

Question 18

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

Answer 18

Along roads or railways

Question 19

How are height networks classified?

Answer 19

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

Question 20

What is the US horizontal datum?

Answer 20

NAD83

Question 21

What is the US vertical datum?

Answer 21

NAVD88

Question 22

What does GLOSS stand for?

Answer 22

The Global Sea Level Observing System

Question 23

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

Answer 23

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

Question 24

What is a horizontal network?

Answer 24

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

Question 25

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

Answer 25

Horizontal geodetic datum

Question 26

Why do separate vertical and horizontal networks exist?

Answer 26

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

Question 27

What are some disadvantages of 3D networks?

Answer 27

- incomplete in themselves | - few common monuments

Question 28

What is another why to define a Datum?

Answer 28

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)

Question 29

How to define the North American Datum?

Answer 29

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

Question 30

Which ellipsoid did NAD27 use? And where is initial point

Answer 30

Clarke 1866 | Meades Ranch, Kansas

Question 31

Why the update from NAD27 to NAD83?

Answer 31

- 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

Question 32

Which reference system does NAD83 use?

Answer 32

GRS80

Question 33

What is the National Adjustment of 2011?

Answer 33

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

Question 34

What is the NGVD29?

Answer 34

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

Question 35

What is the NAVD88

Answer 35

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

Question 36

Where is the origin point of NAVD88?

Answer 36

Father Point, Quebec, Canada

Question 37

Why switch from NGVD29 to NAVD88?

Answer 37

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

Question 38

National Spatial Reference System of USA

Answer 38

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

Question 39

What is the Geoid?

Answer 39

A physical surface at mean sea level

Question 40

Is Geoid same as mean sea level?

Answer 40

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

Question 41

What is sea surface topography?

Answer 41

Departure of actual mean sea level from the geoid

Question 42

What is the most accurate way to define Geoid?

Answer 42

The best fitting equipotential surface to mean sea level

Question 43

Where is the height of sea surface topo from?

Answer 43

Geoid

Question 44

Range of geoidal height on global scale?

Answer 44

200m

Question 45

Datum at the tide gauges?

Answer 45

Orthometric

Question 46

Not a mathematical surface?

Answer 46

Geoid

Question 47

Physical surface?

Answer 47

Geoid

Question 48

Designed to appx the physical surface of earth?

Answer 48

Telluroid

Question 49

Quasi-Geoid

Answer 49

- Quasi and geoid coincide in open sea | - Math surface - which is why is not equipotential

Question 50

What is height above ellipsoid called?

Answer 50

Quasi-geoid height or height anomaly

Question 51

What is more precise than Geoid?

Answer 51

Quasi-geoid

Question 52

Telluroid

Answer 52

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

Question 53

Quasi-Geoid

Answer 53

Can be derived without assumptions about near surface mass distributions