Gravity Effect, leveling, Earth's shape, size (Pres VI) Flashcards
What is relative vertical positioning?
Determining the vertical position of one point with respect to another.
- with determination of height difference
- trigonometric leveling including deflection of the vertical
Why do we need deflection of the vertical?
To transform between the geoid & ellipsoid
What is the geodetic network?
It’s a set of points representing terrain, then monumented and determine position in some known coordinate system
What are geodetic height networks also known as?
Vertical network
How is Geodetic height network (1D) defined?
By H, which is the height above sea level and orthometric height,
Where is the orthometric height defined from?
Mean sea level
How is the horizontal network (2D) defined?
With known horizontal coordinates. Height is only weakly determined.(only needed to transform from surface to reference ellipsoid)
Where is raw GPS height measured from?
Ellipsoid
What is the 3D network?
x, y, and z combined in the Geodetic network
What are monument points called in Geodetic height networks?
Benchmarks
How are measurements made in Geodetic height networks?
Using trigonometric height difference or differential leveling
Which coordinates are very accurate and which are approximate in Geodetic height networks?
Heights are very accurate and horizontal coordinates are apprx
Height are usually measured from where in Geodetic height networks?
Mean sea level or more precisely the Geoid
What is the geoid called within a Geodetic height network?
Vertical geodetic datum
What is a datum?
A surface on which a coordinate has a constant value
What does H equal on the Geoid
0
How is mean sea level established?
Using tide gauges. Working inland from them
Where are leveling lines usually located due to the nature of the instruments?
Along roads or railways
How are height networks classified?
According to their order of accuracy…1st order, 2nd order, etc
What is the US horizontal datum?
NAD83
What is the US vertical datum?
NAVD88
What does GLOSS stand for?
The Global Sea Level Observing System
What is the Global Core Network (GCN) of GLOSS and what is it for?
It has 290 sea level stations around the world for long term climate change and sea level monitoring
What is a horizontal network?
Monumented points whose geodetic coordinates (x,y) are known with respect to some reference ellipsoid
What is the ellipsoid called in the case of horizontal networks?
Horizontal geodetic datum
Why do separate vertical and horizontal networks exist?
Because of the historical development of different instruments and techniques for determining heights and horizontal coordinates
What are some disadvantages of 3D networks?
- incomplete in themselves
- few common monuments
What is another why to define a Datum?
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)
How to define the North American Datum?
Coordinate system and set of all points and lines whose coordinates, lengths, and directions have been determined by measurement or calculation
Which ellipsoid did NAD27 use? And where is initial point
Clarke 1866
Meades Ranch, Kansas
Why the update from NAD27 to NAD83?
- 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
Which reference system does NAD83 use?
GRS80
What is the National Adjustment of 2011?
Lat and longs adjusted for passive (in ground) monuments, but NOT orthometric height.
What is the NGVD29?
A vertical control datum established for vertical control in the US. Mean sea level fixed by 26 tide gauges
What is the NAVD88
The vertical control datum established in 1991 for Canada-Mexico-US.
Where is the origin point of NAVD88?
Father Point, Quebec, Canada
Why switch from NGVD29 to NAVD88?
- Many km of leveling has been added.
- 1000s benchmarks have been destroyed.
- Crustal motion, glacial rebound, distortion of ground, and subsidence have moved monuments
National Spatial Reference System of USA
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
What is the Geoid?
A physical surface at mean sea level
Is Geoid same as mean sea level?
They don’t quite coincide in real life due to variations in tide changes from temp, salinity, winds, currents
What is sea surface topography?
Departure of actual mean sea level from the geoid
What is the most accurate way to define Geoid?
The best fitting equipotential surface to mean sea level
Where is the height of sea surface topo from?
Geoid
Range of geoidal height on global scale?
200m
Datum at the tide gauges?
Orthometric
Not a mathematical surface?
Geoid
Physical surface?
Geoid
Designed to appx the physical surface of earth?
Telluroid
Quasi-Geoid
- Quasi and geoid coincide in open sea
- Math surface - which is why is not equipotential
What is height above ellipsoid called?
Quasi-geoid height or height anomaly
What is more precise than Geoid?
Quasi-geoid
Telluroid
Needed for physical geodesy
Not equipotential
Designed to appx earth’s surface
Quasi-Geoid
Can be derived without assumptions about near surface mass distributions
