Lecture 8 - (Authoritative) Geodetic Systems/Datums Flashcards
What needs to be defined for a geodetic datum?
The size and shape of the Earth and the origin and orientation of the coordinate systems (to model Earth). Used as the basis for coordinate systems in GIS, precise positioning systems and navigation systems.
What is a horizontal datum?
A model used to measure positions on the Earth, includes the NAD 2020
What is a vertical datum and its 3 types?
Defines the elevations of points on the sea level.
Tidal: the average water level at gauges over time
Gravimetric: based on a geoid
Geodetic: through a process of geodetic leveling among bench marks with at least one datum origin point to serve as the absolute level of the vertical datum.
Four defining parameters of a reference ellipsoid.
Semi-major axis, flattening factor of the earth, nominal mean angular velocity, geocentric gravitational constant.
What is the EGM96?
Earth Gravitational Model 1996. A geopotential model of the Earth consisting of spherical harmonic coefficients complete to degree and order 360. Used surface gravity data/calculate gravitational force and the vertical correction between the WGS84 ellipsoid and the true geoid.
Summarize CGVD2013.
Canadian Geodetic Vertical Datum of 2013. Defined by the equipotential surface (the costal mean sea level for North America). It came from an agreement between the US and Canada. Realized by the geoid model CGG2013. It provides the separation between the GRS80 ellipsoid and the surface in NAD83 reference frame.
Is it correct one does not need to perform any leveling observation anymore because of the release of CGVD2013?
No.
If CGVD28 has few meters accuracy and are rounded to closest meter, the difference between them can be ignored.
If CGVD28 heights are precise (<10cm) and cover a large region, the difference must be considered.
If CGVD28 heights are precise (<2cm) and cover a small region, the difference must be considered but generally the same difference is sufficient for the region.
Summarize the used geodetic datums in North America.
WGS60, WGS72, WGS84, NAD83
Do all GNSS use the same geodetic datums?
No. Each GNSS system has its own definition of ECEF terrestrial reference system (TRS). The system definitions are pretty much identical.
GPS (WGS84), GLONASS, Galileo, Beidou.
Datum = coordinate frame + reference ellipsoid.
What is the 7 parameter datum transformation?
3 translations / 3 rotations / 1 scale factor
What is Molodesky datum transformation?
7+3 parameter transformation between a local datum and a geocentric global datum.
How to determine as set of local datum transformation parameters?
Model type.
Summarize WGS84
World Geodetic System of 1984. An Earth centered, earth fixed terrestrial reference system and geodetic datum. Based on a consistent set of constants and model parameters describing Earth. Compatible with ITRS and follows the IERS.
What are the axis’ and origin of WGS84?
Origin: Earth’s center of mass.
Z-axis: The direction of the IERS Reference Pole (IRP)
X-axis: Intersection of the IERS Reference Meridian (IRM) and plane passing through the origin and normal to Z-axis.
Y-axis: Completes a RH, earth-centered, earth-fixed orthogonal coordinate system.
What are the axis’ and origin of WGS84?
Origin: Earth’s center of mass.
Z-axis: The direction of the IERS Reference Pole (IRP)
X-axis: Intersection of the IERS Reference Meridian (IRM) and plane passing through the origin and normal to Z-axis.
Y-axis: Completes a RH, earth-centered, earth-fixed orthogonal coordinate system.
