Geod quiz 1 - Sheet1 Flashcards
definition of geodesy
the science of the measurement and mapping of earth’s surface
global geodesy includes:
the determination fo the shape and size of earth, its orientation in space, its gravity field
what is a geodetic survey
the determiniation of the earth’s surface and gravity field over a region (usually a country/ group of countries)
what needs to be considered in geodetic surveys?
the earth’s curvature and gravity field
what is plane surveying
when the details of the earth’s surface are determined on a local level (ignoring curvature and gravity effects)
geodesy applications
agriculture, aviation, space, public safety, survey and mapping, roads and highways
wht does tectonic geodesy include?
crustal deformation, plate boundaries, microplates, earthquakes, post seismic relaxation, volcanoes
hazards applications of geodesy
real time gnss/gps data, real time algorithims for seismic and volcanic hazards, sea level rise calculations, ice mass variations, land water storage
what do reference frames describe
orientation of the earth in space, earth’s surface geomoetry, the gravity field
what was the first idea about earth’s shape
a disk encircled by oceans (homer in the illiad, 800BC)
what shape did pythagoras propose?
spherical earth (580-500BC)
when was the spherical model accepted and why
around aristotle’s time (382-322BC), because of 1) the round shadow of earth in lunar eclipses and 2) the apparent rising of an aproaching ship on the horizon
who is the founder of geodesy and what did he do?
eratosthenes of alexandria, deduced the earth’s radius from measurements, developed arc measurement method. found that raus of sun descended vertically into a well in syene at summer solstice, then comapred to alexandria. estiamted 6366km (actual= 6367 km)
who suggested ellipsoidal earth
newton (ssuggested flattening of earth)
what are the WGS84 and ITRF?
really similar global ellipsoidal models. both are geocentric (relative to the center of earth’s mass), WGS= world geodetic service, ITRF = international terrestrial ref. frame
what is the NAD84
north americal ellipsoidal model (north american datum = NAD). NON geocentric
how do we take present day geodetic surveys?
1) control points (might be in shallow concrete) 2) geodetic benchmarks (installed in very stable bedrock or on a deep steel rod)
who is gladys west?
mathmatician that layed groundwork for gps. she made a model of the earth accounting for gravitational, tidal, and other forces
who were laplace, gauss, and bessel
dudes who determined that the ellipsoid was NOT accurate
how did laplace, gauss, and bessel figure our that the elliposoid wasn’t accurate?
they used the deflection of vertical (aka the deviation of a plumbline from the ellipsoidal normal)
what is the geoid and who defined it
the mathmatical surface of the earth. all points on a geoid have the same effective potential. gauss and bessel defined it
how is the geoid currently measured?
recent satellite missions (Gravity FIeld and Steady STate Ocean Circulation (GOCE) and GRACE)
what is a terrestrial refernce system
a spatial refernece system co-rotating with the Earth
what’s special about a TRS
positions of points attached to the solid earth have coords. which undergo only small variations with time (due to geophysical effects - tectonic or tidal deformations)
what are reference systems good for?
describing the positions and motions of the earth, other celestial bodies, artificial satellites, the surface fo the earth, the stationary and time variable parts of earth’s gravity field
what are refrence systems represented by?
coordiante systems
how does time play into reference systems?
the mutual motion of the earth and other celest. bodies, temporal variations of the earth’s shape, gravity field, and orientation
how are reference systems and frames related?
refernce systems are realized through reference frames. ref. systems are theoretical, ref. frames are REAL and ppl have access to them
what do reference frames consist of
a set of well determined, fixed points or objectss (given by their coords and -if necessary- their velocities)
what are time systems based on
processes of quantum physics, motions in the solar system, daily rotation of earth
what is the equation for flattening?
F = (a-b)/a
why are time systems important
need uniform time scale to: model motion of artificial satellites, describe motion of earth w/ respect to inertial space, date all measurements and results
what is the difference between control points and benchmarks
benchmarks are in bedrock (representative of tectonic motion), control points are in shallow conrete (measure shallow surface processes)
how does sea level factor into the geoid defintion?
Mean sea level is used ( without tides/ waves)
what is the difference between ref. systems and frames?
ref system = theoretical, frames = application (you tie measurements to reference frames)
what defined mass?
plank’s constant expressed in Joules
how is the realtization of the meter possible?
atomic clocks and GPS satellites
what is sexagesimal graduation
system of measurement using degrees, minutes, and seconds to measure angles and time
what are some key equations in sexagesimal graduation
1 degree = 60 arcmin, 1 arcmin= 60 arcsec, 2pie = 360, angle in degrees = (180/pie) angle in radians
what are other important constants in geodesy
speed of light, gravitational constant (henry cavendish figured out G)
what do most geodetic measurements use for positioning
time or frequency measurements of electromagnetic waves
what are forier’s theroem and equations
he said that any wave form in the time domain can be represented by the weighted sum of sins and cosines. F(v) = integral of neg infinity to infinity of f(t) times e^(-2pi times i times vt)
what is a big takeaway about general relativity
the observed gravitational effect between masses results from their warping of space time. so, systems of space time coords withing ht egravity field must be defined and used for modelling
are ref. systems defined within the frame of relatitivty theory
yes, now they are
when do you have to pay attention to relativistic effects?
when you are off of earth - satellite orbits, space geodetic observations (on earth you don’t need to worry about it)
what is the definition of the SI second?
86,400 s/ day
what time does the origin of international atomic time (TAI) correspond to?
Jan 1, 1958
what are some versions of universal time?
UT1 and UTC
what’s the difference between UT1 and UTC?
UT1: principal form of universal time, cocmputed of observations of distant quasars using long baseline inferometry. UTC; (coordinated universal time) approximates UT1, international standard for civil time, ticks SI seconds and is in step with TAI
do the UTC and TAI account for leap seconds?
yes
how do we get TAI?
by weighing the averages of time kept by over 400 atomic clocks (cesium) in different labs
what is terrestrial time? and what is it used for
an astronomical time standard ddefined by the international astronomical union. it’s used for the relativistic treatment of space geodetic techniques ALSO it is basically equal to international atomic time (TAI)
is terrestial time (TT) adjusted so that it agress with the SI unit of a second on the geoid?
yes
what is GPS time? (GPST)
runs parallel to TT, doesn’t have leap seconds, implemented by atomic clocks in the GPS ground control stations and GPS satellites. is ahead of UTC (bc no leap seconds)
how many dimenstions are ref. systems?
4d
what are refernece coordiante systems defined by?
the origin and orientation of planes or axes of a cartesian coord. system
how do you go from cartesian coordinates to spherical coordinates?
that matrix with cos(v)cos(lamda), etc.
what are earth centered earht fixed coordiantes (ECEF)
show the relationship to the geodetic lat, long, and height. consideres earth’s flattening
what is precession
a secular effect due to gravitation of the moon and sun on the equitorial buldge of earth. it rotates between vega and the north star. it creates torque
what does precession cause
the vernal equinox moves clockwise along the ecliptic with one rotation every 25,800 years.
what is nutation
rotation of the true pole about precession (precession is superimpsoed by nutation), slight fluxuation during precession due to small oscillations
what defines the rotation of the earth
a vector directed to the north pole of the instantaneous axis of rotation AND the angular velocity (w)
how long is one siderial day
86,164.10s
what is the equation for angular velocity
delta theta/ delta time (anngle/time)
what is a sidereal day
the time it takes for the earth to complete on erotation about its axis with respect to the ‘fixed’ stars (like the sun)
what causes the direction and magnitude of the rotational axis vector to change with time (with repect to solid earth)
time variable lunar, solar and planetary gravitation AND mass redistributions in the atmo and hydrospheres over tectonic plate movements AND post-glacial isostatic adjustment AND mantle convection DUE TO LIQUID OUTER CORE MOTION
what are the earth rotation parameters
polar motion (wobble) and the earth rotation angle
what is polar motion (wobble)
the motion of the earth’s rotation (spin) axis with repect to the earth fixed ref. system
what is the chandler wobble
a free oscillation period of abt 435 days
why does the chandler wobble occur?
the spin axis of the earth doesn’t aligh exactly witht he polar axis of moment moment of inertia
what is the annual wobble? why does it occur?
a wobble that superimposes the chandler wobble. it is caused by mass redistributions and mass motions in the earth
what is the secular motion wobble? why does it occur?
supeimposes chandler wobble. due to glacial isostatic adjustment. ALSO caused by sea level changes, large scale tectonic movements, mass schifts inthe earth’s interior, and polar ice melting
what casues periodic, quasiperiodic, and irregular variations with respect to polar motion
ocean tides, atmospheric and hydrologic processes, oceanic and continental water variations
what casues other free motions with respect to polar motion
the misalignment of rotational axis and figure axes. related to the flattening of the earth’s mantle, inner, and outer core
what do earth orientation parameters describe and how many are there
the irregularities of the earth’s rotation. there are five (celestial pole offsets, pole coordinates, universal time (UT1))
what is the ephemerides
the positions of naturally occuring and artificial satellites in the sky at a given time
what is a celestial reference system characterized by (CRS)
newton’s laws of motion
what is a dynamical CRS based on? and how have they been realized?
the ephemerides of solar system bodies. they’ve been realized through various optical measurements and radio data
what is a kinematic CRS defined by?
the positons and proper motions of stars or quasars
what ar the X Y and Z coords in a CRS and what do they entail
Z coincides with earth’s rotational axis (north pole), X and Y axis span equitorial plane (y on equator, x pointing out from equator- like radius). X points to the Vernal Equinox
what is the vernal equinox
the point on the celestial sphere where the path of the sun crosses the celestial equator from south to north in March.
what are hour circles
great circles perpendicular to the celest. equator
whar are celestial parallels
small circles parallel to the equator
what are declination and right ascention
declination: angle above/ below the equitorial plane. right ascention= angle in equitorial plane measured CW from vernal equinox
how is a terrestial ref system represented by
3D cartesian coords (X,Y,Z)
what iss the geocenter
the earth’s center of mass (including atmosphere and oceans)
how are the axes aligned in the terrestial reference system?
Z axis aligned with mean terrestial north pole, mean equitorial plane is perp to Z axis and includes X and Y axes
what is a terrestial system used to do?
describe positions on/ clsoe to the earth’s surface, national surveys, geoinformation systems, navigation, a frame for determining earth’s gravity field and other geophysical properties
what celestial frame is used?
represented by the IAU; international celestial reference frame
what is the international terrestrial reference system
a world spatial reference system co-rotating with the earth. it provides a set of coords of some points on earth’s surface
who maintains the world spatial reference system
intrantional earth rotation and reference systems service (IERS)
how many satellites do GPS, Galileo, and Glonass have
Gps has 6, the other two have 3
what system has the most satellites
galileo (27), gps has 24 and glonass havs 21
how do we utelize satellite laser ranging
satellites orbit earth, control stations on earth detect laser with satellite, then we can calulate the distances betweeen control stations
what is DORIS
a french system where sations on earth send signals to satellites
what is the International Terrestial Reference Frame and what does it do
the mathmatical realtization of the ITRS. it provides coords in geocentric ECEF cartesian positions (as well as their 3D velocities)
what is a helmert transformation
transformation between different systems consisting of a shift in origin, change in orientation, and scale factor
what ITRF is used now?
ITRF2020 (some ppl still use ITRF14)
when does ITRF improve
when the number of sites with long time series increases, new techniques appear, estiamation procesures are improved
why do we use the ITRF?
because it provides the msot accurate global ref. frame available, GNSS orbits use it, if everyone uses it then it’s easy to comapre data bc it’s standardized
what is the no net rotation frame (NNR) and what does it do
represents velocities without referrring to a certain plate. it’s achieved by aligning the velocity field to the HORIZONTAL movement s of a plate tectonics model. HAS NO IMPACT ON RELATIVE PLATE VELOCITIES
what are fixed plate, regional, and local reference frames
used for defining relative plate motions
how do you chose a reference frame
based on geophysical objectives, maybe like the local surroundings of a volcano, one side of a fault, or the upper plate of a subduction zone
what is the major plate reference frame
relative to eurasia, nubia, NA, SA, etc plates.
what are regional ref. frames
smaller coherent regions (central valled of CA)
what are local ref. frames
sites near (but outside the influence of) a volcano, geothermal field. etc.
