scope of the geodesy

Sunday, September 18, 2011

Geodesy, the science of precise measurement of the Earth's shape and gravitational field, has at its root many of the methods employed by the local surveyor or cartographer, but is distinguished from these by the scale of interest: in general geodesy refers to the analysis of observations made over distances at which the curvature of the Earth becomes significant. Modern geodesy utilizes surface-to-surface and ground-based astronomical observations, and increasingly also data from signals transmitted and received by artificial Earth-orbiting satellites, to quantify the form of our planet.

Geodesy and gravity

Physical geodesy is that branch of the subject concerned with the measurement of the Earth's gravity field and the geoid, the surface of equal gravitational potential that in the oceans is close to mean sea level. On the planetary scale the geoid is approximated by an ellipsoid of revolution about its minor axis with a degree of flattening of around 1 part in 300 caused primarily by the Earth's rotation. Permanent deviations of the geoid from an ellipsoid occur as a result of non-uniform mass distribution within the Earth, and also temporarily over the oceans by up to 1 m in response to winds, tides, and ocean currents. Both types of perturbation may be of interest to Earth scientists, but to geodesists the term ‘geoid’ refers to the stable long-term features of the equipotential surface. Geoids are expressed in terms of the geoid–ellipsoid separation or geoid height, the perpendicular distance to the geoid from a given reference ellipsoid. Historically, different countries tended to use their own reference ellipsoids with differing origins and major and minor axes, chosen so as to minimize the geoid height in the region of interest, but the increasing use of space geodetic data makes it more appropriate nowadays to use a common global reference ellipsoid such as WGS-84 or GRS-80 with its origin at the centre of mass of the Earth. In this case, geoid heights may reach several tens of metres.

The geoid can be derived from gravity measurements at known positions over land or sea, or more directly by using precise radar altimetry readings taken from low-orbiting satellites over the oceans. The latter method has proved particularly efficient in determining the short-wavelength components of the geoid. A satellite orbiting at an altitude of 100 km and emitting microwave radar pulses with a beam width of 1° will illuminate patches of the sea surface with radii of about 3–5 km. The radar altimeter measurement is affected by the roughness of the ocean surface and by atmospheric refraction, and so the accuracy of the measurements is limited to approximately 0.2 m, comparable with the precision to which the satellite's position can be tracked. In contrast, obtaining a gravimetric geoid from terrestrial gravity measurements is much more complicated and error-prone. The spatial resolution of gravimetric geoids is limited to length-scales smaller than the area over which gravity observations are given, and the observations themselves must be subjected to elaborate corrections to remove the effects of matter situated above the points at which gravity was measured.

Terrestrial gravity measurements can be made in an absolute sense by measuring the time for a pellet to fall a fixed distance in an evacuated chamber, or in a relative sense by using mechanical force-balance instruments. Relative gravimeters can be of the static variety, in which the change in gravity from the nominal value displaces a mass on a spring by an amount proportional to the change, or the more accurate astatic variety in which the displacement for a given gravity perturbation is much larger. The mean value of the acceleration due to gravity at the Earth's surface is approximately 9.8 m s−2. Gravity anomalies are commonly expressed in units of gal (named after Galileo; 1 gal = 0.01 m s−2) or gravity units (1 gu = 10−6 m s−2 = 0.1 mgal). The precisions of modern astatic relative gravimeters are of the order of 0.1–0.01 mgal but are subject to long-term drift in excess of 0.05 mgal/month, whereas absolute gravimeters can reach accuracies of 0.001 mgal and do not suffer from temporal drift. Seaborne gravity observations are less accurate than those on land, first because of the pitch and roll of the ship, which cause unwanted accelerations of the gravimeter (although gyroscopic stabilizing platforms can limit these), and secondly because of the Eötvös effect. This effect is caused by the east–west component of the ship's velocity, which modifies the centrifugal acceleration due to the normal rotation of the Earth about its axis. An uncertainty in ship speed of 1 km hr−1 results in an error in the Eötvös correction of up to 4 mgal at the Equator and less at higher latitudes. With modern GPS navigation systems (see below) providing ship velocity, accuracies one order of magnitude better than this can easily be achieved. Airborne gravity measurements also require an Eötvös correction and are significantly degraded by vibration and acceleration of the aircraft. Another problem with airborne measurements is that the aircraft's altitude must be known to within 1 m to attain gravimetric precision of the order of 1 mgal.

Positions and reference frames

Fundamental to positional geodesy is the reference frame in which the resulting coordinates are expressed. Early marine navigators could compute their astronomic latitude relatively easily, by observing the highest and lowest angles above the horizon attained by the Sun and other stars. Astronomic longitude is a slightly trickier problem because an accurate determination relies on the ability to ascertain the local time with respect to the time at the origin of longitude (by modern convention the Greenwich meridian). Until accurate chronometers were developed in the eighteenth century, this could be achieved only by means of complex calculations based on the eclipses of stars and planetary bodies. Although astronomical measurements can give independent positions at any point on the Earth, their disadvantage is that the observations are taken with respect to the local vertical (the direction of the gravitational force, perpendicular to the geoid). Because the geoid is not a perfect ellipsoid and undulates gently, the directions of the vertical in two places can in fact be parallel, resulting in identical astronomical coordinates for the two locations. Correction must be made for this deflection of the vertical to give geodetic coordinates (latitude, longitude, and height above the ellipsoid); these coordinates, being relative to a given ellipsoid, are unique. The parameters of the ellipsoid used must then be specified together with the coordinates.

Because, as described above, many countries used their own ellipsoidal parameters (major and minor axis radii and location of the centre) to achieve as close as possible a match between the geoid and ellipsoid in their region, ellipsoidal coordinates are not universal. An alternative way of describing the position of a point is by using Cartesian geocentric coordinates with respect to three mutually perpendicular axes with their origin at the centre of mass of the Earth. The Z-axis is chosen to coincide with the Earth's rotation axis, the X-axis intersects the Equator at the Greenwich meridian, and the Y-axis is at 90°E. The rotation axis changes continuously in a way that is not entirely predictable in the short term as a result of day-to-day and seasonal redistribution of mass within the atmosphere and oceans; it also changes more predictably in the longer term as a result of the Chandler wobble with a period of 14 months and as a result of nutation of the Earth (small oscillations of its axis) with a period of 18.6 years. In practice, therefore, the Z-axis is defined by the mean rotation axis over a fixed interval. The X- and Y-axes are no longer defined directly by observations at Greenwich, but in an implied sense through the choice of coordinates for several sites around the globe.

When dealing with astronomical and satellite measurements it is necessary to consider not just the Earth-fixed reference frame referred to above but also a suitable celestial reference frame in which the distant stars are (approximately) fixed and the equations of motion of satellites can be written without including any external rotational terms (an inertial frame). In practice such a frame is not perfectly realizable, because even the most distant stars exhibit small proper motions with respect to each other, and because it is not yet possible to model accurately all the resistive forces acting on near-Earth satellites. According to the problem in question, it may be more appropriate to consider an Earth-centred or Sun-centred (barycentric, fixed to the centre of mass of the Solar System) reference frame. In either case, the system is defined with respect to the ecliptic, the plane of the Earth's orbit around the Sun, and the Earth's Equator. The line of intersection of these two planes marks the equinoxes, the points at which the Sun apparently crosses the Equator as it passes from one hemisphere to the other. The vernal equinox (also known as the ascending node), where the Sun passes from the southern to the northern hemisphere, is chosen as the fundamental axis of the reference frame. Like the Earth's rotation axis, the actual Equator and ecliptic vary with time, and so the celestial reference frame is defined using mean values of these entities, observed over a fixed period.

Terrestrial geodetic measurements

Geodetic positioning by surface-to-surface observations has in some respects been superseded by space geodesy (in particular GPS, see below), but for small- and medium-scale positioning it often remains the best method. Also, many geophysical studies, in particular those of Earth rotation and lithospheric deformation, benefit from the longer history of terrestrial geodetic measurements. The first true geodetic measurements, dating back to the time of ancient Greece, were astronomical in nature. Eratosthenes (a contemporary of Archimedes) derived a remarkably accurate value for the Earth's circumference by comparing the zenith angle of the noonday sun at Alexandria with that at Aswan. Advances in timekeeping, telescopes, star catalogues, and the modelling of atmospheric refraction of light mean that astronomical positions this century are accurate to within 0.3≤ of latitude or longitude, an absolute positional accuracy of 10 m, which is insufficient for survey purposes. The difference in position between two nearby points can, however, be measured more precisely: the azimuth from one to the other can be measured to within 0.5≤ of arc, corresponding to 25 mm over a 10-km baseline. Many modern survey networks have such azimuth observations to provide orientation control.

Triangulation, first proposed by the sixteenth-century Dutch geodesist Gemma Frisius, was until the development of satellite geodesy the main method by which regional geodetic networks were measured. In its simplest form, triangulation relies on the fact that if the angles at the vertices of a set of abutting triangles are known, and also the length and orientation of one side of a triangle, then the positions of all the other vertices can be calculated using simple trigonometric relations. This method obviously requires that the vertices are intervisible, which is why triangulation pillars are usually sited on the tops of mountains or on tall buildings. The angle measurements are made with a theodolite, which is essentially a telescope free to rotate in the horizontal and vertical planes and with graduated circles so that the angle can be read off to within up to 0.1≤. Atmospheric refraction of light is a significant source of error, and to counter this several sets of measurements are taken, often at night when the temperature structure of the atmosphere is more stable.

Historically, the main advantage of triangulation over traverse surveys (in which successive bearings and distances are used to proceed from the known point to each unknown point in turn) was that there is a need for only one distance measurement in the entire network, although many networks had more than one to counteract errors. Distance measurements were previously made with Invar tape, designed to have a low coefficient of thermal expansion, but this method entailed laborious corrections for gravitational sag of the tape and the underlying topography between the two ends of the baseline. Since the 1950s electronic distance measurement (EDM) systems, using microwave radar or visible light, have made it relatively easy to measure the distance between intervisible points. Atmospheric refraction errors are mitigated by using two frequencies of light, because at optical frequencies the atmosphere is dispersive (i.e. it delays different signals by an amount depending on their wavelength, which can later be compensated). In this way the distance-dependent errors of EDM can be limited to about 1 ppm. Using EDM, trilateration, which is similar to triangulation but uses distance measurements instead of angles, becomes feasible.

The vertical component of position can be measured either by vertical triangulation (trigonometric levelling), which is severely limited by atmospheric refraction errors because the density of the atmosphere changes rapidly upwards, or more accurately by spirit levelling. The difference in height between successive points a few tens of metres apart is measured using a telescopic level and graduated rod and the process is repeated along a traverse or loop. Accuracies of up to 0.1 mm per kilometre of traverse can be achieved.

At the length-scales involved in geodesy, ordinary plane trigonometry cannot be used and the computations must be performed for a curved reference surface. This is a major reason why an appropriate ellipsoid must be chosen to approximate the geoid in the region. All these terrestrial methods are affected by the deflection of the vertical explained above, and this must be estimated either independently or, as part of the complete geodetic problem, together with the positions of the points. Because trigonometric and spirit levelling give the orthometric height (relative to the geoid), not the ellipsoidal height, the difference between the two (i.e. the geoid height or geoid–ellipsoid separation) must also be computed.

Space geodesy and satellite positioning.
Soon after the discovery of extraterrestrial radio sources in the 1930s, it was realized that these signals could be used to determine the distances between radio telescopes. Use of this technique of very-long-baseline interferometry (VLBI) for geodesy did not come to full fruition until the late 1960s, when precise atomic clocks made it possible to record and time-tag independently the signals received at widely separated points. The time difference between the arrivals of a wavefront at each telescope is proportional to the component in the direction towards the radio star of the baseline joing the two telescope. Measurements are made in the S- and X-bands (2–8 GHz). At these frequencies, signal delay caused by charged particles in the ionosphere is a problem which can be overcome by measuring at more than one frequency because the effect is dispersive. More significant is the delay caused by the troposphere, in particular tropospheric water vapour, which is non-dispersive and must be estimated using models of pressure, temperature, and humidity variation with altitude.

History of GPS (Global Positioning System) ~

Wednesday, September 14, 2011

NAVSTAR GPS or GPS is an acronym often abbreviated of Navigation System with Time and ranging Global Positioning System. GPS is a navigation system and satellite-based positioning can be used by many people at once in all weather, and is designed to provide position and velocity three-dimensional rigorous and time information, continuously throughout world.

GPS is an artificial satellite of the United States initially used for military interests, but on its development until This satellite is currently also used for the public interest. 

The background of the development of satellites (GPS) because:

1. Satellite is relatively short age
2. The number of small satellites
3. The number of orbits that fewer and more difficult
4. Using one of the reference datum (WGS '84)
5. Military field applications, digital maps, civil, etc.

Currently the GPS satellites that operate at least amount to 24 satellites, which orbit each satelinya has 6 orbits of a flat trajectory - average have a lifetime maximum of 7.5 years. 

GPS Systems

  • Hardware: Satellite, Receiver
  • Software: Software saatpeluncuran, while processing software, the software calculates accuracy, data processing software,
  • Brainware: Formula math and logic-mathematical logic.
  •  Application

Pertubasi: disruption of satellites that orbit the satellite trajectory is not the same as the modeling of the satellite orbit is ellipse. Disturbance can be either gravity, gravity months, and others. Formula pertubasi usually formulated in Deferensial Order 2:

Coordinates on the GPS system

1.reviewed from Satellite:

  • Sky Coordinate System
Origin: coincides with the center of mass of the Earth (geosentrik)
Positive Z axis: toward the north pole sky (KUL)
Positive X-axis: the direction of the Vernal Equinox

Positive Y-axis: perpendicular to the axis X and Z, so that the full form right-handed system

Kordinal sky system parameters:
1. Asensio rekta (α)
2. Declination (δ)
3. The distance of celestial bodies to the earth's center (r)

Sky Coordinates image system:


α = Asensio rekta
δ = declination
r = distance
P = Pole northern sky (KUL)
M = Earth
VE = Vernal Equinox (the point of intersection between the equator of the heavens with the ecliptic
S = sky objects (stars, sun, moon, etc.)

  • Coordinate System Orbit
  • 3D Coordinate System Kartesi

2.reviewed from the Observer:
  1. Terrestrial Coordinates System
Terrestrial Coordinates System (SKT)

Origin: coincides with the center of mass of the Earth (geosentrik)

  • Z axis positive: through the north pole CTP (Conventional Terrestrial Pole) who was adopted from the CIO that is the north pole of the earth averages 1990-1905
  • Positive X-axis: the Greenwich meridian
  • Positive Y-axis: perpendicular to the axis X and Z, so that the full form right-handed coordinate system
SKT Parameters:
  • geodetic latitude
  • geodetic longitude
  • Height above the reference ellipsoid
  • Origin (geosentrik)

To make the geodetic latitude should have the following requirements:
  • Field of reference ellipsoid (have half the long axis and half short axis)
  • The existence of the coordinate system
  • Prime Vertical: Normal allipsoid (line perpendicular to the ellipsoid)geodetic longitude Prime Vertical: Large angle formed from field to field meridian meridian O.

        observer.Geodetic latitude: angle difference between the fields of the equator with the prime meridian vertical to the field observer.

Image Coordinate System Terrestrial:
N = radius of curvature of the main vertical
λ = geodetic longitude
φ = geodetic latitude
h = height of the reference ellipsoid
O = origin (center of mass of the earth)
S = monitoring stations
CTP = Conventional Terrestrial Pole
P = the north pole of the earth for a moment
Xs, Ys, Zs = coordinates of monitoring stations kartesi

         2. Toposentris Coordinate System

            Coordinates System coordinates kartesi Toposentrik is sisitem 3D berorigin at the point of the observer (on earth) with reference to the ellipsoid normal line direction of strand or strands dititik origin.

Coordinates System Parameters Toposentrik:
* Azimuth
* elevation
* Origin (located on the surface of the earth)

Azimuth: The angle formed between the north with the geodetic reference point.
Elevation: The angle formed from horizonpengamat to the target object in the form of vertical angle
Origin: on the earth's surface titikpengamat
Positive W axis: coincides with the normal ellipsoid, positive towards the zenith
Positive U-axis: positive toward the north geographic
Positive V-axis: positive eastward, complete right-handed system.

image Caption:

Caption:λA: geodetic longitude of point A
φA: geodetic latitude of point A
O: origin the center of the earth
HA: high against the reference ellipsoid
A: The origin on the surface of the earth

        3. Map Coordinate Systems
         4. 3D Coordinate System Kartesi

Systems and Sub Systems GPS

Divided into 3 parts:


USER or user is utilizing all the parties GPS satellites to needs.

2. CONTROL: of the five station locations
remain on earth, can be monitored satellite health that includes speed satellites,trajectory of its orbit

Sub has the main task of this system include: Satellite monitoring and control system continuously, the determination GPS system time, satellite predict efemeris, and behavior recording devices time on the satellite and periodically update the navigation message for each satellite.

3. ORBIT (published later........)

GGOS Atmosphere

GGOS Atmosphere

In modern geodesy and in particular in space geodetic techniques various effects of the atmosphere have to be considered. The atmosphere delays (or advances) radio signals emitted by satellites, e.g. of the GNSS (Global Navigation Satellite Systems), or by distant radio sources observed by VLBI (Very Long Baseline Interferometry), atmosphere pressure loading causes deformation of the Earth's surface up to more than one centimeter (see figure), Earth gravity observations from dedicated satellites have to be reduced for atmospheric influences, and a considerable part of the variations of Earth rotation (polar motion, length of day) is due to processes in the atmosphere. Thus, the atmosphere plays an important role for the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) with its central theme 'Global deformation and mass exchange processes in the System Earth'.

Rethinking Geodesy and Geomatics Education in Indonesia (RG2E) in this modern era

This paper wanted to inspire lovers of knowledge in particular fields of Geodesy and Geomatics. Especially on Geodesy and Geomatics Education in Indonesia.

Why? The facts show an indication that the education of Geodesy and Geomatics has not been able to form a knowledgeable human being who is able to solve problems of mankind with the knowledge of Geodesy and Geomatics acquired during his education.


From Wikipedia, the free encyclopedia

Daftar isi

Geodesy in the view of the lay branch of science is the study of geosciencemapping the earth. Geodesy is one of the oldest branch of science dealing with the earth.


Derived from the Greek Geodesy, Geo (γη) = Earth and daisia ​​/ daiein (δαιω) =split, said geodaisia ​​or geodeien means dividing the earth. Actually the term"Geometry" is enough to mention the science of measuring the earth, where thegeometry comes from the Greek, γεωμετρία = geo = earth and metria =measurement. It literally means measurement of the earth. But the geometric terms(more precisely the spatial or spatial science) which is the basis for studying thescience of geodesy has been commonly referred to as a branch of mathematics.


Classical definition
According to Helmert and Torge (1880), Geodesy is the science of measuring andmapping the earth's surface which also includes the sea floor.

modern definition

According to the IAG (International Association Of Geodesy, 1979), Geodesy is thediscipline of the study of measurement and perepresentasian of the Earth andother celestial objects, including gravity field, respectively, in three-dimensional space that changes with time.

In the report the National Research Council USA, the definition of Geodesy can be read as follows:  a branch of applied mathematics That determines by observationsand measurements the exact positions of points and the figures and areas of largeportions of the earth's surface, the shape and size of the earth, and the variations ofterrestrial gravity.

In different languages​​, geodesy, a branch of applied mathematics, which is done by taking measurements and observations to determine:
The exact position of the points on the earth
The size and extent of most of the earth
The shape and size of the Earth and variations in Earth's gravity
This definition has two aspects, including:
Scientific aspects (aspects of the determination of form), related to geometry and physical aspects of earth and earth's gravity field variations.
Applied aspects (aspects of positioning), associated with measurements andcareful observation points or wider than a large part of the earth. Applied aspects of these later became known as mapping or surveying and geodetic techniques.
Geodetic techniques now no longer just associated with surveying and mapping.The development of digital computer technology has expanded the scope ofgeodetic science and engineering expertise. Map has been managed as ageographic information computing. That is why the international community hasadopted the new terminology: Geomatics or Geoinformatika.

History of Geodesy                                                  
Since time immemorial, Geodetic Science used by humans for purposes ofnavigation. Significantly, the mapping of the earth as the science of Geodesy has been begun since the flood of the Nile (2000 BC) by the kingdom of Ancient Egypt.Geodesy developments are more significant when men learn the shape of the Earth & Earth's size is more in the Greek figures, Erastotenes known as the father of geodesy. Until geodetic techniques serve as an academic discipline almost every state. Currently, due to advances in information technology, the moreextensive coverage of geodetic science.