International Association of Geodesy A Constituent Association of the International Union of Geodesy and Geophysics Permanent GPS Station in southern Greenland, WhatWhat isis Geodesy?Geodesy? part of the GNET Project (Photo: M. Bevis) The Earth is not composed of solid rock, but During the evolution of geodetic science, it due to the increase of temperature towards its has been discovered that the shape of the Earth centre it is built up from a mixture of viscous is not round, but it ‘bulges’ outward along the and solid materials. Thus the shape of the equator. Even this is an over simplification. Earth is oblate, because the centrifugal force The Earth has many ‘hills and troughs’ in the stemming from its rotational motion thrusts gravity potential field, indicating significant departures from symmetric mass distribution the masses out at the equatorial bulge. within the solid Earth. This is superimposed on the topographic variability we are all familiar What about the hills, mountain ranges, with. and deep sea trenches? Why do we need accurate maps? These ‘topographic features’ are quite small compared to the size of our planet. Of course Recall the old saying: they are measured and drawn on maps, but “You can not tell where you are going un- in Geodesy their contribution to the shape of less you know where you have been.” the planet is quite small. Maps play an important role in our lives. These topographic features range from about We need them to find our way in the world – to 9 km above sea level (Mount Everest) to navigate. In ancient times mankind used approximately 12 km below sea level (Mariana landmarks to navigate their way to friends, to Trench). The diameter of the Earth is roughly towns and ports, or other places of interest. 12756 km. Today we often use urban landmarks, such However, as the accuracy of our observation as shopping malls, churches, skyscrapers, techniques improves, the geodetic influence of road intersections, and other points of interest. topographic features become more and more But what happens, if we are not familiar with important. For example, the gravitational the town or rural area? effects of the topographic masses are no longer negligible for geodetic investigations. Then we need accurate maps to navigate, but also to indicate geometric relations Quite simply: Geodesy is the science between points or landmarks depicted on the concerned with the study of the shape, size map, to display information on areal extent of and gravity field of the Earth. different vegetation landuse, to design large infrastructure and transportation problems, and so on. The Earth seems to be round, is it not? In order to create accurate maps we need a well-defined spatial reference system. A spatial In reality the Earth is not a perfect sphere. reference system defines the coordinate system Knowledge of the exact shape of the Earth and datum in which all landmarks have a provides us with accurate maps of the Earth. unique “address” or position (i.e., coordinates). 1 Photo: iStockPhoto (dra_schwartz, hidesy, WhatWhat isis Gravitation?Gravitation? Zhenikeyev) FromFrom thethe AppleApple ...... Detailed knowledge of the Gravitational Field caps (e.g., melting due to the Greenhouse of the Earth does improve our lives, and Effect) may reduce this compression, and provides us to benefit both economically and thus change the gravitational field. socially. It helps us understand climate change, sea level rise, and geological hazards These effects are only examples. However we (e.g., earthquakes, volcanoes); and it underpins can see how complicated our planet is, and can practically all satellite applications, as well as get a sense of how these components interact precise navigation systems. with one another. Since our planet is composed of different In the previous page we defined Geodesy as materials, its gravitational field shows local the science of studying the Shape, Size and variations (called “anomalies”). Observing these Gravity Field of the Earth. But what is the Grav- anomalies can help us to explore the sub- ity Field, and why is it important? surface mass distribution of our planet. In this way, we may look into the interior of the Earth. Gravity in a nutshell What affects the gravitational field? Gravitation is a force that pulls or attracts all bodies in the universe that have a mass The gravitational field is affected by many towards each other. Planets remain in their factors, including: orbits around the Sun due to this force. The gravitational force depends on the amount of mass density variatons inside the Earth; mass of the bodies. In Geodesy we usually distinguish “gravity” as gravitation plus cen- the oceans - the level of oceans changes trifugal acceleration due to Earth's rotation due to temperature, ocean currents, etc. because traditionally we can measure gravity, Changes in the level of oceans also change not gravitation, at points on the rotating Earth. the gravitational field. the Sun, Moon and other planets - the Newton's apple gravitational effects of the Sun, Moon and the planets vary according to their posi- Sir Isaac Newton postulated that the Earth's tion relative to the Earth. gravitation causes such things as the fabled apple to fall, yet it is the same force that keeps ice sheets - they compress the Earth's planets in their orbits. For example, he figured crust beneath them. Changes in the ice out that the Moon would travel in a straight line without the attraction from the Earth pulling it into a curved orbit. 2 ...... ToTo thethe SatellitesSatellites Why do we need to study the gravita- world, these observations are highly confiden- tional field? tial because gravity measurements lead exploration geophysicists to oil fields and Gravitation affects almost everything in our mineral deposits, and assist military planners lives. From precise clocks to hydroelectric to guide missiles to their intended targets. dams, from the tides in the oceans to blood circulation. Satellite missions such as CHAMP (CHAllenging Minisatellite Payload), GRACE We study the Earth's gravitational field to (Gravity Recovery And Climate Experiment) and learn more about our planet. This knowledge GOCE (Gravity Field and Steady-state Ocean can lead us to new ways of doing things with Circulation Explorer) help us to study the global greater awareness towards sustainability of our gravitational field of the Earth. These Low-Earth Orbiters (LEOs) provide geodesists with a environment for future generations. global, homogeneous coverage of gravitational How can the satellites help? measurements. The effect of the Sumatra earthquake on If we want to study the gravitational field glo- gravity recovered from GRACE observations bally, terrestrial gravity data have severe limi- (NASA/JPL). tations, mostly because they do not cover the whole globe with a homogeneous set of mea- surements. Moreover, in some places in the GRACE satellites observing the gravity field of the Earth 3 (Image courtesy of NASA/JPL) GeodesyGeodesy ofof TodayToday When checking the meaning of the term (illustrated in the figure by the measure- Geodesy in the major encyclopedias, we find ment of “lunar distances”). it is usually described as the science concerned 4) Geodesy provides the link to Astronomy (in- with the Shape, Size, and Gravity Field of the dicated by the measurement of so-called Earth. Webster’s dictionary even defines it as “lunar distances”, allowing the reconstruc- a branch of applied mathematics! tion of Universal Time (UT) in connection Geodesy today is much more than that. It is with astronomical almanacs). a geoscience that treats the Earth as a complex dynamic system, a body consisting of many The Space Age, and the development of space layers, surrounded by atmosphere and oceans. geodetic observation techniques associated Geodesy is required to model the system’s with it, revolutionized our understanding of behaviour in space and time so as to provide Geodesy: Distance and distance difference mea- the basic framework needed across a wide surements complemented and, to a certain range of Earth sciences. For example, it does extent, replaced the measurement of angles, not make sense to speak of global sea level making possible the establishment of truly glo- rise without referring to the cm-accurate (time- bal reference systems (geometric and gravita- variable) terrestrial reference system, or the tional). The growth of space geodesy have precise global gravity field determined from yielded positioning at the millimetre-level rou- satellite tracking, or the sea surface tine, enabling the detection of crustal move- topography maps established by a long ment and strain with unprecedented accuracy sequence of altimeter satellite missions. and high time resolution. Geodesy continues to provide, with state- The title page of Peter Apian’s work of-the-art measurement tools, the scientific Introductio Geographica, published in 1533, basis of Navigation, e.g., by the exploitation of proves, on the other hand, that Geodesy is a the existing and planned Global Navigation rather old science (the term Geodesy may not Satellite Systems (GNSS), such as GPS, yet have existed at that time). GLONASS, and GALILEO, and the link to fun- The illustration at the top of the page indicates damental Astronomy via the global terrestrial that: reference system, by monitoring the rotational motion of our planet, by space geodetic tech- 1) State-of-the-art measurements and their niques, and by establishing the celestial refer- correct mathematical treatment were, and ence system and UT (reflecting Earth rotation). are, of central importance. Modern Geodesy is experiencing spectacu- 2) The same measurement techniques are lar growth and each year new frontiers of Earth used for the most demanding as well as science knowledge are being explored, such as “everyday” purposes. temporal variations of positions and gravity 3) Geodetic techniques provide the basis for (land deformations, post-glacial rebound, sea applications of crucial importance and rele- level rise, etc.).