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Geoinformatics-8-2009
article an evolution of the millennial pair (pt. 2) geolocation and time this is the second and last part of the article that was printed in the previous issue of geoinformatics (issue nr. 7, october/november 2009). part 1 was about a short history of relations between geolocation and time, describing the following topics: the shape and the size of earth, latitude, longitude and time, from the spherical to the ellipsoidal earth, transition to the geoid and the third dimension in geodesy. part 2 continues with the fourth dimension in geodesy and describes shortly the main topics of geodesy and its scientific field in the beginning of 21st century, which are the cornerstones that allow modern geodesy to provide globally the best possible service to the various daily life fields of work and living of an individual and to the society as a whole. by joc triglav measurement and presentation of earth's surface and its external gravitational field, including its temporal changes. modern geodesy is based on three pillars: geometrical shape of the earth as a function of time, orientation of earth in space as a function of time and earth gravitational field as a function of time. the four-dimensional aspect of geodesy allows geodesists to employ improved modelling of their observations in evaluation and presentation of the slow long-term changes of the earth and its gravity field. this way a firm scientific foundation is set for other natural and social sciences, which can use it as a skeleton for the development of their geospatialtemporal fields of work in order to provide access to a common digital model of temporally geolocated information on the earth. the already achieved high accuracy and the anticipated further progress open new fields for research and multidisciplinary applications in the 21st century. figure 10.vlbi is an advanced space geodetic technique that can measure a distance of thousands of kilometers between its antennas with an accuracy of few millimeters, by receiving radio signals from deep space as far as several billion light years away. (credits: http://vldb.gsi.go.jp). 2.5 fourth dimension in geodesy the development of geodetic methodology has made its first steps into fourdimensional geodesy at the end of 19th and in the beginning of 20th century with the detection of earth polar motion and the observations of the earth tides as well as with measuring crustal motions and deformations due to earthquakes and postglacial rebound. then in the second half of the previous century, space geodetic methods like laser ranging and vlbi techniques (figure 10) have developed and are being applied to support the scientific field of geodynamics, which includes the study of the interior structure and composition of the earth, its crustal motions and deformations, the rotational dynamics (figure 11), and the terrestrial potential fields. thus, the concept of the figure of the earth has definitely widened from three-dimensional rigidity to four-dimensional time-dependence. technology developments of the last decades especially in the field of laser technology, signal processing, atomic clocks, time transfer, it developments, etc., were supporting the development of space geodetic techniques and the remarkable progress in their accuracy as shown in table 1. this development brings us to the definition of geodesy as a science of 3 geodesy and its scientific field in the beginning of 21st century geodesy was the science and profession, which widely opened an insight into the secrets of mathematics, geometry and trigonometry to geodesists and navigators in the beginning of 17th century with the manuals written in english language. until then these secrets were wrapped in latin and greek writings, which were accessible to only a very narrow group of scholars. the primary task of geodesists in the past periods of development of geodesy and society was at the basic level of measuring the size and shape of the earth and its gravitational field. using geodetic application technologies the geodesists were able to measure, monitor, supervise and register the data on geolocation of objects in the agreed reference systems of countries, regions, continents or the world, with the most accurate mutual mathematical relations established. for centuries, the role of geodesy was in production of plans and maps, therefore the majority of public still understands this as its main goal. in present time, the above mentioned (see 2.5) three pillars are equally important in spite of the fact that a large part of information provided by geodesy is still mainly in the domain of geolocating. geolocating as a subject of this paper belongs to the first above mentioned pillar of geodesy; it is that task of geodesy, table 1. progress in which is best understood by most of the people. accuracy of space geolocating is absolute determination of coordigeodetic techniques 36 december 2009
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