What is Geodesy?

Geodesy is the discipline that deals with the measurement and representation of the earth and other celestial bodies and their respective gravity fields, in a three-dimensional time varying space.

What is Geomatics?

Geomatics is the modern scientific term referring to the integrated approach of measurement, storage and display of the descriptions and location of earth-based data, often termed spatial data. These data come from many sources, including earth orbiting satellites, air and sea-borne sensors and ground based instruments. It is processed and manipulated with state of the art information technology using computer software and hardware. It has applications in all disciplines which depend on spatial data, including navigation, geology and geophysics, mining, civil engineering, oceanography, land development and land ownership and tourism. It is fundamental to all the geoscience disciplines which use spatially related data.

What is concerned with?

Everyday the results of the work of Geomatics Engineers are found around us, from the legal security of your property boundaries, the appropriate location of mobile telephone transmitters, the safe navigation of ships, to the protection of environmental resources. When the questions -what is it, where is it, and how much is there- are asked, the geomatics engineer provides the answer. All decisions that are made by planners, engineers and decision makers regarding the earth, its environment and resources, initially require studies and analyses of models of the earth in the form of maps, plans, earth images and digital information. Geomatics is therefore an activity based on information technology which is concerned with collecting spatial information by survey measurements and analysis, management and manipulation of these data.

Some Applications of Geodesy

Before examining the applications of geodesy, first let us clarify the relations between geodesy and surveying; in most languages, no real distinction is made between the two. The distinction inherent in the English language probably the cause more problems than it solves. Be that it may, surveying is the practice of positioning, and geodesy is the theoritical foundation of surveying.

For centuries, the role of geodesy was to serve mainly mapping - an end many people still regard as the major purpose of geodesy. This reduction of geodesy to control surveying whose sole function is to provide position control for mapping, simply is not correct. Although a significant part of the information provided by geodesy falls within the realm of positioning, an equally substantial contribution is made elsewhere.

Let us now turn to the disciplines where geodetic information, positions or other is needed, here they are:

Mapping: It is well understood that there is a need for an areal network of appropriately distributed points (geodetic control) of known horizontal and vertical positions for the production of maps ranging from small scale maps of entire countries to large scale maps used by municipalities. The establishment of this control is clearly an important geodetic task.

Urban Management: In the urban environment, the locations of man's creations, such as underground utilities, must be defined and documented for future reference.

Engineering projects: During the building of large structures, such as dams, bridges and factories, it is necessary to lay out the various components of these structures in predeterminated locations. For this purpose, coordinates of one kind or another are used, so the availability of control points is naturally desirable. As well, it is often necessary to know the movements of the ground and water levels prior to, during and after the construction. In the case of dams, water tunnels, irrigation projects and the like, the exact shape of the equipotential surfaces of the gravity field should be known. The determination of the movements and the shape of the equipotential surfaces are also geodetic undertakings.

Boundary demarcation: The rigorous definition of international and intranational (provincial or state) boundaries is of paramount importance. Emphasis has also recently been placed on speedy and accurate description of oil and gas leases, even in such remote and inhospitable parts of the world as the Arctic, the North Sea and various continental shelves. The positioning and staking out of these boundaries is most economically done by relating them to a framework of points with known horizontal coordinates - the geodetic network.

Ecology: It has been realized in the past few decades that it is necessary to study the effects of human actions on environment. One such effect is the movement of the ground caused by the removal of underground resources (including water, oil and minerals) or subsurface disposal of wastes. The detection and monitoring of these movements is a geodetic problem.

Environmental management: It has been recognized that the establishment of the environmental data banks, to serve as integrated information systems for transportation, land use, community and social services, land titles extracts, assessment of tax data and population statistics, should be based on land parcels whose locations are uniquely defined in terms of coordinates. Again, it is advisable that these coordinates be referred to a geodetic network.

Geography: All the positional needed in geography is provided by geodesy. Eventhough the accuracy of positional and other geometrical information used by geographers is generally much lower than that needed in the fields described above, this information is a global character that only geodesy can satisfy.

Planetology: It can be argued that this is a part of either astronomy or geophysics. Be that as it may, planetology uses methods for studying the geometry, gravity fields and deformations of planets that are identical with the extraterrestrial methods used in geodesy. Thus, practically all of geodesy is applicable to planetology. Because of this special affinity between planetology and geodesy, some geodesists regard the determination of the shape and size of planets and their gravity fields as part of geodesy.

Hydrography: Some consider this field to be a part of oceanography, while others make it a special (marine) branch of surveying; either way, it has a somewhat special relation with geodesy. It may be understood as the practice of positioning at sea, combined with depth sounding, and, as such, applies many geodetic methods.

Symbiotic Related To Some Other Sciences

Clearly, there are many more uses for geodesy than simply mapping. Still other applications of geodesy are found in scientific fields that have a two-way relation with geodesy: while geodesy supplies one kind of information to them, they provide another kind of information for use in geodesy. Such fields are as follows:

Geophysics has a history of probably the closest affiliation with geodesy. So much so that in some countries geodesy is regarded as a branch of geophysics. Because of this close relationship, it is sometimes difficult to distinguish where geophysics ends and geodesy begins: the boundaries are somewhat blurred.

Geophysics, along with many other fields, requires the positions and other geometrical information on the earth's temporal deformations. Geodetic techniques are used increasingly in the detection of tectonic movements. In other part of contemporary geodynamics as well, geodetic data are used to obtain the geometry of the deformations.

Gravity is one of the most important sources of information used in both theoretical and eksploration geophysics. Gravity data is necessary for studying the irregularities in the underground mass density distribution. Since geodesists are also vitally interested in gravity data to study the geometry of the gravity field, both sciences claim a jurisdiction over gravity data collection (gravimetry). A somewhat artificial division would assign global gravity work to geodesy while regional and local gravity measurements would be a geophysical task. The temporal variations of gravity field offer a valuable hint about the physical causes of vertical crustal movements. As such, these data are often exploited in the context of contemporary geodynamics.

In return, geophysics offers an insight into the physical response of the earth to variety of force, into the possible density distribution within the earth, and into the effects of the internal structure of the earth on its motion. This information is needed when various mathematical models (relations) for geodetic purposes are being designed.

Space science, compared with geophysics, is a very young field. Right from the beginning, its relation to geodesy has also been a very close one. The main reason is that the knowledge of the geometry of the earth's external gravity field is essential for predicting the orbits of space vecicles. In addition, locations of satellite tracking stations must be known precisely enough to be of use; these are determined by geodetic means.

On the other hand, space science has developed some very powerful positioning systems that use the earth's artificial satellites, and these are now being used in geodesy to complement the existing terrestrial techniques. The analysis of the observed close satellite orbits now provides the best long wavelength data on the the earth's gravity field, including the value of the flattening of the earth. Tracking of deep space probes gives the best estimates of the value of the mass of the earth.

Astronomy, one of the oldest sciences in existence, and geodesy developed hand in hand for a long time. Although the interdependence of geodesy and astronomy has somewhat diminished in the recent of past, positional visual astronomy still plays a certain role in geodesy. In addition, the future will probably see an increasing involvement of positional radio-astronomy. Another part of astronomy, celestial mechanics, is also needed in geodesy to study the satellite orbits. Geodesy shares with astronomy the interest in lunar laser ranging; the ranges are used in astronomy to compute the lunar orbit and libration, while geodesists use them for position determination. Of common interest too is the monitoring of the rotation of the earth.

Oceanography is another science with which geodesy shares interests. Both geodesy and oceanography are involved in the location and movements of shorelines. Geodesy provides the oceanographers with relative heights of the on-shore water level measuring devices (tide gauges) and their relative vertical movements. Also, the geodetically determined positions of various marine objects, including ice and oceanographical vessels, are value to oceanographers.

Oceanographical information of interest to geodesists includes the dynamics of the sea surface and the deviations of the mean sea surface from an equipotential surface of the earth's gravity field. This information is needed for the establishment of a datum for heights.

Atmospheric science, along with all the aforementioned sciences, uses the geodetic positions and gravity pertaining to meteorological stations and probes. It shares with geodesy an interest in satellite orbit analysis: while geodesy interprets the orbital perturbations in terms of gravitational effects, atmospheric science looks at the effect of the distribution of air density. Geodesy needs realistic models for atmospheric refractivity and the appropriate meteorological datato evaluate atmospheric refraction. Meteorological data are also needed in the analysis of sea level temporal variations, and in special cases, that of the temporal variations of the earth's surface.

Geology requires both horizontal and vertical position for its maps. In return, it provides geodesists with a knowledge of geomorphology and the local stability of different geological formations. The information on stability is a must for any geodesist in charge of selecting suitable sites not only for geodetic monuments, but also for observatories of various kinds.



last revised: 2000.03.07