Precise Geodetic Measurements Research Articles

Cartographic Representation of Glacial Phenomena: Historical and Recent Developments

With the first systematic exploration of glacial phenomena in the middle of the nineteenth century, glaciology became an independent science. In parallel, glacial structures and processes needed to be visualized cartographically. Such representations always reflect the then current states of the technology of glaciological data collection and of cartography. Already in the last century, well-designed and user-friendly glacier maps with a high information content were published with the help of precise geodetic measurements and well-developed cartographic techniques. The focus was upon statistical illustration of the changes in glacier geometry and by glacial-morphological forming. In the 1950s, glaciologists started to represent dynamic parameters such as glacier fluctuations, mass balance, and ice flow. Furthermore, the topographic information of the printed maps was supplemented by orthophotos and satellite images. Today, in the age of digital cartography, screen representations become more and more important. Using digital photogrammetry and remote sensing, new methods of geophysical sounding, or satellite-based global positioning systems (GPS), large quantities of data can be recorded. Using geographic information systems (GIS), these data can be manipulated, modelled, compiled as digital elevation models (DEM), analysed, and finally visualized interactively as high-quality maps or as perspective views. Future trends point towards comprehensive, interactive glacial information systems with integrated functions for database query, modelling, and visualization.

Derivation Of Refractive Index And Temperature Gradients From Optical Scintillometry To Correct Atmospherically Induced Errors For Highly Precise Geodetic Measurements

Refraction effects of optical beams are generally caused by an inhomogeneous propagation medium and are a major source of systematic errors in the precise optical determination of angles and distances in the atmospheric surface layer. In this contribution a method for deriving vertical temperature and refractive index gradients from optical scintillation is presented. Knowledge of these gradients is required for the compensation of atmospherically induced errors for highly precise terrestrial geodetic measurements, like direct transfer and levelling. The advantage of the present optical method is, that temperature and refractive index gradients can be derived as line-averaged values over the propagation path, which is not possible by meteorological point measurements. Field observations have been carried out with a displaced-beam scintillometer over flat terrain and under different atmospheric conditions in order to verify this method. The experiments show, that this method allows to derive accurate correction values for precise terrestrial geodetic measurements.

Educational Outreach Strategy involves K‐12 students in earthquake hazard research

NASA has been making precise geodetic measurements to assess earthquake hazard associated with the subduction zone process in Alaska since 1984. In 1995, a new strategy for making geodetic measurements was implemented that included local residents in the measurement program, and in 1997, the program was expanded to include teachers and students at five high schools. For the inhabitants of Alaska, understanding the hazards and underlying physical processes associated with earthquakes has immediate personal relevance. Large earthquakes occur yearly, great earthquakes have occurred during the lifetime of many residents, and the weekly testing of the tsunami warning system in coastal towns is a reminder of the devastation caused by earlier tsunamis generated near Alaska and a cautionary note about future inundation.

Present‐day crustal deformation in southern California

The effects of laterally homogeneous mantle electrical conductivity have been included in steady. Using an extensive set of precise geodetic measurements, we have developed a detailed picture of present‐day deformation rates in southern California. This large set of measurements, amounting to nearly 2000 repeated distance measurements over the period 1973 to 1991, comes from the U.S. Geological Survey's Geodolite trilateration program, involving their combined Anza, Joshua Tree, and Salton networks. Building on previous results from these data, we are able to present the deformation field as estimates of the rate of horizontal strain accumulation in small four‐station subnetworks of the overall 89‐station network. Using this technique, the spatial details of the 18‐year average strain rate field can be determined. By correlating these spatial details with the tectonics of the region we are able to understand better how deformation is partitioned across this highly complex margin between the Pacific and North American tectonic plates. Some of the more interesting findings of this study are that (1) the vast majority of strain rate estimates show a pattern of nearly pure shear as would be expected in a transcurrent environment, (2) the fastest accumulation of surface strain in southern California is along the San Jacinto Fault west of the Salton Sea, not along the San Andreas Fault, (3) strain accumulation rate along the length of the San Jacinto Fault increases toward the southeast as the fault enters the Imperial Valley, (4) a large area near the southern end of the Salton Sea, where the San Andreas Fault meets the Brawley Seismic Zone, is undergoing areal dilatation, which is in part consistent with the formation of crust at a spreading center, and (5) deformation at the transition zone between the San Andreas Fault and the Eastern California Shear Zone also appears to be the result of crustal spreading.

Application of the global positioning system to crustal deformation measurements: 3. Result from the southern California borderlands

Five years of measurements from the Global Positioning System (GPS) satellites collected between 1986 and 1991 are used to investigate deformation in the offshore regions of southern California. GPS provides the first practical technique to make precise geodetic measurements in the region. The geodetic network is situated along the California coastline from Vandenberg (120.6°W, 34.6°N) to San Diego, with additional sites on Santa Cruz, San Nicolas, Santa Catalina, Santa Rosa, and San Clemente Islands. The precision of horizontal interstation vectors is subcentimeter, and the interstation vector rate between OVRO and Vandenberg agrees with the very long baseline interferometry derived rate to within one standard deviation. No significant motion is observed in the western Santa Barbara Channel between Vandenberg and Santa Rosa Island, 0.5 ± 1.6 mm/yr, where the quoted uncertainties are one standard deviation. Motions in the eastern Santa Barbara Channel are consistent with compressional deformation of 6 ± 1 mm/yr at N16 ± 3°E. This motion is in agreement with seismicity and an independent geodetic analysis for the period 1971–1987 (Larsen, 1991). San Clemente Island is moving relative to San Diego at the rate of 5.9 ± 1.8 mm/yr at a direction of N38 ± 20°W. The motion between San Nicolas Island and San Clemente Island, 0.8 ± 1.5 mm/yr, is insignificant.

A scheme for reducing the effect of selective availability on precise geodetic measurements from the Global Positioning System

From March to August 1990, the signals transmitted by the Block II satellites of the Global Positioning System (GPS) were dithered under a policy of “Selective Availability” (SA). The dithering appears as a ∼10−10 deviation of the satellite oscillator frequency, which, when accumulated over several minutes, can produce an error of ∼100 cycles (∼20 m) in the model for carrier beat phase. Differencing between simultaneously sampling receivers minimizes the error. If, however, the receivers do not sample simultaneously, it is necessary to model the frequency deviation, which we estimate from the phase observed at a station with a stable local oscillator. We apply such a model to data collected in March 1990 by TI4100 and Minimac receivers sampling at times separated by 0.92 s. Applying the algorithm significantly improves the root mean square (RMS) scatter of the estimated relative position vectors. The RMS scatter from a data set including dithered satellites is similar for both simultaneously and non‐simultaneously sampling receivers, a result which indicates that SA can be adequately modeled.

Present state of geodynamic properties of Kalabsha area, nortwest of Aswan Lake, Egypt

Aswan Lake is the second largest man-made lake in the world. Its filling started 1964 and reached the maximum water level in 1978. An earthquake of magnitude 5.5 took place in 1981 along the most active fault near the lake (Kalabsha fault). This earthquake was follwed by a tremendous number of smaller events that continue till now. Seismicity and the underground water table around the lake are monitored continuously through a radio-telemetered network. A local geodetic network was established around parts of the active faults in the northwestern part of the High Dam Lake, for monitoring vertical and lateral movements. The Kalabsha local geodetic network (the first one) was established around an active part of the Kalabsha fault in 1983. Precise geodetic measurements have benn carried out twice a year since 1984. On the basis of the repeated geodetic measurements, seismicity of the area and geophysical as well as geological data, the present state of the geodynamical properties of the Kalabsha area is studied. Remarkable horizontal movements were detected; they are correlated with the seismicity of the area and are attributed to the differential loading by the lake. The Kalabsha fault is a right-lateral strike-slip motion on an E-W plane. The magnitude of the movements detected along the fault is variable for the different epochs of measurements and is correlated with both seismicity and water loading in the lake.

MEASUREMENTS FOR THE CONSTRUCTION OF THE TEST SLAB OF THE LABORATORY OF REINFORCED CONCRETE STRUCTURES OF THE UNIVERSITY OF THESSALONIKI

Abstract In this paper the methodology of measurement and construction of the test slab of the Laboratory of Reinforced Concrete Structures of the University of Thessaloniki is described. A total of 180 steel pipes of anchorage were placed in the test slab forming an 1.0 m × 1.0 m grid. The accurate positioning of the pipes was possible with the help of high precision geodetic measurements. Free network adjustment was used for an evaluation of the accuracy obtained. Accuracies of ± 1.0mm in plan and plusmn; 0.4mm in elevation were achieved in the positioning of the pipes.

MEASUREMENTS FOR THE CONSTRUCTION OF THE TEST SLAB OF THE LABORATORY OF REINFORCED CONCRETE STRUCTURES OF THE UNIVERSITY OF THESSALONIKI

In this paper the methodology of measurement and construction of the test slab of the Laboratory of Reinforced Concrete Structures of the University of Thessaloniki is described. A total of 180 steel pipes of anchorage were placed in the test slab forming an 1.0 m × 1.0 m grid. The accurate positioning of the pipes was possible with the help of high precision geodetic measurements. Free network adjustment was used for an evaluation of the accuracy obtained. Accuracies of ± 1.0mm in plan and plusmn; 0.4mm in elevation were achieved in the positioning of the pipes.

Study of recent crustal movements on apsheron peninsula by geodetic methods

In active tectonic areas with rapid Recent Vertical Movements of the Earth's Crust (RVMEC), where movements are complicated by their technogenic origin, adjustments of precise geodetic measurements are impossible to carry out using the existing technological diagrams and specified tolerances, due to the fact that the measured parameters have varied in time. This article covers experience in the compositions of the RVMEC map, using the results of three first-order relevellings. Using this map, methods for adjustments of levelling networks and compilations of dynamic cataloques of heights, where field measurements have been updated, have been developed. The RVMEC map is used for geological and geophysical data interpretation in conjunction with results of geodetic measurements. The article gives parameters of abnormal values for vertical movements of separate sites, rates of general subsidence of the southern part of the Apsheron Peninsula and correlations of gravimetric measurement data.

Strategies for high‐precision Global Positioning System orbit determination

High‐precision orbit determination of Global Positioning System (GPS) satellites is a key requirement for GPS‐based precise geodetic measurements and precise low‐Earth orbiter tracking. We explore different strategies for orbit determination with data from 1985 GPS field experiments. The most successful strategy uses multiday arcs for orbit determination and incorporates fine tuning of spacecraft solar pressure coefficients and stochastic station zenith tropospheric delays using the GPS data. Average rms orbit repeatabilities for five of the GPS satellites are 1.0, 1.2, and 1.7 m in altitude, cross‐track, and downtrack components, when two independent 5‐day fits are compared. Orbits predicted up to 24 hours outside a 7‐day arc show average rms component differences of 1.5–2.5 m when compared to independent solutions obtained with a separate, nonoverlapping 5‐day arc. For a 246‐km baseline, with 6‐day arc carrier phase solutions for GPS orbits, baseline repeatability is 2 parts in 108 (0.4–0.6 cm) for east, north, and length components and 8 parts in 108 for the vertical component. For a 1314‐km baseline with the same orbits, baseline repeatability is about 2 parts in 108 for the north component (2.5 cm) and 4 parts in 108 or better for east, length, and vertical components. When GPS carrier phase is combined with pseudorange, the 1314‐km baseline repeatability improves further to 5 parts in 109 for the north (0.6 cm) and 2 parts in 108 for the other components (2–3 cm).

Geodetic evidence for rifting in Afar a brittle-elastic model of the behaviour of the lithosphere

An important episode of rifting occurred in November 1978 in southwest Afar, in the first subaerial section of the accreting plate boundary between the African and Arabian plates. Horizontal rifting of more than 2 m took place, with vertical displacements of about 1 m, earthquakes of magnitude up to 5.3, and a fissural volcanic eruption of basaltic lavas. Very precise geodetic measurements were carried out in order to study this crisis and strains of the order of 3 × 10 −4 were measured, both tensile and compressive. This paper presents an analysis of the mechanical behaviour of the lithosphere. It is shown that an elastic-brittle model with a rebound mechanism fits very well the data, and it is suggested that such a model, with magma injection in the resulting open fissures, should be used to describe accretion at plate boundaries.

Range difference multilateration for obtaining precision geodetic and trajectory measurements

The theoretical aspects of a new multilateration technique suitable for precision geodesy and orbit determination applications are examined. The multilateration technique considered herein makes use of the differential time of arrival of signals at an ensemble of ground stations from a spacecraft or aircraft as the fundamental data type. It will be demonstrated that simultaneous measurements give rise to a system of equations which upon solution permits the determination of the three-dimensional vehicle coordinates plus the three-dimensional coordinates of the station net relative to an arbitrarily adopted origin (which may be taken to be one of the stations). A solution to these equations can be obtained without any a priori knowledge of the locations of the stations and vehicle. The necessary conditions for obtaining all of these coordinates in the same solution are discussed, and it is indicated that at least five stations are required in the station ensemble. A subsequently performed study of the mathematical degeneracies indicates that the five-station configuration becomes singular if the stations are in the same plane. As a result, stations spaced closely together on the Earth's surface will yield ill-conditioned solutions. This degeneracy can be overcome by the addition of one more station. Hence, the system recommended herein will be comprised of six stations simultaneously receiving radio pulses emitted from an arbitrary vehicle. The formulation of the mathematical equations is undertaken along with the appropriate method of solution. An error analysis is also developed. Some preliminary system considerations such as data type synchronization and ionospheric and tropospheric corrections to the data types are examined. It is demonstrated that synchronization of the data types presents no problem and that the radio data types can be calibrated to accuracies of better than five centimeters if suitable instrumentation is available. The system proposed herein is a convenient alternate to a range-only multilateration system because obtaining radio range as opposed to differential range (not range-rate) will always be less accurate. The pulsed laser range multilateration system already has been discussed by the authors in another paper.

Precise Geodetic Measurements and Their Relation to Seismological Investigations

Precise Geodetic Measurements and Their Relation to Seismological Investigations

Precise geodetic measurements and their relation to seismological investigations

The use of precise measurements is becoming more general in efforts to interpret the phenomena connected with changes in the Earth's surface. It may be that the use of these precise measurements will eventually lead to a more exact knowledge as to how the Earth s surface originally became so irregular with great areas standing high, as continents, and other areas depressed, as ocean basins.The science of seismology is adding much to our exact data. The locations of the epicenters of earthquakes and the determination of their intensity and the rate of transmission of the earthquake‐shocks through the Earth's materials at different depths furnish information of the highest value.