scholarly journals A numerical test of the topographic bias

2018 ◽  
Vol 8 (1) ◽  
pp. 14-17 ◽  
Author(s):  
L.E. Sjöberg ◽  
M.S.S. Joud
Keyword(s):  
Density Distribution ◽  
Simple Formula ◽  
Numerical Test ◽  
Physical Geodesy ◽  
Gravity Anomalies ◽  
Analytical Continuation ◽  
External Potential ◽  
The Earth ◽  
Topographic Bias ◽  
The Common

Abstract In 1962 A. Bjerhammar introduced the method of analytical continuation in physical geodesy, implying that surface gravity anomalies are downward continued into the topographic masses down to an internal sphere (the Bjerhammar sphere). The method also includes analytical upward continuation of the potential to the surface of the Earth to obtain the quasigeoid. One can show that also the common remove-compute-restore technique for geoid determination includes an analytical continuation as long as the complete density distribution of the topography is not known. The analytical continuation implies that the downward continued gravity anomaly and/or potential are/is in error by the so-called topographic bias, which was postulated by a simple formula of L E Sjöberg in 2007. Here we will numerically test the postulated formula by comparing it with the bias obtained by analytical downward continuation of the external potential of a homogeneous ellipsoid to an inner sphere. The result shows that the postulated formula holds: At the equator of the ellipsoid, where the external potential is downward continued 21 km, the computed and postulated topographic biases agree to less than a millimetre (when the potential is scaled to the unit of metre).

scholarly journals On the topographic bias and density distribution in modelling the geoid and orthometric heights

2018 ◽  
Vol 8 (1) ◽  
pp. 30-33 ◽  
Author(s):  
Lars E. Sjöberg
Keyword(s):  
Density Distribution ◽  
Mass Distribution ◽  
Geoid Height ◽  
Analytical Continuation ◽  
Orthometric Height ◽  
Gravimetric Geoid ◽  
Mountainous Regions ◽  
Imprecise Information ◽  
Topographic Bias ◽  
The Difference

Abstract It is well known that the success in precise determinations of the gravimetric geoid height (N) and the orthometric height (H) rely on the knowledge of the topographic mass distribution. We show that the residual topographic bias due to an imprecise information on the topographic density is practically the same for N and H, but with opposite signs. This result is demonstrated both for the Helmert orthometric height and for a more precise orthometric height derived by analytical continuation of the external geopotential to the geoid. This result leads to the conclusion that precise gravimetric geoid heights cannot be validated by GNSS-levelling geoid heights in mountainous regions for the errors caused by the incorrect modelling of the topographic mass distribution, because this uncertainty is hidden in the difference between the two geoid estimators.

WHITE ANTS DESTROYING LIVING TREES AND CHANGING THF FOLIAGE, IN CAMBRIDGE, MASS

1885 ◽  
Vol 17 (7) ◽  
pp. 134-136
Author(s):  
H. A. Hagen
Keyword(s):  
Dead Wood ◽  
The South ◽  
The Earth ◽  
The Common

The common white ant, Termes flavipes, destroys dead wood, stumps of trees and timber, just as does its nearest relative, T. lucifugus, in Europe. Of the latter species some cases are reported where living pines and oaks have been destroyed in the South of France. For T. flavipes, only one case is known, in which living grape vines in a hot house in Salem were injured. (S. H. Scudder. Proc. Boston, N. H. S., vol. 7, p. 287). Now the earth in the hot houses here in Cambridge is largely infested by white ants, but as far as I know, no destruction of plants has been observed. I was very much interested by the information from Mr. F. W. Putnam that in a garden in Irwing street living maples were largely infested by white ants. The evidence of the truth of this information was apparent by the first glance at the trees.

scholarly journals XXIII. On the degree of salubrity of the common air at sea, compared with that of the sea-shore, and that of places far removed from the sea

1780 ◽  
Vol 70 ◽  
pp. 354-377 ◽  
Keyword(s):  
The Earth ◽  
The Face ◽  
The Common ◽  
In The Beginning ◽  
Bodies Of Water

Sir, As you had recommended to me the examination of the air at sea by the nitrous test, I followed your advice in my return to the Continent in the beginning of November last: and I embraced that opportunity with the more eagerness, as I knew that you had given credit to the account of several consumptive people having recovered their health by going on sea voyages, after the common means for curing that distemper had failed. I was in hopes likewise to find in this inquiry, a confirmation of what you conjectured in you Anniversary Discourse in the year 1773, viz . that great bodies of water, such as seas and lakes, are conducive to the health of animals, by purifying and cleansing the air contaminated by their breathing in it: so that the salutary gales, by which this infected air is conveyed to the waters, and by them returned again to the land, though they do rise now and then to storms and hurricanes, must nevertheless induce us to trace and to reverse in them the ways of a beneficent Being, who, not fortuitously, but with design, not in wrath, but in mercy, thus shakes the waters and the air together, to bury in the deep those pestilential effluvia which the vegetables upon the face of the earth are insufficient to consume.

scholarly journals Corrected μβ for stars of Hipparcos catalogue from independent latitude observations over many decades

2011 ◽  
pp. 35-41 ◽  
Author(s):  
G. Damljanovic ◽  
I.S. Milic
Keyword(s):  
International Organizations ◽  
Sufficient Accuracy ◽  
Proper Motions ◽  
Earth Orientation Parameters ◽  
Method Of Calculation ◽  
The Earth ◽  
Hipparcos Catalogue ◽  
International Polar ◽  
The Common ◽  
Orientation Parameters

During the last century, there were many so-called independent latitude (IL) stations with the observations which were included into data of a few international organizations (like Bureau International de l'Heure - BIH, International Polar Motion Service - IPMS) and the Earth rotation programmes for determining the Earth Orientation Parameters - EOP. Because of this, nowadays, there are numerous astrometric ground-based observations (made over many decades) of some stars included in the Hipparcos Catalogue (ESA 1997). We used these latitude data for the inverse investigations - to improve the proper motions in declination ?? of the mentioned Hipparcos stars. We determined the corrections ??? and investigated agreement of our ?? and those from the catalogues Hipparcos and new Hipparcos (van Leeuwen 2007). To do this we used the latitude variations of 7 stations (Belgrade, Blagoveschtschensk, Irkutsk, Poltava, Pulkovo, Warsaw and Mizusawa), covering different intervals in the period 1904.7 - 1992.0, obtained with 6 visual and 1 floating zenith telescopes (Mizusawa). On the other hand, with regard that about two decades have elapsed since the Hipparcos ESA mission observations (the epoch of Hipparcos catalogue is 1991.25), the error of apparent places of Hipparcos stars has increased by nearly 20 mas because of proper motion errors. Also, the mission lasted less than four years which was not enough for a sufficient accuracy of proper motions of some stars (such as double or multiple ones). Our method of calculation, and the calculated ?? for the common IL/Hipparcos stars are presented here. We constructed an IL catalogue of 1200 stars: there are 707 stars in the first part (with at least 20 years of IL observations) and 493 stars in the second one (less than 20 years). In the case of ?? of IL stars observed at some stations (Blagoveschtschensk, Irkutsk, Mizusawa, Poltava and Pulkovo) we find the formal errors less than the corresponding Hipparcos ones and for some of them (stations Blagoveschtschensk and Irkutsk) even less than the new Hipparcos ones.

scholarly journals DETERMINATION AND MAPPING OF THE TERRESTRIAL GRAVITY ANOMALIES IN THE MOUNTAINOUS AREAS OF IRAQ

2021 ◽  
Vol 6 (24) ◽  
pp. 213-225
Author(s):  
Shazad Jamal Jalal ◽  
Tajul Ariffin Musa ◽  
Ami Hassan Md Din ◽  
Wan Anom Wan Aris
Keyword(s):  
Gravity Data ◽  
Physical Geodesy ◽  
Gravity Anomalies ◽  
Gravity Survey ◽  
Mountainous Areas ◽  
Entire Study ◽  
Terrestrial Gravity ◽  
The Past ◽  
Regional Gravity ◽  
Entire Study Area

Gravity data and computing gravity anomalies are regarded as vital for both geophysics and physical geodesy fields. The mountainous areas of Iraq are characterized by the lack of regional gravity data because gravity surveys are rarely performed in the past four decades due to the Iraq-Iran war and the internal unstable political situation of this particular region. In addition, the formal map of the available terrestrial gravity which was published by the French Database of Bureau Gravimetrique International (International Gravimetric Bureau-in English) (BGI), introduces Iraq and the study area as a remote area and in white color because of the unavailability of gravity data. However, a dense and local (not regional) gravity data is available which was conducted by geophysics researchers 13 years ago. Therefore, the regional gravity survey of 160 gravity points was performed by the authors at an average 11 km apart, which was covers the whole area of Sulaymaniyah Governorate (part of the mountainous areas of Iraq). In spite of Although the risk of mine fields within the study area, suitable safe routes as well as a helicopter was used for the gravity survey of several points on the top of mountains. The survey was conducted via Lacoste and Romberg geodetic gravimeter and GPS handheld. The objective of the study is to determine and map the gravity anomalies for the entire study area, the data of which would assist different geosciences applications.

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