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Geodesy
Published in Basudeb Bhatta, Global Navigation Satellite Systems, 2021
The word geodesy comes from Greek, literally meaning ‘dividing the earth’. The first practical objective of geodesy was the provision of an accurate framework for the control of national topographical surveys, and hence it is the foundation of a nation’s maps (Smith 1997). To prepare the map, geodesy must define the basic geometrical and physical properties of the figure of the earth. The scientific objective of geodesy has therefore always been to determine the size, shape, and gravitational field of the earth (Clarke 2001; DMA 1984; Smith 1997; Seeber 1993; Hofmann-Wellenhof and Moritz 2005). Geodesy can be defined as the science which deals with the methods of precise measurements of elements of the surface of the earth and their treatment for the determination of the geographic positions on the surface of the earth including the gravity field of the earth in a three-dimensional time varying space. It also deals with the theory of size and shape of the earth. Satellite geodesy is the measurement of the form and dimensions of the earth, the location of objects on its surface and the figure of the earth’s gravity field by means of satellite technique (Seeber 1993). Traditional astronomical geodesy, that includes astronomical positioning, is not commonly considered a part of satellite geodesy. This chapter will discuss the essentials of geodesy to understand GNSS positioning.
Small baselines techniques of time series InSAR to monitor and predict land subsidence causing flood vulnerability in Sidoarjo, Indonesia
Published in Geomatics, Natural Hazards and Risk, 2022
Noorlaila Hayati, Amien Widodo, Akbar Kurniawan, I Dewa Made Amertha Sanjiwani, Mohammad Rohmaneo Darminto, Imam Satria Yudha, Josaphat Tetuko Sri Sumantyo
Time Series of InSAR shows an indication of subsidence in the southwest. However, the profile derived from the GPS measurements shows the highest deformation value compared to InSAR time series calculation. Considering the different techniques of satellite geodesy, this variation occurred because InSAR Techniques uses pulse waveform, and the spatial resolution is processed 30 m apart while GPS observations are based on points, not area. Second, there is a temporal difference between InSAR and GPS observation. InSAR used time series analysis for approximately 4 years from March 2017 to December 2020. Conversely, the GPS campaign was held from March to May 2021. Therefore, a simple adjustment was deployed to obtain an average deformation value for 2 months (compared to GPS observation). Finally, the accuracy and precision of GPS are more compatible compared to InSAR.
A geodetic study of the Alpine Fault through South Westland: using campaign GPS data to model slip rates on the Alpine Fault
Published in New Zealand Journal of Geology and Geophysics, 2018
Chris J. Page, Paul H. Denys, Chris F. Pearson
This region has been the subject of geodetic investigations since the 1980s (Blick 1986; Pearson 1990) that were based on the analysis of triangulation measurements, but subject to large measurement errors. The advent of satellite geodesy resulted in improved measurement precision that enabled studies such as Pearson et al. (2000) to examine slip rates on the Alpine Fault with a best fit model that accommodates c. 75% of the relative plate motion and a locking depth of c. 10 km. For the central South Island, Beavan et al. (1999) showed that the majority of the observed velocity signal (50–70%) is uniform slip along strike of the Southern Alps with a shallower locking depth of 5–8 km, which is consistent with higher crustal temperatures associated with a thinner crust. On the eastern side of the Southern Alps and away from the Alpine Fault, Denys et al. (2014, 2016) showed the spatial variation in strain accumulation within the Otago fault system.