Maka setiap cerapan akan memberikan data-data untuk dibuat pelarasan. Hasilnya lebih baik dan tepat daripada kaedah terabas. Pengukuran Graviti Relatif Secara Praktik Kaedah Gelung Kehadapan Forward Looping Method Kaedah ini biasa digunakan bagi menubuhkan jaringan graviti kejituan tinggi dimana pada setiap titik pengukuran dilakukan sebanyak tiga kali lihat rajah dibawah. Pengukuran Graviti Relatif Secara Praktik sambKaedah Gelung Kehadapan Sama seperti cerapan ukur aras, pengukuran boleh ditamatkan pada titik rujukan yang pertama ataupun pada titik rujukan yang lain di dalam kawasan kerja.
Kaedah ini mempunyai kelebihan kerana cerapan dapat memberikan maklumat drif yang lebih baik pada titik-titik yang dicerap dan bilangan cerapan untuk setiap titik adalah mencukupi bagi melakukan pelarasan yang baik dan menghasilkan keputusan yang jitu. Kekurangan kaedah ini hanyalah mengambil masa dan kos yang tinggi. Pengukuran Graviti Relatif Secara Praktik Kaedah Bintang Kaedah bintang ini melibatkan satu titik rujukan dimana cerapan dilakukan di stesen ini setiap kali melakukan cerapan dititik sekitarnya lihat rajah dibawah.
Kaedah ini boleh memberikan maklumat drif alat yang baik kerana cerapan dititik rujukan dilakukan berulangkali. Namun demikian, dari segi prosedur dilapangan, pencerapan ini akan mengambil masa yang agak panjang terutama apabila titik rujukan terletak jauh daripada titiktitik tersebut. Pembetulan diberikan untuk selisih sistematik seperti berikut : i.
Pembetulan Ketinggian Alat. Pembetulan Pengukuran Graviti ii. Pembetulan Pasang Surut Bumi. Pembetulan pasang surut bumi perlu dilakukan terhadap pengukuran jitu. Pembetulan Pengukuran Graviti iii. Drif adalah satu fenomena yang menyebabkan bacaan kaunter alat gravimeter berubah dengan masa.
Ini adalah kerana spring gravimeter sentiasa berada pada keadaan tegang dan secara perlahanlahan akan memanjang. Perubahan suhu boleh menyebabkan berlakunya drif pada alat gravimeter. Kesan drif boleh diminimakan dengan mengawal suhu bahagian-bahagian mekanikal alat yang berfungsi didalam bekas hampagas dan suhu spring.
Secara praktik garis null garis seimbang alat gravimeter dapat dilihat berubah mengikut masa disebabkan oleh deformasi sistem elastik alat gravimeter yang berubah. Penurunan Data Cerapan Graviti Secara amnya perubahan graviti adalah disebabkan oleh rupabentuk dan topografi bumi yang tinggi rendah.
Penurunan graviti ini perlu dibuat kepada satu aras rujukan sebelum data graviti dapat ditafsirkan untuk kajian yang bermakna. Terdapat berbagai kaedah penurunan graviti, tetapi kaedah utama yang digunakan dalam geodesi ialah :. Tiga tujuan utama penurunan graviti adalah : Interpolasi dan extrapolasi graviti Kajian struktur dalaman bumi.
Tujuan i dan ii adalah untuk memenuhi keperluan geodetik, sementara tujuan iii adalah untuk kegunaan geofizik dan geologi. Anomali Graviti Anomali graviti ditakrifkan sebagai nilai skala yang bersamaan dengan perbezaan antara magnitud graviti sebenar yang diukur g , diatas geoid dan graviti normal diatas elipsoid geosentrik, iaitu : Sesi Julai Sesi Dec Jika cepatan graviti, g diukur di beberapa tempat berhampiran permukaan laut tetapi pada latitud berlainan, didapati nilainya akan bertambah dari khatulistiwa menuju ke arah kutub.
Dimana 0 ialah graviti normal di Khatulistiwa. The known parameters are: gravity measured at the surface point P , gP, and normal gravity on the reference spheroid. Gravity on the geoid is computed by downward continuing gP to the geoid, and K is computed from the mathematically defined gradient of normal gravity above the reference spheroid. Matlamat penurunan cerapan graviti menggunakan kaedah Bouguer adalah untuk menghilangkan kesan keseluruhan jisim topografi diluar permukaan geoid. Bouguer menggunakan konsep plet plate yang dipanggil plet Bouguer, dimana andaian dibuat bahawa kawasan disekitar stesen A adalah mendatar dan jisim diantara geoid dan permukaan bumi mempunyai ketumpatan yang tetap iaitu.
Gabungan penganjakan jisim topografi dan penggunaan penurunan udara bebas dipanggil penurunan Bouguer lengkap. Hasilnya ialah graviti Bouguer diatas geoid, iaitu; g B! Prosidur diatas boleh disempurnakan lagi dengan mengambilkira penyimpangan topografi sebenar dari kepingan Bouguer.
Prosedur ini dipanggil pembetulan rupabumi terrain correction. Maklumat Graviti Dari Cerapan Satelit Satelit rekaan yang digunakan bagi tujuan geodetik beredar mengelilingi bumi di dalam sistem orbitalnya pada ketinggian dimana pengaruh medan graviti bumi memberi kesan terhadap laluan orbitnya.
Kesan tarikan graviti ini akan menghasilkan orbit terganggu bagi satelit tersebut. Oleh itu melalui data cerapan satelit, anomali graviti boleh dinilai dari orbit satelit yang mengelilingi bumi. Open navigation menu. Close suggestions Search Search. User Settings. Skip carousel. Carousel Previous. Carousel Next. What is Scribd? Explore Ebooks. Bestsellers Editors' Picks All Ebooks. Explore Audiobooks.
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Significant Achievements in Satellite Geodesy Jump to Page. Search inside document. Sesi Julai Sesi Dec Dengan gabungan maklumat pengukuran arka dipermukaan bumi, penentuan saiz bumi dapat dilakukan. Click here to sign up. Download Free PDF. Physical Geodesy. JP Tianco. A short summary of this paper. Download Download PDF. Translate PDF. Jose Paolo B. R Helmert, Geodesy, over the years of continuous research and studies involving the topic, has expanded towards many branches that deal with more specific topics and fields.
These branches include geometric geodesy, geodetic astronomy, satellite geodesy, and physical geodesy. Each of these branches is related to one another yet they are unique in their approach towards the more specific concepts of geodesy. Physical geodesy is the branch of geodesy that deals with the physical properties of the gravitational field of the earth. The geoid is the equipotential surface of the Earth at the mean sea level. This means that without the tides, waves, and oceanic currents, the ocean surface coincides the geoid.
The geopotential is the differences in potential of the gravitational field of the Earth while the geoid is the equipotential surface surface with constant geopotential of the geopotential.
Gravity and the gravitational field are significant and rather basic concepts used in geodesy because of the dependence of the geopotential to these concepts. The gravitational force vector is defined with respect to a potential in the following equation: It is also important to consider that the Earth is not just a stagnant body and that it rotates around an axis.
This rotation of the Earth causes a centrifugal force that also has a great effect on the geopotential. This force can also be derived from a potential and is described by the following equations.
Equation 1 shows the original equation that defines the centrifugal force and Equation 2 shows the force as its x, y, and z components gradient and omega as the angular velocity. The resultant of the gravitational force vector and the centrifugal force vector is the actual gravity vector.
Consequently, the potential of gravity, W, is the sum of the potentials of the two force vectors mentioned above. These two are only a few of the many concepts related to physical geodesy. Some of these concepts are discussed and dissected in a more in depth manner in the following parts of this paper.
Significance The study of physical geodesy has numerous advantages in different fields of earth sciences, engineering, urban planning, etc. Below is a summary of the importance of physical geodesy to other related fields. These examples are further discussed in another section of this paper. Maps and surveys done using geodetic from the geopotential control points are more reliable. Also, gravity is one of the most important sources of information used in geophysics.
Also, it provides geologists with knowledge of the location and stability of certain geological formations. Physical geodesy in the Philippines is a scarcity, to say the least. Only a few researches and projects have been inclined into further improving the state of physical geodesy of the country. One report submitted to the United Nations Economic and Social Council prepared by the Philippines described the state of surveying, mapping, and physical geodesy in general, of the country.
The report first described the general geographic features of the Philippines land area, highest point, terrain classification, etc. The report focused on the Geodetic Network Infrastructure of the Philippines, citing the National Mapping and Resource Information Authority NAMRIA as the primary agency to carry out water, coastal and land surveys as well as mapping, remote sensing, and other related services for both the government and the general public.
The report also discussed PRS92, the GPS stations that were set up all over the country first order, second order, 36 third order and how these stations are interconnected to form a network of stations. One of the researches done in the country regarding physical geodesy is entitled Research and Development in Support of the Implementation of the Philippine Reference System of Results and Recommendations done by Enrico C.
Paringit, Diony A. Ventura, and Jose Galo P. Isada published in October This local geoid model must be developed and validated from the global geopotential models. The whole survey that was done made use of the PRS92 as its single reference frame. One particular and practical impact the project had was through the standardization of surveys on land parcels throughout the country.
Prior to PRS92, land parcels in the country were surveyed through either triangulation from the previously determined Luzon datum of found in Balanacan, Marinduque or through azimuth derived from astronomic observation. Because they are obtained from two different reference systems, the azimuth obtained from astronomic observation and the azimuth obtained from GNSS-based geodetic measurements have their own differences.
The project provided a comparative analysis between the azimuths obtained from the two different reference systems. The analysis showed adjustments or transformations that can be done to relate the old data astronomic to the newer reference system geodetic.
This made it possible to relate the old azimuths to more popular reference systems with the use of both GNSS and PRS92 which is a more practical and accurate approach to land parcel surveying. According to results and findings of the 8-month long period of field observations, the geodetic control network of the Philippines is not as accurate as how the country needs it to be. A static network, especially for the Philippines, is highly undesirable. Also, for a more accurate and efficient reference system, it was highly recommended that gravity anomaly and deflection of the vertical must be verified to the quantities obtained through field observations.
Two more concepts that are used in physical geodesy, and consequently, important to introduce, are the concepts of level surfaces and plumb lines. Level surfaces are surfaces whose potential is constant. Level surfaces are sometimes referred to as equipotential surfaces.
An example of an equipotential surface or a level surface is the geoid. Two vectors, whose scalar product is computed to be 0, are said to be orthogonal to one another. This is true to the relationship of the gravity vector and the equipotential surfaces such as the geoid. The imaginary lines orthogonal to the geoid are called the plumb lines. Plumb lines are important because their intersections with points on the surface of the Earth are computed to be the orthogonal height H of these points.
The orthogonal height is the difference in elevation computed from the geoid. The figure below shows the relationship between level surfaces, the geoid, orthogonal height and the gravity vector. Figure 3: Level surfaces and plumb lines Moritz, There are other types of gravity calculations and the experimentation and procedures that govern them. These calculations vary from one procedure to the other in terms of approximations in the values of the gravity field, geoid model, and other concepts that rely on these data with measurements.
For example, for gravity computations, the level ellipsoid is more often used. Level ellipsoids or reference ellipsoids are, unlike other geometric models, geocentric. Geocentric models have their centers coincided with the mass center of the Earth. This is because the gravity vector, from our basic Science courses, points directly towards the mass center of the Earth. The ellipsoid takes the place of the geoid and, consequently, is assumed to be a level surface with constant potential.
Mathematical calculations involving this ellipsoid is much easier and to conduct and formulas can be understood and formulated more clearly. Gravity, height, and potential calculations are done with respect to this level ellipsoid using the same concepts of level surfaces and plumb lines. The gravity obtained from the level ellipsoid is called the normal gravity. This ellipsoid and the normal gravity field are determined by four constants: Reference Ellipsoid constants These four constants, again, vary from one calculation procedure to the other.
Calculations using these varied constants also have their own differences.
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