Physical Geodesy 2nd Semester Midterms PDF

GEODESY Geodesy is the science of accurately Engineering Surveys measuring and understanding three fundamental properties of the Earth: its Satellite Geodesy geometric shape, its orientation in space, and its gravity field— as well as the c...

GEODESY Geodesy is the science of accurately Engineering Surveys measuring and understanding three fundamental properties of the Earth: its Satellite Geodesy geometric shape, its orientation in space, and its gravity field— as well as the changes of these properties with time. WHAT IS PHYSICAL GEODESY? When measuring positions and relations between the points on the surface of the earth Geodesy taks its etymology from two Greek (as well as above and below) we are subject to words, GEO meaning EARTH and DAIESTHAI all kinds of physical influences from the physical meaning TO DIVIDE. environment. Our instruments obey some physical laws and APPLICATIONS OF GEODESY laws which we must try to understand in order to be able interpret our measurements. Primary or Zero order Triangulation, trilateration, and traverse. PHYSICAL INFLUENCE The measurement of height above sea- level by triangulation or spirit levelling. Astronomical observations of Latitude, Longitude and azimuth to locate origin of surveys, and to control their direction. Crustal movement. To detect changes in the relative positions on the ground, and their heights above sea level. Observation of the direction of gravity by astronomical observations for latitude and longitude. WHAT IS GRAVITY? Observation of the intensity of Gravity by the pendulum and other apparatus. To deduce the exact form of Earth’s sea level equipotential surfaces at all heights. Legend has it that Isaac Newton formulated Polar motion studies gravitational theory in 1665 or 1666 after watching an apple fall and asking why the Earth tides apple fell straight down, rather than sideways or even upward. The separation between the Geoid and the mean sea level "He showed that the force that makes the apple fall and that holds us on the ground is the same WHAT IS GEOID? as the force that keeps the moon and planets in their orbits," The geoid is the equipotential surface of the Earth at the mean sea level. This means that “WHAT GOES UP, MUST COME DOWN.” without the tides, waves, and oceanic currents, the ocean surface coincides the geoid. The Gravity is the invisible force by which a planet or geoid is greatly affected by the Earth’s gravity other body draws objects toward its center. The field or more commonly known as the force of gravity keeps all the planets in orbit geopotential. around the sun. Albert Einstein described gravity as a curve in space that wraps around an object—such as a star or a planet. If another object is nearby, it is pulled into the curve. Anything that has mass also has gravity. Objects with more mass have more gravity. Gravity also gets weaker with distance. So, the closer objects are to each other, the stronger their gravitational pull is. Earth's gravity comes from all its mass. All its mass makes a combined gravitational pull on all the mass in your body. That's A geoid is the irregular-shaped “ball” that what gives you weight. And if you were on a scientists use to more accurately calculate planet with less mass than Earth, you would depths of earthquakes, or any other deep weigh less than you do here. object beneath the earth's surface. While we often think of the earth as a sphere, WHAT IS PHYSICAL GEODESY? our planet is very bumpy and irregular. the study of the physical properties of the The radius at the equator is larger than at the gravity field of the Earth, the geopotential, with poles due to the long-term effects of the earth's a view to their application in rotation. And, at a smaller scale, there is geodesy. topography—mountains have more mass than a valley and thus the pull of gravity is regionally stronger near mountains. DEFINITION OF PHYSICAL GEODESY This irregular shape which is called "the geoid," Physical geodesy is the branch of geodesy that is a surface which defines zero elevation. Using deals with the physical properties of the complex math and gravity readings on land, gravitational field of the Earth. Physical geodesy surveyors extend this imaginary line through the uses the geoid as a primary model for the continents. This model is used to measure Earth’s shape and its other properties. surface elevations with a high degree of accuracy. When a more accurate reference figure is The Earth’s gravity field determines the geoid, required, an ellipsoid of revolution is used as a which is the equipotential surface at mean sea representation of Earth’s shape and size. level. If the oceans would be at rest—no waves, no currents, no tides—the ocean surface would coincide with the geoid. In reality it deviates by WHAT IS AN ELLIPSOID? up to 1m. The difference is called sea surface topography. It reflects the dynamical Ellipsoid, closed surface of which all plane cross equilibrium in the oceans. Only largescale sections are either ellipses or circles. An ellipsoid currents can sustain these deviations. is symmetrical about three mutually perpendicular axes that intersect at the center GEOPHYSICS The Earth’s gravity field reflects the internal mass BASIC CONCEPT OF PHYSICAL GEODESY distribution, the determination of which is one of the tasks of geophysics. By itself gravity field knowledge is insufficient to recover this distribution. A given gravity field can be produced by an infinity of mass distributions. Nevertheless, gravity is an important constraint, which is used together with seismic and other data. Physical Geodesy utilizes measurements and HYDROLOGY characteristics of the Earth’s gravity field as well as theories regarding this field to deduce the Minute changes in the gravity field overtime— shape of the geoid and in combination with arc after correcting for other time-variable effects measurements, the Earth's size. With the like tides or atmospheric loading—can be sufficient information regarding the Earth’s attributed to changes in hydrological gravity field, it is possible to determinized parameters: soil moisture, water table, snow undulations, gravimetric deflections, and the load. For static gravimetry these are usually Earth’s flattening. nuisance effects. Nowadays, with precise satellite techniques, hydrology is one of the main aims of spaceborne gravimetry. Despite a LINK OF PHYSICAL GEODESY TO EARTH low spatial resolution, the results of satellite SCIENCES gravity missions may be used to constrain basin- scale hydrological parameters. OCEANOGRAPHY  GLASIOLOGY AND SEA LEVEL GEOTHECNICAL ENGINEERING To gain knowledge about the subsurface structure, gravimetry is a valuable tool for certain geotechnical (civil) engineering projects. One can think of determining the depth-to-bedrock for the layout of a tunnel. Or making sure no subsurface voids exist below the planned building site of a nuclear power plant. Glaciology and sea level. The behavior of the HEIGHTS AND SURFACES Earth’s ice masses is a critical indicator of global climate change and global sea level behavior. Thus, monitoring of the melting of the Greenland and Antarctica ice caps is an important issue. GEOPHYSICAL PROSPETING The geoid approximates mean sea level. The shape of the ellipsoid was calculated based on the hypothetical equipotential gravitational surface. A significant difference exists between this mathematical model and the real object. However, even the most mathematically sophisticated geoid can only approximate the real shape of the earth. Since gravity contains information on the SURFACES subsurface density structure, gravimetry is a standard tool in the oil and gas industry (and 1. Earth Surface – this is the surface that we see other mineral resources for that matter). It will and walk on, where our equipment is being set always be used together with seismic profiling, up on or originally referenced to this surface. test drilling and magnetometry. The advantages of gravimetry over these other 2. Geoid Surface – this is related to the techniques are relatively inexpensive, non- gravitational field of the Earth and also known destructive (one can easily measure inside as the mean sea level. buildings), compact equipment, e.g. for borehole measurements Gravimetry is used to 3. Ellipsoidal Surface – this is not a physical localize salt domes or fractures in layers, to surface, but a mathematical surface. It is the estimate depth, and in general to get a first best fit of an ellipsoid to idea of the subsurface structure. the surface of the Earth. Often used for GPS and other Satellite measurements.  - END - Reference and Supplementary Material: HEIGHTS Lesson presentation by Engr. Mikaela Grace C. Lugma, RGE. 1. Orthometric Height (H)– the vertical distance H along the plumb line from a place of interest (Note: This trans is not for sale. Any attempt to acquire it without to the geoid, the vertical datum that consent is considered property infringement.) approximates mean sea level. 2. Geoid Height (N) – is the difference of the vertical distance between the reference geoid Prepared by: and the ellipsoid. 3. Ellipsoidal Height (h) – the difference of the vertical distance between a point on the Earth's John Paul Santos Surface and the ellipsoid. BSGE 2 ‘25