Topic 1. Introduction to Physical Geodesy

• The foundations of GNSS heavily rely on the concepts of elevation, and reference systems.
• Geodesy is one of the Earth Sciences and comes from the modern latin word “geodaesia”.
• Geo means Earth and daiein means to divide.
• “geodaesia” means Dividing the earth.
• Geodesy is less well known, although it is the oldest of the geosciences.
• The shape and size of the earth is a major topic in Geometric Geodesy.
• The gravity field of the earth is a major topic in Physical Geodesy.
• Point positioning is a major topic in Satellite Geodesy.
• According to the classical definition of FR HELMERT (A1880), geodesy is the “science of the measurement and mapping of the earth’s surface.”
• According to the Geodetic Glossary (NGS 2009), geodesy can be defined as “The science concerned with determining the size and shape of the Earth.”
• Geodesy is the science that locates positions on the Earth and determines the Earth’s gravity field.
• According to Smith (1997), geodesy is the science that determines the figure of the earth and the interrelation of selected points on its surface by either direct or indirect techniques.
• Geodesy is a branch of applied mathematics that includes observations for determining the size and shape of the earth and defining coordinate systems for 3-dimensional positioning the variation of phenomena near to or on the surface such as gravity, tides, earth rotation, crustal movement and deflection of the plumbline.
• Geometric Geodesy is concerned with describing locations in terms of geometry.
• Coordinate systems are one of the primary products of geometrical geodesy.
• Physical geodesy is concerned with determining the Earth’s gravity field, which is necessary for establishing heights.
• Satellite geodesy is concerned with using orbiting satellites to obtain data for geodetic purposes.
• Geodetic Astronomy is concerned with the study of determining position using measurements made by observing celestial bodies.
• Integrated Geodesy is a relatively new branch concerned with the unification of the physical and geometric components of the Earth.
• Physical Geodesy is concerned with determining the Earth’s gravity field, which is necessary for establishing heights.
• Satellite geodesy is concerned with using satellite positioning systems in determining position and defining the Earth’s size and shape.
• Gravity is one of the most important sources of information used in geophysics.
• In Geodetic Observations, the two most important physical influences are Refraction and Gravity.
• Physical geodesy has numerous applications, in various fields of earth sciences, mathematics, and other related fields.
• The ancient Greeks measured the heights of pyramids by sighting through high quality optical devices called oculi, which allowed them to measure high places more accurately than the Egyptians.
• Data from physical geodesy suggests that the outermost layers of the Earth are the ones adapting and adjusting to these changes so that isostasy can be restored.
• Since physical geodesy takes into consideration the “true shape” of the Earth, environmental concerns can be easily mapped out and identified using data obtained from physical geodesy.
• One interesting application of physical geodesy in the field of geology is the theory of isostasy.
• The history of physical geodesy begins with work by early Greek and Chinese mathematicians.
• The ancient Egyptians developed a technique of measuring the horizontal length of a pyramid by dragging a rope across it in different places, thus establishing the length of the sides twice.
• Geophysics deals with the physical response of the earth to a variety of forces and the internal structure of the earth affecting its motion.
• It is desirable that the reference be as close to the topographic surface as possible so that the individual points can be related to the reference surface in a simple manner.
• When measuring the positions and relations between the points on the surface of the Earth, we are all subject to all kinds of “Physical Influences” from the environment.
• Greek words “iso” and “stasis” meaning “equal standing”.
• The Theoretical understanding of the gravity field, its determination, and its relevance to the geometrical investigations (that constitutes to the main topic of surveying) is in the field of Physical Geodesy.
• Those “Physical Influences” are: Gravity Force, Coriolis Force, Air Refraction, Influences of Temperature Variations.
• In general, isostasy states that the lighter crust of the Earth “floats” above the Earth’s dense underlying mantle.
• Physical geodesy provides data of comparison for deviations from the mean sea level from an equipotential surface of the earth’s gravitational field (e.g., Geoid).
• The GNSS is a product of both physical geodesy and satellite geodesy.
• In geodesy, “close” to the topographic surface of the earth, is chosen as a reference surface.