Earth_model...2.ppt
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The document discusses the modeling of the Earth, emphasizing the distinctions between various models including ellipsoids and geoids. It outlines the challenges in achieving consistent measurements globally due to regional variations in survey methods and data analysis. Key globally applicable ellipsoids like GRS80 and WGS84 are highlighted, along with the concepts of orthometric height, ellipsoidal height, and geoidal separation.
Earth_model...2.ppt
- 2. Modeling the Earth • The best model of the Earth is 3D globe • For measuring the Earth, Globes have certain drawbacks
- 3. Modeling the Earth • At any point on Earth there are three important surfaces, the Ellipsoid, the Geoid, and the Earth surface
- 5. Ellipsoid Mathematical surface obtained by revolving an ellipse around earth’s polar axis
- 6. Ellipsoid Model of the Earth’s Shape
- 7. Ellipsoid • Different ellipsoid were adopted in various parts of the world Why difference in Ellipsoidal Estimates? • Because there were different sets of measurements used in each region or continent • These measurements often could not be tied together or combined in a unified analysis • Due to differences in survey methods and data analyses.
- 8. Local or Regional Ellipsoid • Origin, R1, and R2of ellipsoid specified such that separation between ellipsoid and Geoid is small • Examples: Clarke 1880
- 9. Global Ellipsoid • Global ellipsoid selected so that these have the best fit “globally”, to sets of measurements taken across the globe • Example: Geodetic Reference System of 1980 (GRS 1980), World Geodetic System 1984 (WGS84)
- 11. Globally Applicable Ellipsoids • Extremely precise measurements across continents and oceans were possible using data derived from satellites, lasers, and broadcast timing signals • This has led to the calculation of globally applicable ellipsoids such as GRS80, or WGS84
- 12. Geoid Definition: “Three-dimensional surface along which the pull of gravity is constant” OR “A continuous surface which is perpendicular at every point to the direction of gravity”
- 13. Geoid • True shape of the Earth varies slightly from the mathematically smooth surface of an ellipsoid • Differences in the density of the Earth cause variation in the strength of the gravitational pull, in turn causing regions to dip or bulge above or below a reference ellipsoid • This undulating shape is called a Geoid
- 15. Measuring Heights • Orthometric Height/Elevation: Vertical distance above a geoid • Ellipsoidal Height: Heights above the ellipsoid • Geoidal Height/Geoidal Separation: The difference between the ellipsoidal height and geoidal height at any location – Geoidal heights vary across the globe – The absolute value of the geoidal height is less than 100 meters at most of the Earth locations – The geoid is not a mathematically defined surface rather it is a measured and interpolated surface
- 17. References • Bolstad Text Book • http://courses.washington.edu/gis250/lessons/projection/