Geodesy

Lec 9/9/08 The rise of ?science? in mapmaking efforts Maps = documents to record observations by mariners Portuguese map of the world (1529) is the best of the era Rise of map projections (formal mathematical system for transforming a globe to a flat plane) in 1626 Defensible; based on math Graticule: lines of latitude and longitude 17th and 18th centuries: rise of thematic mapping (ex: isoline, choropleth, dot maps) Concepts of distribution and space arose during the Enlightenment: vegetation maps, rock types, climate, population density National Mapping Agencies arise 18th -19th centuries: advancement in surveying technology ? better data upon which to make maps Big Ideas: Societal ideas/beliefs influence maps Map-making technology Exploration improves data acquisition and map making GEODESY: the field of study investigating Earth?s size and shape. Geodesy must be established in order to create a coordinate system used to make a map Spherical Earth 6th century BCE: Pythagoras thinks earth must be ?the perfect shape? and therefore a sphere Aristotle (4th century BCE): as ships sail away they disappear hull first, mast last? therefore Earth must be spherical Greeks accepted spherical shape but size disputed Eratosthenes first calculated the Earth?s circumference: radius is 6371 km, circumference is 40,031.2 km Ellipsoidal Earth Late 1600s: Sir Isaac Newton thinks Earth is an ellipsoid: bulging at center, flat at poles Predicted a flattening of 1/300 (actually 1/298) Confirmed by Global expeditions of Earth and remote sensing Ellipsoid: basis for WGS 72 and 84, and the Clark 1866 ellipsoid ? datums common in GIS files Shape determined by length of equatorial (semi-major) and polar (semi-minor) axis Polar flattening: f=(a-b)/a Geoidal earth: irregular ellipsoid Sea-level equipotential surface: surface on which gravity is everywhere equal to its strength at mean sea level (over both land and water) Deviates from ellipsoid by up to 100 meters due to tidal and gravitational forces Used to calculate the elevation of sea level (average surface of the ocean) Geoid rises generally over the continents (thick crustal regions) and falls over the oceans (thinner crust) Improvements in Geoid Measurement due to grace expedition (measures ?Gravity anomaly?) Surface highest at red spots Density of land mass determines gravitational pull Geodetic Datum: model that describes the location direction and scale relationships with the surface of the Earth Datum = anchor point that defines the location direction and scale relationships with the surface of the earth Horizontal datum provides precise description of latitude and longitude based on base value points Common datums: NAD (North American Datum); Meade?s Ranch, KS = base value for NAD27 NAD83 uses GRS80 reference ellipsoid centered at Earth?s center of mass Latitude and longitude measurements differ greater than 400 meters in some locations (between the 2 datums) Datums are most important for large and medium scale mapping: applications that require a higher coordinate accuracy Small scale maps don?t change much by changing the datum Geographic Coordinates Entire coordinate geometry is keyed on these points: Axis of Rotation: Earth?s axis, which is aligned with the North Star and passes through the North and South Poles Equator: intersections of surface with imaginary plane perpendicular to bisecting the Earth?s axis of rotation Latitude: location on Earth?s surface between the equator and the Earth?s poles Function of angle between horizon and the North Star Parallels: imaginary plane parallel to the equator forming a small circle of parallel lines Longitude Made of meridians ? plane intersecting the axis of rotation in a line: will intersect the surface of Earth in a circle Half of a circle, between the two poles = meridian Lines of constant longitude between two poles To measure, you need a constant starting point (prime meridian) Divides east and west hemisphere Very difficult to determine Harrison invented a clock to determine time difference and thus solve the problem of longitude Graticule: imaginary network of parallels and meridians Human construct based on uniquely defined reference points System isn?t immutable (ex: Paris was defined as Prime Meridian before Greenwich) Characteristics: Parallels are parallel Parallels are equally spaced on meridians. Meridian and great circles appear straight when viewed orthogonally (eye in the same plane) Meridians converge at poles Meridian is equally spaced on each individual parallel but spacing decreases towards poles Meridian and parallels are equally spaced at the equator Parallels and Meridian intersect at a right angle Poles = points