Transcript 4Location
Where in the world are we? … • A system for describing location on the earth • What is the shape of the earth … • and how earth’s shape affects the process of determining location Location in 2 dimensions + The Cartesian coordinate system 3 (3,2) 2 1 + (0,0) 1 Y axis - X axis - 2 3 Location on a rotating sphere Starting points: • Origin (center of the globe) • Axis of rotation Great Circle Meridians: the “y” grid lines Meridian: a great circle that passes through the earth’s poles Prime meridian: Arbitrary starting point Longitude Longitude: angular location, on the earth’s surface, from the Prime Meridian Circle has 360 degrees total: Meridians range from 180° east to Prime 180° west Meridian 60° Location on a rotating sphere: “x” axis Great Circle that is perpendicular to the axis of rotation the equator Parallels: the “x” grid lines Small Circle Parallel: • A small circle that is parallel to the equator (and at right angles to the axis of rotation) Latitude North Pole 90° Latitude: angular location on the earth’s surface, between the equator and one of the poles 60° Equator 0° 90° South Pole Longitude and latitude: Measures of angle Longitude Latitude Meridians & parallels: Lines of a constant lat. or long. Parallel Meridian Geographic Coordinate System: • Parallels & meridians (the graticule) • Latitude and longitude A system for defining location on a sphere! But is the earth really a sphere? The earth’s true shape: an ellipsoid? N Rotation of the earth generates centrifugal force Polar flattening S So … Ellipsoidal shape of earth does not affect latitude and longitude, but it does affect the spacing of parallels & meridians! North Pole 0° Equator Length of 1°: from Equator: 68.7 miles from N or S Pole: 69.4 miles Extremely accurate maps must base measurement of distances on an ellipsoid rather than a spherical shape. Don’t need to know the math but need to know the concept The earth’s true shape: A geoid? When doing precise mapping, it turns out that we need to consider a “bumpiness” to the earth’s surface. Not mountains! This bumpy shape is caused not by mountains, but by variations in the density of the earth’s crust. These variations cause variations in the effect of gravity. Thus, a geoid is a model of the earth’s surface taking into account variations in gravity. Variations in gravitational pull Geoid: A model of the earth’s surface showing variations in gravity. Given that the shape of the earth is so complex ... How do we figure out where, precisely, we are?