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 (3,2) X axis Y axis 123 (0,0)
Location on a rotating sphere Starting points: Origin (center of the globe) Axis of rotation
Great Circle
Meridian: a great circle that passes through the earth’s poles Prime meridian: Arbitrary starting point Meridians: the “y” grid lines
Longitude Longitude: angular location, on the earth’s surface, from the Prime Meridian Circle has 360 degrees total: Meridians range from 180° east to 180° west Prime 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: angular location on the earth’s surface, between the equator and one of the poles Latitude North Pole 90° South Pole 0° Equator 60°
Longitude and latitude: Measures of angle Longitude Latitude
Meridians & parallels: Lines of a constant lat. or long. Meridian Parallel
Geographic Coordinate System: A system for defining location on a sphere! But is the earth really a sphere? Parallels & meridians (the graticule) Latitude and longitude
The earth’s true shape: an ellipsoid? Rotation of the earth generates centrifugal force Polar flattening N S
So … Ellipsoidal shape of earth does not affect latitude and longitude, but it does affect the spacing of parallels & meridians! Don’t need to know the math but need to know the concept 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. North Pole 0°Equator
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. 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. Not mountains!
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?