1/28/16. EGM101 Skills Toolbox. Oblate spheroid. The shape of the earth Co-ordinate systems Map projections. Geoid

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1 EGM101 Skills Toolbox Oblate spheroid The shape of the earth Co-ordinate systems Map projections The geoid is the shape that the surface of the oceans would take under the influence of Earth's gravitation and rotation alone, in the absence of other influences such as winds and tides. Geoid Specifically, the geoid is the equipotential surface that would coincide with the mean ocean surface of the Earth if the oceans and atmosphere were in equilibrium, at rest relative to the rotating Earth. The World Geodetic System (WGS) is a standard for use in cartography, geodesy, and navigation. It comprises a standard coordinate system for the Earth, a standard spheroidal reference surface (the datum or reference ellipsoid) for raw altitude data, and a gravitational equipotential surface (the geoid) that defines the nominal sea level. The latest revision is WGS 84, established in 1984 and last revised in WGS 84 is the reference coordinate system used by the Global Positioning System. 1

2 The Hypsographic Curve Illustrates the relationship between height of land and depth of oceans Shape of curve supports theory of plate tectonics 2 flat and 3 sloped areas -= uneven distribution of area at different depths and elevations Hypsographic curves for other planets (derived from satellite data) used to determine if plate tectonics actively modifies surfaces 3 (Major) Ocean Floor Provinces 1. Continental Margins 2. Deep-Ocean Basins 3. Mid-Ocean Ridges 2

3 Coordinate systems Coordinate Systems are ways of splitting up the world in order to form transferable units (numbers) that relate to points on a map. They are very useful, as a set of coordinate values can be given to almost anyone in the world and they can relate the coordinate values to a real life place. Different coordinate systems have different methods of splitting up the earth. A coordinate system enables every location on the earth to be specified by a set of coordinates of known location on a grid. There are two major global coordinate systems: The Geographic Coordinate System i.e. latitude and longitude pairs. The Universal Transverse Mercator System i.e. UTM coordinates. Latitude-Longitude system Meridian Parallel Universal Transverse Mercator (UTM) system 3

4 The Universal Transverse Mercator (UTM) projected coordinate system uses a 2-dimensional Cartesian coordinate system to give locations on the surface of the Earth. It is a horizontal position representation, i.e. it is used to identify locations on the Earth independently of vertical position, but differs from the traditional method of latitude and longitude in several respects. The UTM system is not a single map projection. The system instead divides the Earth into sixty zones, each a six-degree band of longitude, and uses a secant transverse Mercator projection in each zone. Map projections A map projection is any method of representing the surface of a sphere or other shape on a plane. Map projections are necessary for creating maps. All map projections distort the surface in some fashion. Depending on the purpose of the map, some distortions are acceptable and others are not; therefore different map projections exist in order to preserve some properties of the sphere-like body at the expense of other properties. There is no limit to the number of possible map projections. 1569: Flemish geographer and cartographer Gerardus Mercator created a map using a mathematical formula to project points on the earth s surface onto a map based on their latitude and longitude. Called the Mercator projection became the standard means of making maps for navigation, because the directions of the compass corresponded to directions on the map. North, east, south, and west were straight lines on paper, just as they are on the earth s surface. The problem with the Mercator projection is that it distorts areas and distances. The North and South Poles are stretched all the way across the top and bottom of the map, and regions to the far north and south appear much larger than they actually are. This isn t a problem for navigation Google Maps uses a Mercator projection even today and the distortion is negligible for maps of small regions. But it can give a false impression of the relative sizes of various countries and continents. For example, on the following map below, Greenland is larger than Africa. The creation of a map projection involves three steps: 1. Selection of a model f or the shape of the Earth or plane tary body (usually choosing between a sphere or ellipsoid). Because the Earth's actual shape is irregular, information is lost in this step. 2.Transformation of geographic c oordinate s (longitu de and lat itude) t o plane coordinates (eastings and northings or x,y). 3. Reduction of the scale (it does not matter in what order the second and third steps are performed). 4

5 Mollweide Projection Equal-area, pseudocylindrical map projection generally used for global maps of the world Robinson Projection 5