Coordinate System of the Earth PDF

Coordinate system of the Earth 1 Shape of the earth There are three commonly used approximations of the shape of the earth: sphere, ellipsoid, and geoid. The sphere is the simplest approximation of the earth’s shape as it is defined by a single radius....

Coordinate system of the Earth 1 Shape of the earth There are three commonly used approximations of the shape of the earth: sphere, ellipsoid, and geoid. The sphere is the simplest approximation of the earth’s shape as it is defined by a single radius. A parameter called the flattening (f), defined by (a-b)/a The ellipsoid is closer to the earth’s shape as defined by semi-major and semi- minor axes. The geoid is the closest approximation of the earth’s shape based on gravity measurements that help define global mean sea level. The reasons why the sphere and ellipsoid are still used are that they are much easier to use for calculations and when defined properly can be exceptionally accurate. 2 3 Ellipsoids The ellipsoid is the most commonly used of the three approximations of the earth’s shape today. There is not a single ellipsoid that fits earth well; therefore, there have been many official ellipsoids throughout the 19th and 20th centuries. Different ellipsoids were adopted because different measurements were used in different countries when defining these ellipsoids, and geodetic surveys were isolated by large bodies of water. Ellipsoid created by one country may approximate that portion of the earth very well, but may not be a suitable approximation for another portion of the earth. 4 Datum A datum is a mathematical model of the Earth, which serves as the reference or base for calculating the geographic coordinates of a location. A shift of the datum will result in the shift of positions of points. 5  Datums serve as starting points of reference for surveying and mapping as they link the physical earth to a mathematical coordinate. A datum is a reference surface for measuring locations on the earth. A datum has two major components: the specification of an ellipsoid, which is an ellipsoid that has been surveyed and defines the origin and orientation of latitudes and longitude lines. Datums: There are three common datums that you will use in GIS: North American datum of 1927 (NAD 27), North American datum of 1983 (NAD 83), and World Geodetic System of 1984 (WGS 84). 6 What is a coordinate system? ▪ A coordinate system is a method for identifying the location of a point on the earth. ▪ Coordinate systems -(x,y,z) coordinate systems for map data. ▪ In GIS, data is always stored in some coordinate system. ▪ Coordinate systems enable geographic datasets to use common locations for integration. ▪ A coordinate system is a reference system used to represent the locations of geographic features, imagery, and observations such as GPS locations within a common geographic framework. ▪ Coordinate Reference System (CRS), also called Spatial Reference System (SRS) in QGIS. 7 Types of coordinate systems ✓ In GIS, data is always stored in some coordinate system. ✓ We can differentiate between three main coordinate system types: 1.Geographical coordinate systems (in which spherical coordinates are measured from the earth's center) – for example, WGS-84 used by GPS 2.Planimetric (Projected) coordinate systems (in which the earth's coordinates are projected onto a two-dimensional planar surface) – for example Arc 1960 / UTM Zone 37S, Adindan / UTM Zone 37N 3.Non-geographic, case-specific coordinate systems – for local surveying work. 8 Geographical Coordinate Systems A geographic coordinate system (GCS) uses a three-dimensional spherical surface to define locations on the earth. ❑ A point is referenced by its longitude and latitude values. ❑ Longitude and latitude are angles measured (in degrees) from the earth's center to a point on the earth's surface. 9  Parallels (lines of latitude) and meridians (lines of longitude), from a graticular network. For most geographic coordinate systems, the prime meridian (0 degrees) is the longitude that passes through Greenwich, England. Positive Longitude values increase towards East and negative Longitude values decrease towards West. The line of latitude midway between the poles, the horizontal axis, is called the equator and defines the line of zero latitude. Positive Latitude values increase towards North and negative Latitude values decrease towards South. The origin of the graticule (0,0) is defined by where the equator and prime meridian intersect. 10  Latitude and longitude values are traditionally measured either in decimal degrees or in degrees, minutes, and seconds (DMS). Latitude values are measured relative to the equator and range from -90° at the South Pole to +90° at the North Pole Longitude values are measured relative to the prime meridian, ranging from -180° when traveling west to +180° when traveling east. In GIS, longitude = X and latitude = Y coordinate. Although longitude and latitude can locate exact positions on the surface of the globe, they are not uniform units of measure. Only along the equator does the distance represented by one degree of longitude approximate the distance represented by one degree of latitude. IMPORTANT! Because degrees of latitude and longitude don't have a standard length, you can’t measure distances or areas accurately or display the data easily on a flat map or computer screen. 11 Map Projection ❖ To represent 3D space (the earth's surface) 2D space (a map), a mathematical transformation called a projection must be performed. ❖ Projected coordinate system is thus always based on some geographical coordinate system. ❖ A spheroid can’t be flattened to a plane any easier than flattening a piece of orange peel—it will rip. ❖ Representing the earth’s surface in two dimensions causes distortion in the shape, area, distance, or direction of the data. ❖ A projection can keep any one of these four properties correct, but causes distortions in all other three. ❖ Many different map projections exist. You should still be aware of the properties of the selected projection and the influence of those properties on your maps and geographic analysis 12 Map Projection 13