Coordinate system Geographical coordinate system

UTm Universe transvers mercator, Geographic coordinate system
geoid, planar projection, cylindrical and conical projection, longitudinal, latitude, UTM zones 60 zones

1. Coordinate System D. Naresh Kumar Assistant Professor Civil Engineering Department St. Martin’s Engineering College

2. Geographic Coordinate Systems A geographic coordinate system is a reference system for identifying locations on the curved surface of the earth. Locations on the earth’s surface are measured in angular units from the center of the earth relative to two planes: the plane defined by the equator and the plane defined by the prime meridian (which crosses Greenwich England). A location is therefore defined by two values: a latitudinal value and a longitudinal value. D. Naresh Kumar St. Martin's Engineering College

3. latitudinal lines are shown on the left and examples of longitudinal lines are shown on the right. D. Naresh Kumar St. Martin's Engineering College

4. A GCS is defined by an  ellipsoid ,  geoid  and  datum Sphere and Ellipsoid Assuming that the earth is a perfect sphere greatly simplifies mathematical calculations and works well for small-scale maps (maps that show a  large  area of the earth). However, when working at larger scales, an ellipsoid representation of earth may be desired if accurate measurements are needed. An ellipsoid is defined by two radii: the semi-major axis (the equatorial radius) and the semi-minor axis (the polar radius). D. Naresh Kumar St. Martin's Engineering College

5. Geoid Representing the earth’s true shape, the  geoid , as a mathematical model is crucial for a GIS environment. However, the earth’s shape is not a perfectly smooth surface. It has undulations resulting from changes in gravitational pull across its surface. These undulations may not be visible with the naked eye, but they are measurable and can influence locational measurements. Note that we are not including mountains and ocean bottoms in our discussion, instead we are focusing solely on the earth’s gravitational potential which can be best visualized by imagining the earth’s surface completely immersed in water and measuring the distance from the earth’s center to the water surface over the entire earth surface D. Naresh Kumar St. Martin's Engineering College

6. Datum The alignment can be local where the ellipsoid surface is closely fit to the geoid at a particular location on the earth’s surface geocentric  where the ellipsoid is aligned with the center of the earth. How one chooses to align the ellipsoid to the geoid defines a  datum . D. Naresh Kumar St. Martin's Engineering College

7. Local Datum` Geocentric Datum D. Naresh Kumar St. Martin's Engineering College

8. Projected Coordinate Systems The surface of the earth is curved but maps are flat. A projected coordinate system (PCS) is a reference system for identifying locations and measuring features on a flat (map) surface. It consists of lines that intersect at right angles, forming a grid. Projected coordinate systems (which are based on Cartesian coordinates) have an origin, an  x  axis, a  y  axis, and a linear unit of measure. Going from a GCS to a PCS requires mathematical transformations. The myriad of projection types can be aggregated into three groups:  planar ,  cylindrical  and  conical . D. Naresh Kumar St. Martin's Engineering College

9. Planar Projections A planar projection (aka Azimuthal projection) maps the earth surface features to a flat surface that touches the earth’s surface at a point ( tangent  case), D. Naresh Kumar St. Martin's Engineering College

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11.  Cylindrical Projection A cylindrical map projection maps the earth surface onto a map rolled into a cylinder (which can then be flattened into a plane). The cylinder can touch the surface of the earth along a single line of tangency (a  tangent  case), D. Naresh Kumar St. Martin's Engineering College

12. This is a popular projection used in defining the Universal Transverse Mercator (UTM) and State Plane coordinate systems. The UTM PCS covers the entire globe and is a popular coordinate system in the US. It’s important to note that the UTM PCS is broken down into zones and therefore limits its extent to these zones that are 6° wide.  D. Naresh Kumar St. Martin's Engineering College

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14. Conical Projection A conical map projection maps the earth surface onto a map rolled into a cone. Like the cylindrical projection, the cone can touch the surface of the earth along a single line of tangency (a  tangent  case), D. Naresh Kumar St. Martin's Engineering College

15. Geographic Coordinate System Longitude  specifies positions east and west as the angle between the  prime meridian  and a second  meridian  that intersects the point of interest. Longitude ranges from +180 (or 180° E) to -180° (or 180° W). 180° East and West longitude together form the International Date Line. Latitude  specifies positions north and south in terms of the angle subtended at the center of the Earth between two imaginary lines, one that intersects the  equator  and another that intersects the point of interest. Latitude ranges from +90° (or 90° N) at the North pole to -90° (or 90° S) at the South pole. A line of latitude is also known as a  parallel . At higher latitudes, the length of parallels decreases to zero at 90° North and South. Lines of longitude are not parallel but converge toward the poles. Thus, while a degree of longitude at the equator is equal to a distance of about 111 kilometers , that distance decreases to zero at the poles. D. Naresh Kumar St. Martin's Engineering College

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