Coordinate Systems key concepts. Projections and Coordinate Systems Data Quality Meta Meta Data

Transcription

1 Coordinate Systems

2 Coordinate Systems key concepts Projections and Coordinate Systems Data Quality Meta Meta Data

3 Projections and Coordinate Systems: Geographic Coordinate System Uses Uses 3D spherical surface to define locations Often Often incorrectly called a datum Includes Includes angular unit of measure, prime meridian and datum Point Point referenced by longitude/latitude Angles Angles measured by degrees

4 Parallels Lines of Latitude Latitude lines are parallel The equator defines the line of zero latitude Every degree of latitude is theoretically equal Parallels run east/west; measure distances north and south of equator

5 Meridians Lines of Longitude Meridians converge at the poles Line of zero longitude is called the Prime Meridian Distance of 1 longitude decreases toward the poles Meridians run north/south; measure distance east & west of Prime Meridian

6 Graticular Network Network of Lat/Long called a graticule Origin of graticule (0,0) where Equator and Prime Meridian intersect 4 geographic quadrants based on compass bearings from Origin

7 Degrees, Minutes, Seconds (DMS) Point on Earth s surface referenced by Lat/Long values Lat/Long based on 360 Each degree has 60 minutes Each minute has 60 seconds

8 Decimal Degrees (DD) Similar Similar to DMS Minutes Minutes and seconds expressed as decimal values ESRI ESRI products require DD in geodatasets

9 Converting from DMS to DD 37 36' 30" (DMS) Divide Divide each value by the number of minutes or seconds in a degree: 36 minutes =.60 degrees (36/60) 30 seconds = degrees (30/3600) Add up the degrees to get the answer: = DD

10 Spheres and Spheroids Sphere Spheroid Shape and size of GCS surface defined by sphere or spheroid. Mathematical calculations easier on a sphere. Sphere can be used for small-scale maps (< 1:5,000,000) Spheroid gives better accuracy for large-scale maps (>1:1,000,000

11 Major and Minor Axes of Ellipse Major Semiminor Axis Minor Axis Axis Semimajor Axis Shape of ellipse defined by two radii. Longer radius: Semimajor Axis Shorter radius: Semiminor Axis Rotating spheroid around semiminor axis creates a spheroid

12 Spheroids for Accurate Mapping Earth has been surveyed many times Surveys result in many spheroids Spheroid chosen to fit one country Best fit for one regions not same for another region Earth is neither perfect sphere nor spheroid Changing coordinate system s spheroid changes all previously measured values

13 Datums Spheroids approximate earth s shape Datum Datum defines position of spheroid relative to center of the earth Datum Datum defines origin and orientation of lat/long lines Local Local datum aligns spheroid to fit surface in a particular area

14 Datum Comparisons Local geographic Coordinate system Earth s surface Earth-centered datum Local datum Earth-centered geographic Coordinate system

15 North American Datums NAD27 uses Clarke 1866 spheroid Origin Meade Ranch Kansas Manually calculated control points NAD83 Based on earth and satellite observations Uses GRS80 spheroid Origin is earth s center of mass Previous control points shift as much as 500

16 Projected Coordinate Systems Defined Defined on flat, 2D surface Has Has constant lengths, angles and area Always Always based on geographic coordinate system X,Y X,Y coordinates on grid

17 What is a Map Projection? Transformation of 3D surface to 2D flat sheet Causes distortion in the shape, area, distance or direction of data Uses mathematical formulas to relate spherical coordinates to planar coordinates Different projections cause different distortions Map projections designed for specific purpose i.e. large-scale data in limited area

18 Relevance to GIS maps are a common source of input data for a GIS 1) often input maps will be in different projections, requiring transformation of one or all maps to make coordinates compatible 2) thus, mathematical functions of projections are needed in a GIS often GIS are used for projects of global or regional scales so consideration of the effect of the earth's curvature is necessary monitor screens are analogous to a flat sheet of paper 1) thus, need to provide transformations from the curved surface to the plane for displaying data

19 Conclusion What What is a coordinate system? A coordinate system is a grid that may be used to define where a particular location is Connection between projection and coordinate system The projection defines the coordinate system by defining the 2-D 2 D surface of the earth