Georeferencing Ming-Chun Lee

The GIS Data Model Layers Integrate Using Explicit Location on the Earth’s Surface Themes have to be georeferenced.

Georeferencing Identifying Locations on the Earth Using Locations on a Map or GIS Aspects of Georeferencing Coordinate Systems Geographic Coordinate System Projected Coordinate System Projections Identifying Locations on the Earth Using Locations on a Map or GIS You connect a location on a map to a location in the real world Aspects of Georeferencing, what you need to do georeferecning: Coordinate Systems Geographic Coordinate System Projected Coordinate System Projections

Coordinate Systems A Method of Locating Objects on the Earth’s Surface Examples: Geographic (Global) Coordinate System Projected (Cartesian) Coordinate System A Method of Locating Objects on the Earth’s Surface Examples: Geographic (Global) Coordinate System Projected (Cartesian) Coordinate System Even though we said, projected CS, allow you to locate a feature on a map, but you still need to reference this point to a location in the real world, so your ultimate goal is still to Locating Objects on the Earth’s Surface

Geographic Coordinate System Geographic Coordinate System (GCS) uses a three dimensional spherical surface to define locations on the earth. Geographic Coordinate System (GCS) uses a three dimensional spherical surface to define locations on the earth. A GCS includes an angular unit of measure, a prime meridian, equator, and a datum (based on a spheroid).

Geographic Coordinate System In the spherical system, horizontal lines are lines of equal latitude, or parallels Vertical lines are lines of equal longitude, or meridians. These lines encompass the globe and form a gridded network called a graticule In the spherical system, horizontal lines are lines of equal latitude, or parallels Vertical lines are lines of equal longitude, or meridians. These lines encompass the globe and form a gridded network called a graticule

Geographic Coordinate System A point is referenced by its longitude and latitude values. Longitude and latitude are angles measured from the earth’s center to a point on the earth’s surface. The angles often are measured in degrees (or in grads). A point is referenced by its longitude and latitude values. Longitude and latitude are angles measured from the earth’s center to a point on the earth’s surface. The angles often are measured in degrees (or in grads).

Measuring Latitude North Pole (90 º N) 60 º N Latitude Measures the Angular Distance from the Equator 60 º Equator South Pole (90 º S) Side View

Measuring Longitude 135 º E Longitude Measures the Angular Distance from the Prime Meridian (Greenwich, UK) North Pole Equator 135 º 70 º 70 º W Prime Meridian (0 º) Top View

Measuring Latitude & Longitude Measurement in Degrees, Minutes, Seconds 1 Degree = 60 Minutes 1 Minute = 60 Seconds 1 Degree = 3600 Seconds Forms of Latitude & Longitude: DMS: 135º45'23" N Decimal: 135 + 45/60 + 23/3600 = 135.7564º N

Global vs. Cartesian Coordinates Spherical Effect on Distances Variable Distances Effect on Areas Minimal Distortion Cartesian Planar Effect on Distances Constant Distances Effect on Areas High Distortion a Edges

Projected Coordinate Systems A projected coordinate system is defined on a flat, two-dimensional surface. A projected coordinate system is always based on a geographic coordinate system that is based on a sphere or spheroid. In a projected coordinate system, locations are identified by x,y coordinates on a grid A projected coordinate system is defined on a flat, two-dimensional surface. A projected coordinate system is always based on a geographic coordinate system that is based on a sphere or spheroid. In a projected coordinate system, locations are identified by x,y coordinates on a grid

Projection Whether you treat the earth as a sphere or a spheroid, you must transform its three-dimensional surface to create a flat map sheet. The Process of Systematically Transforming Positions on the Earth’s Spherical Surface to a Flat Map While Maintaining Spatial Relationships Whether you treat the earth as a sphere or a spheroid, you must transform its three- dimensional surface to create a flat map sheet. The Process of Systematically Transforming Positions on the Earth’s Spherical Surface to a Flat Map While Maintaining Spatial Relationships

Projection Types and Effects Equal Area Projections Preserve Areas of Features Conformal Projections Preserve Shapes of Small Features Show Local Directions Correctly Equidistant Projections Preserve Distances to Places from One Point True Direction Projections Preserve Bearings either Locally or from Center of Map

Geometric Models for Projection Conical Tangent along a Specific Latitude Screen is a conic surface. Lamp at the center of the earth Cylindrical Tangent along Equator or a Longitude Screen is a cylindrical surface. Lamp at the center of the earth Planar/Azimuthal/Zenithal Tangent at a Single Point Screen is a flat surface tangent to the earth. Lamp at various positions Conical Tangent along a Specific Latitude Screen is a conic surface. Lamp at the center of the earth Cylindrical Tangent along Equator or a Longitude Screen is a cylindrical surface. Lamp at the center of the earth Planar/Azimuthal/Zenithal Tangent at a Single Point Screen is a flat surface tangent to the earth. Lamp at various positions

Albers (Conical)

Mercator (Cylindrical)

Lambert Azimuthal Equal Area (Planar)

Map Projection and Distortion For Geometric Properties Affected by Projection: Shape or Angle Area Distance Direction All projections produce some distortion

Common Coordinate Systems Universal Transverse Mercator (UTM) State Plane (SP)

Universal Transverse Mercator Series of Cylindrical Projections, Tangential to a Longitude Line 60 Zones, One Every 6 Degrees Longitude Each Zone: Width is 6º in Longitude Height is from 80º S to 84º N in Latitude

State Plane Planar Projection Most States Have Two or More Zones Tangent at Different Points for Each State Most States Have Two or More Zones

State Plane Coordinates Developed in order to provide local reference systems that were tied to a national datum. In the United States, the State Plane System was developed in the 1930s and was based on the North American Datum 1927 (NAD27). State Plane System has been superseded by the NAD-83 System, maps in NAD-27 coordinates (in feet) are still in use. The State Plane System 1983 is based on the North American Datum 1983 (NAD83). NAD 83 coordinates are based on meters.

Datums Datums are Origin Points for Coordinate Systems, i.e. a Single Reference Point on the Surface of the Earth with the Coordinate Value of (0,0) Common Datums: North American Datum 1927 (NAD27) North American Datum 1983 (NAD83)