1. Georeferencing
Ming-Chun Lee

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

3. 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

4. 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

5. 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).

6. 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

7. 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).

8. 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

9. 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

10. 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: / / = º N

11. 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

12. 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

13. 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

14. 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

15. 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

16. Albers (Conical)

17. Mercator (Cylindrical)

18. Lambert Azimuthal Equal Area (Planar)

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

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

21. 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

23. 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

24. 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.

25. 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)