Introduction to Georeferencing

Introduction to Georeferencing

• Georeferencing associates data with a specific location on Earth using coordinate systems
• Enables geographic information systems (GIS) to accurately map features

Key Properties of Georeferencing Systems

• Uniqueness: Locations must be uniquely identifiable, like differentiating locations with the same name (e.g. Springfield, MO vs Springfield, UK)
• Persistence: Locations remain consistent over time
• Spatial Resolution: Specifies level of detail (e.g. "Norman" is more precise than "Oklahoma")
• Metric vs. Non-Metric:
  • Metric: Uses coordinate systems for distance calculations (e.g., latitude/longitude)
  • Non-Metric: Uses relative locations (e.g., "next to the river")

Geographic Coordinate System (GCS)

• Uses latitude and longitude lines to define locations
• Components:
  • Latitude (Parallels): Measures north-south position, ranging from 0° at the equator to ±90° at the poles
  • Longitude (Meridians): Measures east-west position, ranging from 0° at the Prime Meridian (Greenwich, England) to ±180°
• Units: Angular degrees, minutes, and seconds (or decimal degrees)
  • Example: Norman, Oklahoma = 35°13'N, 97°26'W
• Terms to Remember:
  • Meridian: Line of constant longitude
  • Parallel: Line of constant latitude
• Unprojected Nature of GCS: Cannot accurately calculate areas or distances without projection

Map Projections

• Converts Earth's 3D surface to a 2D map
• Introduces distortions

Projection Properties

• Equal-Area (Equivalent): Preserves area, but distorts shape
• Conformal (Orthomorphic): Preserves shape, but distorts area
• Equidistant: Preserves distances (only in specific areas)
• True-Direction (Azimuthal): Preserves angles/directions
• Compromise: Balances distortion among properties

Projection Factors

• Developable Surfaces: Shapes onto which Earth is projected
  • Cylindrical: Gridlines form rectangles (e.g., Mercator)
  • Conical: Gridlines curve (e.g., Albers Equal Area)
  • Planar: Gridlines radiate from a central point (e.g., Azimuthal)
• Aspect: Orientation of the developable surface
  • Normal: Aligned with the poles
  • Transverse: Aligned with the equator
  • Oblique: At an angle
• Viewpoint: Light source for projection
  • Gnomonic: From the Earth's center
  • Stereographic: From the far side of the globe
• Case: Intersection of developable surface with Earth
  • Orthographic: From infinity
  • Tangent: Touches at one line or point
  • Secant: Cuts through the Earth

Common Map Projections

• Albers Equal Area: Preserves area, distorts shape, distance, and direction; Conic projection; used for regional/national maps
• Lambert Conformal Conic: Preserves shape and direction, distorts area; Conic projection; used for navigation, State Plane Coordinate System
• Mercator: Preserves direction and shape; distorts area at high latitudes; Cylindrical projection; used for navigation, world maps

Projected Coordinate Systems (PCS)

• Grid superimposed on a map projection, specifying locations in a plane
• Components:
  • Ellipsoid: A mathematical model of Earth's shape (e.g., GRS80, WGS84)
  • Datum: Links ellipsoid to specific Earth locations (e.g., NAD83)
  • Projection: Converts 3D Earth to 2D coordinates
• Common PCSs:
  • Universal Transverse Mercator (UTM): Divides globe into 60 zones, uses meters for coordinates, popular for GPS mapping
  • State Plane Coordinate System (SPCS): Divides states into zones, uses different projections to minimize distortion (Lambert or Mercator)

Practical Applications

• GCS vs. PCS:
  • GCS (latitude/longitude) is suitable for global contexts
  • PCS (projected) allows accurate distance, area, and direction measurements
• Choosing the right projection depends on map purpose
  • Preserve area for population density
  • Preserve distance for transportation
  • Preserve direction for navigation