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

Explore the fundamentals of georeferencing, which links data to specific locations on Earth through various coordinate systems. This quiz covers key properties, geographic coordinate systems, and the significance of spatial resolution in mapping. Test your knowledge of GIS and its components!