Thinking Geospatially

  • Help computer understand what we need!
  • Geospatial Data Concepts
    • Geospatial Data
      • Location and geometry
      • Attribute Data
        • Information describing the spatial objecets/features
      • Cartographic Data
        • Symbology and arrangement of the representations
  • Reference frameworks
    • Datums, projections
  • Data Acquisition Systems
    • Surveying, GPS
  • Data Models
    • Raster, Vector
  • Functional Models
    • Problem-solving
  • Information Dissemination
    • Visualization, Cartography, Human Interpretation

Geodesy, Datums, and Geographic Coordinate Systems

Geodesy

  • The science of measuring and analyzing:
    • The size of the Earth
    • The shape of the Earth
    • Its orientation in space
    • Its gravitational field
    • Tts magnetic field

The Gravitational Field

  • Knowing the strength of gravity at different places on the surface helps to access the actual shape of the planet
  • The geoid is a measured and interpolated surface representing the 3D surface along which the Earth’s gravitational field is constant
  • Geoids are now created from satellite measurements and surface readings
  • Colors represent deviations from mean sea level

Modeling the Earth

  • Spatial data is measured and calculated based on models of the Earth
  • Coordinates of Latitude and longitude refer to a shape chosen to represent the earth, not the actual surface

Ellipsoid Height

Datum

  • A datum is a mathematical model that standardizes the shape of the Earth for our particular purposes
  • A datum is basis for calculating positions on the Earth’s surface
  • A datum is composed of three elements:
    • An ellipsoid - a surface model of the Earth
    • Orientation - the position of the ellipsoid
    • Point of origin - the point where the Earth and the ellipsoid align

Horizontal and Vertical Datum

  • A horizontal datum standardizes location for horizontal measurements: x,y coordinates
  • A vertical datum standardizes elevation and height measurements: Z coordinate
  • Horizontal datum can now include a z coordinate, a height measurement from the chosen ellipsoid, but a vertical datum gives a more accurate height measurement

Commonly Used Datum

  • NAD27: North American Datum of 1927 (Based on Clarke 1866 ellipsoid)
  • NAD83: North American Datum of 1983 (Based on GRS 1980 ellipsoid)
  • WGS84: World Geodetic System of 1984 (Based on WGS 1984 ellipsoid)
  • Difference up to 250 meters

Choosing a Model (Datum)

  • Because the shape of the Earth is not uniform, we must choose on that best represents the Earth for our particular purpose
  • We use what we have learned from geodesy to make this choice, including the geoid, surface measurements, satellite imagery, etc.

Coordinate System

Two Kinds of Coordinate Systems

  • Geographic Coordinate System (GCS) is locational reference systems based on a graticule over a chosen datum
  • Projected Coordinate Systems(PCS) are locational reference systems based on a planar grid - a grid over a 2D surface, and the 2D surface is a chosen projection

The Graticule

  • The grid of lines of latitude and longitude
  • Latitude
    • Lines are parallel to equator
    • Describe N/S values
  • Longitude
    • Lines of meridian converge at poles
    • Describes E/W values

Latitude

  • The value of one’s latitude is the angle that a line drawn perpendicular to the surface through a point on a spheroid makes with the equatorial plane - not with center of the ellipsoid
  • Called geographic or geodetic latitude
  • Positive values in Northern Hemisphere; Negative values in Southern Hemisphere
  • Latitude goes to 90 degree

Longitude

  • The value of one’s longitude is the angle between the plane of the meridian that passes through a point on the surface of the spheroid and the plane of a chosen prime meridian (where the value of longitude is zero)
  • Called geographic or geodetic longitude OR geocentric longitude since the measurement is at the center of the ellipsoid
  • Positive values in Eastern Hemisphere
  • Negative values in Western Hemisphere
  • Longitude goes up to 180 degree

Degrees Minutes and Seconds

  • Latitude and Longitude are often expressed in units of Degrees, Minutes, and Second (DMS)
  • 1 Degree is broken down into 60 minutes
  • 1 Minute is broken down into 60 seconds
  • Converting it to Decimal Degrees

Geographic Coordinate Distances