gis spatial data and maps
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gis spatial data and maps






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gis spatial data and maps gis spatial data and maps Presentation Transcript

  • GIS Earth Science Department, University of the Western Cape DIGITAL MAPS Dr. A. Siad
    • Spatial data can be described according to:
    • entity
      • distinct object (river)
    • attribute
    • characteristic of entity (discharge)
    • relationship
    • linkages amongst objects (drainage basin)
  • Advantages of digital maps over paper
    • Easier to produce
    • Easier to store
    • Easier to distribute
    • Dimensional stability
  • topology
    • mathematics of spatial relationships among point, line and area objects.
  • Map a graphical representation of the spatial structure of physical and/or cultural environments
    • Map are abstractions since they present the real world in a simplified fashion at a reduced scale.
    • maps are used for:
    • the display of spatial data
    • the analysis of spatial data
  • Cartography
    • is the art and science of map-making
  • Cartographic considerations
    • Map scale
    • Data classification
    • Symbolization
    • Generalization
  • Map scale
    • the ratio between distances measured on a map and the corresponding distances measured on the ground
    • large scale map  displays a small area in greater detail
    • small scale map  displays a large area in lesser detail
    • the proposed use of a map is critical in determining the scale at which it is produced
  • methods for describing map scale:
    • Representative fraction 1 : 3,500,000
    • Graphical (bar)
    • Verbal “one inch equals one mile”
  • classes of maps
    • Reference maps (general purpose)
    • Thematic maps (special purpose)
  • Mapping process
    • Planning stage
      • needs assessment
      • project specifications are established (cost/time)
    • Data acquisition stage
      • primary data/secondary data
    • Cartographic production stage
      • design
      • drafting
      • proofing
      • printing
    • Product delivery stage
  • Georeferencing
    • the representation of the location of real-world features within the spatial framework of a particular coordinate system
  • Relationship between coordinate systems and map projections
    • Map projections define how positions on the Earth’s curved surface are transformed onto a flat map surface
    • Coordinate systems provide a referencing framework by which positions are measured and computed.
  • Plane coordinate systems
    • Plane rectangular system (Cartesian)
    • Plane polar coordinates
  • Cartesian sytem
    • X-axis (easting)
    • Y-axis (northing)
    • 1 st quadrant (+x, +y)
    • 2 nd quadrant (+x, -y)
    • 3 rd quadrant (-x, -y)
    • 4 th quadrant (-x, +y)
  • Plane polar coordinate system (Vector)
    • Polar axis = baseline
    • Radius vector, vectorial angle
    • Positive angles are measured in clockwise direction from polar axis
  • conversions
    • One can convert from plane rectangular coordinate to plane polar coordinates using basic trigonometric functions
  • Geographic Coordinate System
    • Latitude angular distance north or south of baseline (equator)
    • Longitude angular distance east or west of baseline (prime meridian)
    • graticule = grid of latitude and longitude lines (parallels and meridians) on a map or globe
  • Map projection
    • Is a systematic representation of all or part of the surface of a round body, especially the earth, on a plane (Snyder, 1987)
    • a two-dimensional spatial rendition of a the Earth’s surface (Fesperman, 2002)
  • Spatial properties of a map (projection)
    • area
    • direction
    • distance
    • shape
    • Projection type preserves
    • Equal area (equivalent) area
    • conformal (orthomorphic) shape
    • equidistant distance
  • Map projection types
    • a developable surface is one that can be laid flat without distortion
    • Cylindrical
    • Conical
    • Planar
  • Map projection aspect
    • Describes the orientation of the developable surface relative to the globe
    • normal axis of cylinder or cone parallel to earth axis
    • transverse axis of cylinder or cone is perpendicular to earth axis
    • oblique axis oriented between normal and transverse cases
  • Map projection viewpoints
    • Gnomonic
      • Light source at center of globe
      • Great circles are straight lines (navigation)
    • Orthographic
      • Light source at infinity
      • Equal areas preserved
    • Stereographic
      • Light source at far side of globe
      • Planar are conformal, cylindrical are compromised
  • The Ellipsoid
    • the earth is not a sphere, but an oblate spheroid, being slight depressed at the poles
    • satellite-based observations have allowed for the development of global standard in mathematical model
  • The Geoid
    • reference surface for vertical coordinates (above or below the datum)
    • equivalent to mean sea level in the earth’s oceans
    • determined by equal gravimetric potential