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gis spatial data and maps


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gis spatial data and maps

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