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GIS fundamentals - vector

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Introduction to Vector GIS data

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GIS fundamentals - vector

  1. 1. Vector data Dr. Hans van der Kwast OpenCourseWare ocw.unesco-ihe.org
  2. 2. Learning objectives After this lecture you will be able to: • understand vector data models as used in GIS 2
  3. 3. GIS Basics 2 common data models to represent reality in a GIS: • Vector • Raster
  4. 4. Vector data • Vector data represents real world features in a GIS environment • A feature can be anything that you see in the landscape • Features have attributes: text or numerical information that describe the features 4
  5. 5. Vector types Three types: • Points • Lines or polylines • Polygons 5
  6. 6. Points • 1 X,Y location • No area • No length • Display: marker symbol, size, colour • Applies to features: •Without area •Too small to be displayed at current scale
  7. 7. Points • Geometry consists of only 1 vertex (node) 7
  8. 8. Lines or polylines • Series of X,Y points • No area • Have length • Connectivity • Direction • Display: Line stile, thickness, colour, offset • Applies to features: •Without area but with length •Too thin to represent as an area at the current scale
  9. 9. Lines or polylines • Geometry consists of 2 or more vertices 9
  10. 10. Polygons • Enclosed region • Area • Display: Fill pattern, fill colour, outline pattern and colour • Applies to features: •Countries •Provinces •Catchments
  11. 11. Polygons • Geometry consists of 3 or more vertices and the last vertex is the same as the first QGIS training Uganda 11
  12. 12. Vector overlay 12 • Points • Polylines • Polygons
  13. 13. Exercise vector 13 • This map doesn’t have a legend. In order to do proper styling we need to know which features are • Points • Polylines • Polygones Source: OpenStreetMap.org
  14. 14. What can we do with vector data? • Spatial queries, e.g.: • Which houses are within the 100 year flood level of a river? • Where is the best place to put a hospital so that it is easily accessible to as many people as possible? • Which learners live in a particular suburb? 14
  15. 15. Common problems with vector data • Accuracy of vectors depend on scale of digitizing 15 Digitized from 1/1000000 map Digitized from 1/50000 map
  16. 16. Common problems with vector data 16 Slivers
  17. 17. Common problems with vector data 17 Overshoots and undershoots
  18. 18. Vector attributes 18 Attribute data are used to describe features
  19. 19. Vector attributes 19
  20. 20. Vector attributes 20
  21. 21. Attributes for vector symbology 21 Coloured by roof colour Coloured by presence of balcony
  22. 22. Search by attributes 22
  23. 23. Attributes in detail Attribute Table Field 1 : YearBuilt Field 2: RoofColour Field 3: Balcony Record 1 1998 Red Yes Record 2 2000 Black No Record 3 2001 Silver Yes 23 Fields represent properties of the feature. Can you think of other attributes that we can connect to houses?
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  25. 25. Acknowledgements • Examples were taken from: A Gentle Introduction to GIS http://docs.qgis.org/2.6/en/docs/gentle_gis_intro duction/ 25

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