Introduction to Vector GIS data

1. Vector data
Dr. Hans van der Kwast
OpenCourseWare
ocw.unesco-ihe.org

2. Learning objectives
After this lecture you will be able to:
• understand vector data models as used in GIS
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3. GIS Basics
2 common data models
to represent reality in a
GIS:
• Vector
• Raster

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
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5. Vector types
Three types:
• Points
• Lines or polylines
• Polygons
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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. Points
• Geometry consists of
only 1 vertex (node)
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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. Lines or polylines
• Geometry consists of
2 or more vertices
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10. Polygons
• Enclosed region
• Area
• Display:
Fill pattern, fill colour,
outline pattern and colour
• Applies to features:
•Countries
•Provinces
•Catchments

11. Polygons
• Geometry consists of 3 or more
vertices and the last vertex is the
same as the first
QGIS training Uganda 11

12. Vector overlay
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• Points
• Polylines
• Polygons

13. Exercise vector
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• 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. 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?
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15. Common problems with vector data
• Accuracy of vectors depend on scale of digitizing
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Digitized from 1/1000000 map Digitized from 1/50000 map

16. Common problems with vector data
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Slivers

17. Common problems with vector data
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Overshoots and undershoots

18. Vector attributes
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Attribute data are used to describe features

19. Vector attributes
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20. Vector attributes
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21. Attributes for vector symbology
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Coloured by roof colour Coloured by presence of balcony

22. Search by attributes
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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
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Fields represent properties of the feature.
Can you think of other attributes that we can connect to houses?

24. 24

25. Acknowledgements
• Examples were taken from:
A Gentle Introduction to GIS
http://docs.qgis.org/2.6/en/docs/gentle_gis_intro
duction/
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