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# How is the real world represented in GIS?

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Introduction to geographical information systems by Katy Bregazzi, West Midlands Regional Observatory.

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### How is the real world represented in GIS?

1. 1. How is the real world represented in GIS? Katy Bregazzi West Midlands Regional Observatory 0 West Midlands Regional Observatory 2008
2. 2. Synopsis How is the real world represented in GIS? Linking data to a GIS Sources of data Layers 1 West Midlands Regional Observatory 2008
3. 3. How is the Real World Represented in GIS? Map Objects: - Point - Line - Area - Other vector geometry (network, surface) - Grid (raster/image) 2 West Midlands Regional Observatory 2008
4. 4. ‘Objects’ in Maps 3 West Midlands Regional Observatory 2008
5. 5. ‘Objects’ in Maps Point 4 West Midlands Regional Observatory 2008
6. 6. ‘Objects’ in Maps Line 5 West Midlands Regional Observatory 2008
7. 7. ‘Objects’ in Maps Area 6 West Midlands Regional Observatory 2008
8. 8. ‘Objects’ in Maps Co-ordinate Geometry: - Shape and location can be defined in terms of co-ordinate pairs from a given grid system (e.g. British National Grid). Point - An object that occurs in one physical location in space. - Defined by a single pair of co-ordinate values. Line - An object which spans between two or more points. - Defined by a sequence of co-ordinate pairs defining each point through which the line is drawn. Area (or Polygon) - An object which has area. - Defined by a sequence of co-ordinate pairs, with the first and last points joined to make a complete enclosure. 7 West Midlands Regional Observatory 2008
9. 9. ‘Objects’ in Maps – effects of scale Depending on the scale of the map and its intended purpose, ‘real life’ things can be represented by different object types - Railway station Point Area Snow Hill Stn. 8 West Midlands Regional Observatory 2008
10. 10. How is the real world represented in GIS? Some Examples: Vector Data Points Lines Areas Accident Locations; Roads; Motorways Emergency response Signs; Bus stops districts; Parking lots Port locations Rivers; Streams; Canals Lakes; Harbours; Wetlands Rail crossings Rail tracks Rail corridors Utility poles; Pylons Utility lines: pipes/cables Utility service areas Picnic sites; toilets Footpaths; cycle routes Parks; Forests; Fields Raster Data Aerial Photographs Satellite Imagery Ordnance Survey background maps e.g. 1:50K Scanned maps and images 9 West Midlands Regional Observatory 2008
11. 11. How is the real world represented in GIS? Network A feature defined by a series of line segments connected to form a continuous branching system of links. Networks enable the calculation of optimal routes through road networks, or the simulation of flow through rivers or pipes. Surface A feature which requires three dimensions to define it. A series of coordinate pairs define the surface, each pair with a vertical ‘z’ value. The ‘z’ value may represent: height, rainfall, population density etc. 10 West Midlands Regional Observatory 2008
12. 12. How is the Real World Represented in GIS? Map Objects: Other Attributes (fields) Attributes: descriptive information that is known about the feature Text Numbers Pictures River; Name, Length, Chemical Quality, Biological Quality Road; Name, Length, Classification (e.g. A or B road), Speed limit This extra information within a GIS enables some very powerful interrogation/analysis of the data 11 West Midlands Regional Observatory 2008
13. 13. Linking Existing Data to a GIS Can incorporate simple spreadsheet files or more complex relational databases GIS links with the data through the geography/location field/column e.g. Postcode, Region, SOA Example of joining two tables: 12 West Midlands Regional Observatory 2008
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17. 17. Sources of Data Direct (Primary) Satellite Surveying Data Capture (Global Positioning Systems GPS) Total Station Survey Satellite Remote Sensing Aerial Photography Indirect (secondary) GIS Data Capture Incorporation of existing digital data Paper maps -Digitising Corporate Databases -Scanning (e.g. in Access, Oracle etc) Digital Maps (e.g. from Ordnance survey) 16 West Midlands Regional Observatory 2008
18. 18. Sources of Data for Use in GIS Getting hold of digital geographic data can be difficult: - Create it Yourself (field survey, digitise, scan image etc) - Buy it from other organisations - Web services (e.g. ONS) - GIS Software package - may include some basic free data - Obtain specialist GIS data products (e.g. OS digital products; mastermap, landline etc) Consider: availability, existing agreements, time, costs, quality, purpose, data conversion, file sizes & performance, copyright Much existing data held by organisations contains location and is therefore able to be mapped/used in a GIS 17 West Midlands Regional Observatory 2008
19. 19. GIS Layers With acknowledgements to ESRI 18 West Midlands Regional Observatory 2008
20. 20. GIS Layers Map Objects (Records) Stored in GIS Layers (Files) - Open any combination in any order (including raster and vector) - Control style, labels, visibility etc - Temporary drawing/cosmetic layer - Combine layers for display, query and analysis 19 West Midlands Regional Observatory 2008