Digital mapping

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Digital mapping

  1. 1. Spatial models of the landscape: Digital Maps What they are and what they can do
  2. 2. Paper maps: problemsNothing can be changed:• it was designed for someone else• the scale is fixed• it can’t be updated• it can’t be reclassified or re-symbolised
  3. 3. Digital maps: what are they?• A digital map is a computer database of: – The geometry of the features being mapped – The geographical location of the features – Definitions of how features should be shown – Descriptions of what the features are (attributes)• These maps must be linked to a graphics program so they can be displayed and printed• They must be linked to a Geographical Information System (GIS) if they are to be analysed and queried.
  4. 4. Digital map and its data table
  5. 5. Digital maps: the benefits• The database can be constantly updated and corrected• The data can be classified by the user according to their own needs• Graphic symbols can be chosen to suit the job in hand• The map is dynamic and under the control of the user
  6. 6. How computers store spatial data structures• Spatial data is commonly stored in one of two ways: – Vector (based on geographic coordinates) – Raster (based on geographic grids)• Vector structures are used where discrete real-world objects need to be mapped.• Raster structures are used where continuous “fuzzy” data must be mapped.
  7. 7. Data structures: how vector data is displayed Buildings are points Fields are polygons The key shows theRivers are lines class each object is assigned to Buildings Rivers
  8. 8. Vector map of an urban area
  9. 9. Digitising a polygony x
  10. 10. Digitising a polygony
  11. 11. Digitising a polygony x
  12. 12. Digitising a polygony x
  13. 13. Digitising a polygony ID=Field_10 x
  14. 14. Digitising a polygony Storing the data • Feature_type=polygon • ID=field_10 • vertices=9 • coordinates= ID=Field_10 x1,y1;x2,y2;…x9,y9 x
  15. 15. Digitising a polygony Storing the data • Feature_type=polygon • ID=field_10 • vertices=9 • coordinates= ID=Field_10 x1,y1;x2,y2;…x9,y9 ID Crop Area Soil xAttributes which describe Field_09 Wheat 10.45 Haselor Field_10 Barley 15.76 Moretonthe features are stored in Field_11 Barley 12.33 Moretona linked table Field_14 Pasture 24.79 Didmarton
  16. 16. Fully digitised farm map • Each field is a separate polygon • Each footpath is a line • Each road is a lineID Crop Area SoilField_09 Wheat 10.45 HaselorField_10 Barley 15.76 MoretonField_11 Barley 12.33 MoretonField_14 Pasture 24.79 Didmarton
  17. 17. Strengths of vector based digital maps in land and property management• The basic entities (objects) which make up the landscape are defined and held in the database• Unlimited information can be attached to each entity• Vector digital maps can be easily attached to existing databases
  18. 18. Data structures: how raster data is displayedWhole region is dividedinto equal sized cellsEach cell holds onevalue (attribute)Attributes can berepresented by colours
  19. 19. Data structures: how raster data is stored1 62345
  20. 20. Data structures: how raster data is stored1 62 Resolution say: 500 x 500345
  21. 21. Data structures: how raster data is stored1 62 Resolution say: 500 x 5003 Attribute key: 3 = urban4 4 = forest 5 = field 7 = quarry5
  22. 22. Data structures: how raster data is stored1 62 Resolution say: 500 x 5003 4 5 3 3 3 5 5 5 5 Attribute key: 3 = urban4 5 5 3 3 5 5 5 5 7 4 = forest 5 5 3 5 5 5 5 7 7 5 = field 3450,5670 7 = quarry5
  23. 23. Data structures: how raster data is stored1 62 Resolution say: 500 x 5003 4 5 3 3 3 5 5 5 5 Attribute key: 3 = urban4 5 5 3 3 5 5 5 5 7 4 = forest Choose 5 5 3 5 5 5 5 7 7 colours to 5 = field visualise data 3450,5670 7 = quarry How the data is5 visualised as a raster
  24. 24. Data structures: how raster data is stored1 6 How the raster data is stored columns=9,rows=3, resolution-x=500,resolution-y=500,2 start=3450,5670 (co-ordinates of start of grid) Resolution say: 500 x 500 data=4,5,3,3,3,5,5,5,5,5,5,3,3,5,5,5,5,7,5,5,3,5, 5,5,5,7,73 4 5 3 3 3 5 5 5 5 Attribute key: 3 = urban4 5 5 3 3 5 5 5 5 7 4 = forest Choose 5 5 3 5 5 5 5 7 7 colours to 5 = field visualise data 3450,5670 7 = quarry How the data is5 visualised as a raster
  25. 25. Strengths of raster based digital maps in land management• Every bit of land is represented in the image• Data which is continuous can be mapped easily, such as soil fertility, rain fall, elevation• Maps of different themes can be overlapped easily to show correlations and conflicts• Raster maps are not of significant use in property maps except as backdrop images
  26. 26. Representing vegetation cover in a raster structure Study area divided into equal sized cellsEach cell assigned to a class in the map key
  27. 27. OS Digital raster mapRaster map: images built up of cells
  28. 28. OS Digital maps: vector mapsVector map: Features drawn ascoordinates (points on a graph) joined bylines
  29. 29. OS Digital maps: Mastermap SuperPlan digital maps Features digitised as graphical features: Lines and dots MasterMap digital maps Features digitised as “topographicidentifiers” TOIDS: buildings, property boundaries, roads
  30. 30. Historical mapping from the OS• Historical maps can be found as paper maps in libraries, county archives and surveyor’s offices.• Increasingly the OS is digitising its historical collection, which will allow it to be integrated with modern map products
  31. 31. Raster or vector? What are the comparisonsVector Raster– real world entities are – world is broken into retained in the database squares that don’t exist– many themes can be – only one theme can be shown on one map represented on one map– there may be gaps with no – all of the land is given data between entities some value– it is difficult to compare – easy to compare raster different maps maps– good for discrete things – good for things that vary e.g. property boundaries, continuously, e.g. roads temperature, height, soil depth– good for property management – good for environmental management
  32. 32. Summary: which to useBoth – Each digital model has benefits in different situations – Most modern GIS programs can handle both types of map simultaneously – It is possible to convert between one structure and the other, but information is lost with each conversion.

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