Gis Concepts 2/5


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Introduction to basic concepts on Geographical Information Systems
Autor: Msc. Alexander Mogollón Diaz

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Gis Concepts 2/5

  1. 1. Concepts and Functions of Geographic Information Systems (2/5) MSc GIS - Alexander Mogollon Diaz Department of Agronomy 2009
  2. 2. Concepts and Functions of GIS .PPT Topic #1 Topic #2 Topic #3 1 A GIS is an information system GIS is a technology 2 Spatial Data modelling Sources of data for geodatasets Metadata 3 Geo-referencing Coordinate transformations 4 Database management 5 Spatial Analysis
  3. 3. Modelling of geographic reality <ul><li>Geographic reality (GR) can be conceived as </li></ul><ul><ul><li>a collection of discrete, crisp spatial entities (water courses, built up zones, parcel boundaries, …) </li></ul></ul><ul><ul><li>occurring within </li></ul></ul><ul><ul><li>a spatial continuum, the terrain (topographic elevation, bathimetric depth, groundwater depth, cadmium concentration, noise intensity, ... ) </li></ul></ul>
  4. 4. Modelling of geographic reality with GIS <ul><li>Spatial delineation of that part of geographic reality that is of interest </li></ul><ul><li>Selection of entity classes and terrain characteristics which are of interest </li></ul><ul><li>matching the project, the problem at hand or the mission of the organisation </li></ul>
  5. 5. Spatial data modelling with GIS
  6. 6. The gDB as a spatial model <ul><li>The spatial model is materialised in the geographic database (gDB). The gDB is the CORE of every GIS </li></ul><ul><li>The gDB is a collection of vertically integrated geodatasets </li></ul><ul><li>Every geodataset contains (semi-)structured and interpreted data regarding 1 (or a few) entity classes (as object classes) or 1 terrain characteristic (as a surface/DTM) </li></ul><ul><li>Most common geodataset contains object classes reduced to 2 dimensions (planimetric) </li></ul>
  7. 7. The gDB as a spatial model <ul><li>Geodatasets in a gDB must be vertically integrated </li></ul><ul><ul><li>1 spatial reference system for all geodatasets </li></ul></ul><ul><ul><li>similar level of spatial detail for every geodataset </li></ul></ul><ul><ul><li>coinciding objects or parts of objects stored in separate geodatasets must spatially match </li></ul></ul><ul><ul><ul><li>a river line in geodataset 1 which is also a municipality boundary in geodataset 2 must coincide </li></ul></ul></ul>
  8. 8. Geographic database Geodataset 1 Geodataset 3 Geodataset 2 X Y
  9. 9. Two types of geodatasets <ul><li>There are two types of fundamentally different geodatasets: </li></ul><ul><ul><li>Geodatasets containing vectorial descriptions of objects ; Geometric primitive = point </li></ul></ul><ul><ul><li>Geodatasets containing cell-based descriptions of locations ; Geometric primitive = cell </li></ul></ul>
  10. 10. Vectorial versus cell-based spatial entity modelling
  11. 11. Vectorial objects
  12. 12. The vectorial data structure <ul><li>Well suited to model discrete spatial entities as discrete objects </li></ul><ul><li>Close to traditional cartography </li></ul><ul><li>Recognises objects with: </li></ul><ul><ul><li>geometric characteristics (locations - positions- pairs of X-Y coordinates) </li></ul></ul><ul><ul><li>descriptive attributes </li></ul></ul><ul><li>Spatial resolution (polygon or point representation, polygon or line representation, minimal polygon area and line minimal length, geometric detail of boundaries and lines) is determined by: </li></ul><ul><ul><li>The nature of the spatial entities to be modelled </li></ul></ul><ul><ul><li>Intended applications; “Scale” </li></ul></ul><ul><ul><li>Available memory, storage and computing capacity </li></ul></ul><ul><li>Geometric precision is limited only by the algorithms used and the computer capabilities </li></ul>
  13. 13. Cell-based objects
  14. 14. Cell-based equivalent of vectorial objects
  15. 15. The RASTER data structure <ul><li>Regular array of cells, each having a primary attribute </li></ul><ul><li>Objects are not explicitly defined </li></ul><ul><ul><li>labeling of pre-defined cells as part of or covering completely an entity </li></ul></ul><ul><ul><li>Implicit identification of objects as 1 cell or a connected set of cells having the same attribute </li></ul></ul><ul><li>Position of the cell is determined in an indirect way from </li></ul><ul><ul><li>the absolute X-Y coordinates of the origin of the grid </li></ul></ul><ul><ul><li>the row- and column number of the cell </li></ul></ul><ul><ul><li>the width (and height) of the cell </li></ul></ul><ul><li>A cell has an area which corresponds to the resolution of the raster or grid </li></ul>
  16. 16. Spatial resolution of raster-geodatasets <ul><li>Dimensions of elementary cell </li></ul><ul><li>Predefined choice depending upon </li></ul><ul><ul><li>Nature of the spatial entities to be modelled </li></ul></ul><ul><ul><li>Detail of the data which are available about the entities to be modelled </li></ul></ul><ul><ul><li>Intended applications </li></ul></ul><ul><ul><li>Available memory and computing capacity </li></ul></ul>
  17. 17. Vectorial versus cell-based objects Vector <ul><li>Exact modelling of entities “Vector is corrector” </li></ul><ul><li>Close to traditional cartography </li></ul><ul><li>Compact geodatasets </li></ul><ul><li>Efficient coding of spatial relationships </li></ul><ul><li>Complex structure </li></ul><ul><li>Complex overlay </li></ul><ul><li>Complex maintenance </li></ul>
  18. 18. Vectorial versus cell-based objects Raster <ul><li>Simple structure </li></ul><ul><li>Simpler analysis “Raster is faster” </li></ul><ul><li>Simple representation of complex realities </li></ul><ul><li>Geometric accuracy is determined by pre-defined spatial resolution </li></ul><ul><li>Less compact, compression may be needed </li></ul><ul><li>Spatial relationships are not explicitly stored </li></ul><ul><li>Graphic presentation may be less attractive (resolution) </li></ul>
  19. 19. Vector-to-Raster and Raster-to-Vector-conversion <ul><li>Spatial resolution (cell size) is important </li></ul><ul><li>Cel-size can be choosen freely but small cellslead to voluminous (bytes) geodatasets </li></ul>
  20. 20. Spatial (entity) modelling <ul><li>Design and elaboration of a gDB which is suitable for the intended applications, fit for purpose </li></ul><ul><ul><ul><li>Delineation of territory of interest </li></ul></ul></ul><ul><ul><ul><li>Selection –from geographic reality- of the relevant entity classes (terrain characteristics come later) </li></ul></ul></ul><ul><ul><ul><li>Choice of the coordinate reference system </li></ul></ul></ul><ul><ul><ul><li>Choice between the vector- or raster-datastructure for the geodatasets that will accomodate the data about the selected entity classes </li></ul></ul></ul><ul><ul><ul><li>Choice of the spatial resolution (Poly or Point, Poly or Line, minimal area/length; cell size) </li></ul></ul></ul><ul><ul><ul><ul><li>Must be comparable for all geodatasets in the gDB </li></ul></ul></ul></ul><ul><ul><ul><li>Acquisition / Creation of the geodatasets by structuring and transformation of the source data </li></ul></ul></ul><ul><ul><ul><li>Determiniation of the reference-geodataset </li></ul></ul></ul><ul><ul><ul><li>Integration of the geodatasets: vertical, horizontal, semantical </li></ul></ul></ul>
  21. 21. Each gDB contains a reference geodataset (a base map) <ul><li>Geodataset with the highest geometric quality, covering the full study area </li></ul><ul><li>Providing anchor points for vertical and horizontal integration </li></ul>
  22. 22. Geographic database polygon line point TIN categoric raster image lattice X Y
  23. 23. Sources of data for geodatasets <ul><li>Analogue and digital </li></ul><ul><li>Primary and secundary </li></ul>
  24. 24.
  25. 25. Earth Observation
  26. 26.
  27. 27.
  28. 28.
  29. 29.
  30. 30.
  31. 31. The Global Positioning System
  32. 32. Sources of data for geodatasets <ul><li>Analogue, printed maps: A/D conversion required </li></ul><ul><li>Imaging remote observations from earth, air or space of reflected electromagnetic radiation </li></ul><ul><ul><li>Photographical images </li></ul></ul><ul><ul><li>Multi- en hyperspectral images </li></ul></ul><ul><ul><li>Non-optical radiation (infrared, thermal, radar, …) images </li></ul></ul><ul><ul><li>Laser-based images </li></ul></ul><ul><li>Imaging remote observations from earth, air or space of reflected mechanic pressure waves </li></ul><ul><li>Data obtained by surveying </li></ul><ul><li>Field inventories </li></ul><ul><li>GPS measurements </li></ul><ul><li>Enquiries with an indirect spatial component -> national census </li></ul><ul><li>Traditional databases -> register of inhabitants </li></ul>
  33. 33. Documentation of available geo-datasets <ul><li>By means of METADATA: </li></ul><ul><li>METADATA contain description of </li></ul><ul><ul><li>Source of the data, author </li></ul></ul><ul><ul><li>Purpose of acquisition or production </li></ul></ul><ul><ul><li>Quality </li></ul></ul><ul><ul><ul><li>related to pre-processing </li></ul></ul></ul><ul><ul><ul><li>positional, thematic, temporal accuracy </li></ul></ul></ul><ul><ul><ul><li>consistency, completeness </li></ul></ul></ul><ul><ul><li>Spatial reference system </li></ul></ul><ul><ul><li>Definition of included object classes with attributes or terrain characteristics; assessment methods, value domain and units used for attributes </li></ul></ul><ul><ul><li>Explanation of codes </li></ul></ul><ul><ul><li>Format, availability and conditions/restrictions for use </li></ul></ul>
  34. 34. Summary of important items <ul><li>The gDB is a data model of geographic reality </li></ul><ul><li>Spatial data modelling = designing and building an appropriate gDB </li></ul><ul><li>gDB’s/spatial models are collections of vertically integrated geodatasets </li></ul><ul><li>Geodatasets describe 1 object class or 1 terrain aspect </li></ul><ul><li>A geodataset can be vector- or raster-structured </li></ul><ul><li>Geodatasets can be built from diverse data sources </li></ul><ul><li>Metadata of geodatasets are required for efficient use and re-use </li></ul>
  35. 35. Questions or remarks ? Thank you …