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test 2 geo 141 study notes.docx

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Ryerson University
GEO 151
Steven Swales

Entity Definition: - Geographical phenomena can be represented in two dimensions by main entity types- point, lines and areas - Example: Points can represent hotels, lines can represent ski lifts and areas can represent forests - Problems associated with simplifying the complexities of the real world into entity types include the dynamic nature of the real world, the identification of discrete and continuous features, and the scale at which a particular problem needs addressing - The dynamic nature of the world poses two problems for the entity- definition phase of the project 1) how to select the entity type that provides the most appropriate representation for the feature being modelled 2) The second problem is how to represent changes over time - The definition of entity types is also hampered by the fact that many real world features simply do not fit into the categories of entities available. Spatial Data models- -turning special entities into graphical representation is the second stage in designing and implementing a data model - Computers can display spatial entities in 2 ways 1) The raster approach 2) The vector approach - In the raster world, individual cells are used as building blocks for creating images of point, line, area, network and surface entities - In a raster, the shape and charactistics of an entity is created by the linking of cells - the size of the grid cell influences how the entity appears - Avector spatial data model uses Cartesian coordinates to store the shape of spatial entities - in the vector world, the point is the building block from which all spatial entities are constructed - lines and area entities are constructed by connecting a series of points into chains and polygons Spatial Data structures: - Data structures provides the information that the computer requires to reconstruct the spatial data in digital form Raster data structures- - Values 1,2 and 3 gave been used to classify the raster cells but land use present at the location - Size is a major problem with raster data sets as a complex image made up of a mosaic of different features require same amount of storage space as a similar raster map showing the location of a single forest Vector data structures: - Different entity types- point, lines and areas can be defined by coordinate geometry - The geometric representation of entities can be stored into the computer - Limitations of simple vector data structures start to emerge when more complex spatial entities are considered - Point dictionary is when all points in the data structure is numbered sequentially and contains an explicit reference which records which point is associated with which polygon - Topological structures contain information that informs computers where one polygon or line is in respect to its neighbours - Topology is concerned with connectivity between entities and not the physical shape - Apoint is the simplest spatial entity that can be represented in the vector world with togology - For a line to be topologically correct it must consist ordered set of points with def
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