LECTURE 2 SPATIAL DATA MODELS

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1 LECTURE 2 SPATIAL DATA MODELS

2 Computers and GIS cannot directly be applied to the real world: a data gathering step comes first. Digital computers operate in numbers and characters held internally as binary digits. The real-world phenomenon of interest must be represented in symbolic form. The abstraction process of representing any property of the earth s surface in a computeraccessible form involves the use of symbolic models.

3 Models are simplification of reality. A map is a symbolic model, because it is a simplified representation of part of the real world. The components of the model are spatial objects, approximating spatial entities of the real world; they are represented on the map by graphical symbols. The process of defining and organizing data about the real world into a consistent digital dataset that is useful and reveals information is called data modeling. The logical organization of data according to a scheme is known as data model.

4 Data can be defined as verifiable facts. Information is data organized to reveal patterns, and to facilitate search. Spatial information is difficult to extract from spatial data, unless the data are organized primarily by spatial attributes. Spatial objects are characterized by attributes that are both spatial and nonspatial, and the digital description of objects and their attributes comprise spatial datasets.

5 Spatial data can be organized in different ways, depending on the way they are collected, how they are stored, and the purpose they are put. A database is a collection of inter-related data and everything that is needed to maintain and use it. A Database Management System is a collection of software for storing, editing and retrieving data in a database.

6 SPATIAL OBJECTS DIMENSIONALITY GROUPING ZERO DIMENSIONS POINTS ONE DIMENSION LINES TWO DIMENSIONS AREAS THREE DIMENSIONS VOLUMES NATURAL SPATIAL OBJECTS, IMPOSED SPATIAL OBJECTS

7 RASTER MODEL Spatial data of different types can be overlaid without the need for the complex geometric calculations. Each layer of grid cells in a raster model records a separate attribute. The cells (pixels, picture elements) are constant in size and are generally square. It is unnecessary to store the coordinates of each cell as the cells are arranged in a regular pattern It is enough to determine the cell size and the parameters to transform the X and Y coordinates and the cell locations in the raster map (rows/lines and columns). This process is called georeferencing.

8 RASTER MODEL Through georeferencing we calculate the parameters used in the equation to transform between a coordinate system and cell (pixel) location in the image A point is described by the position of a single pixel. The position of each cell is defined by a row and column number. Lines are represented by strings of connected pixels The spatial resolution of a raster is the size of one pixel on the ground At 100m resolution, a square area of 100km on a side requires a raster of 1000 rows by 1000 columns or one million pixels.

9 RASTER MODEL Large storage space specially for high resolution images Levels of details that require high resolution are lost when using coarse resolution Raster data processing is efficient in neighbourhood query, filtering, overlay operations for combining two or more images

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11 VECTOR MODEL In a vector model the position of each spatial feature is defined by (a series of) x and y coordinates. Besides the location, the meaning of the feature is given by a code (label). Points : the location of a point is described by a single xy coordinate pair. Points are stored in point maps by registering their location in x and y coordinates and by coding them (e.g. A rainfall station, elevation, etc.).

12 VECTOR MODEL Segments: a line is stored as a series of x and y coordinates (points) that best reflect its characteristics. The points are connected by straight lines. Line features are called segments. A segment is described by the coordinates of its starting and ending points (its nodes) and the intermediate points. The code of a segment defines what the segment represents (e.g. Road, state boundary, contour value, etc.). Polygons: areas are represented by their borders or boundary lines, which are in fact lines stored in the same way as segments. A polygon is a closed area of one or more Boundary segments and a code. Boundary segments define both the boundary of the Polygon to their left as to their right (topological structure). An area defined by its Boundary line and by its code is called a polygon. The code of a polygon defines the Contents of the polygon (e.g. Forest, city, sandstone, etc.).

13 VECTOR MODEL Vector maps are point, segment, or polygon maps. Most of the maps are obtained by digitizing or by importing them. Vector maps require less disk storage space and are suitable for creating high quality outputs. They are less suitable for a number of GIS operations, especially those dealing with map overlaying. You can rasterize the vector maps into raster maps

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